verify_refname_available(): report errors via a "struct strbuf *err"

[git] / refs.c

#include "cache.h"
#include "lockfile.h"
#include "refs.h"
#include "object.h"
#include "tag.h"
#include "dir.h"
#include "string-list.h"

struct ref_lock {
        char *ref_name;
        char *orig_ref_name;
        struct lock_file *lk;
        unsigned char old_sha1[20];
        int lock_fd;
};

/*
 * How to handle various characters in refnames:
 * 0: An acceptable character for refs
 * 1: End-of-component
 * 2: ., look for a preceding . to reject .. in refs
 * 3: {, look for a preceding @ to reject @{ in refs
 * 4: A bad character: ASCII control characters, "~", "^", ":" or SP
 */
static unsigned char refname_disposition[256] = {
        1, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
        4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4,
        4, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 2, 1,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 0, 0, 0, 0, 4,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 4, 4, 0, 4, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
        0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 3, 0, 0, 4, 4
};

/*
 * Flag passed to lock_ref_sha1_basic() telling it to tolerate broken
 * refs (i.e., because the reference is about to be deleted anyway).
 */
#define REF_DELETING    0x02

/*
 * Used as a flag in ref_update::flags when a loose ref is being
 * pruned.
 */
#define REF_ISPRUNING   0x04

/*
 * Used as a flag in ref_update::flags when the reference should be
 * updated to new_sha1.
 */
#define REF_HAVE_NEW    0x08

/*
 * Used as a flag in ref_update::flags when old_sha1 should be
 * checked.
 */
#define REF_HAVE_OLD    0x10

/*
 * Try to read one refname component from the front of refname.
 * Return the length of the component found, or -1 if the component is
 * not legal.  It is legal if it is something reasonable to have under
 * ".git/refs/"; We do not like it if:
 *
 * - any path component of it begins with ".", or
 * - it has double dots "..", or
 * - it has ASCII control character, "~", "^", ":" or SP, anywhere, or
 * - it ends with a "/".
 * - it ends with ".lock"
 * - it contains a "\" (backslash)
 */
static int check_refname_component(const char *refname, int flags)
{
        const char *cp;
        char last = '\0';

        for (cp = refname; ; cp++) {
                int ch = *cp & 255;
                unsigned char disp = refname_disposition[ch];
                switch (disp) {
                case 1:
                        goto out;
                case 2:
                        if (last == '.')
                                return -1; /* Refname contains "..". */
                        break;
                case 3:
                        if (last == '@')
                                return -1; /* Refname contains "@{". */
                        break;
                case 4:
                        return -1;
                }
                last = ch;
        }
out:
        if (cp == refname)
                return 0; /* Component has zero length. */
        if (refname[0] == '.')
                return -1; /* Component starts with '.'. */
        if (cp - refname >= LOCK_SUFFIX_LEN &&
            !memcmp(cp - LOCK_SUFFIX_LEN, LOCK_SUFFIX, LOCK_SUFFIX_LEN))
                return -1; /* Refname ends with ".lock". */
        return cp - refname;
}

int check_refname_format(const char *refname, int flags)
{
        int component_len, component_count = 0;

        if (!strcmp(refname, "@"))
                /* Refname is a single character '@'. */
                return -1;

        while (1) {
                /* We are at the start of a path component. */
                component_len = check_refname_component(refname, flags);
                if (component_len <= 0) {
                        if ((flags & REFNAME_REFSPEC_PATTERN) &&
                                        refname[0] == '*' &&
                                        (refname[1] == '\0' || refname[1] == '/')) {
                                /* Accept one wildcard as a full refname component. */
                                flags &= ~REFNAME_REFSPEC_PATTERN;
                                component_len = 1;
                        } else {
                                return -1;
                        }
                }
                component_count++;
                if (refname[component_len] == '\0')
                        break;
                /* Skip to next component. */
                refname += component_len + 1;
        }

        if (refname[component_len - 1] == '.')
                return -1; /* Refname ends with '.'. */
        if (!(flags & REFNAME_ALLOW_ONELEVEL) && component_count < 2)
                return -1; /* Refname has only one component. */
        return 0;
}

struct ref_entry;

/*
 * Information used (along with the information in ref_entry) to
 * describe a single cached reference.  This data structure only
 * occurs embedded in a union in struct ref_entry, and only when
 * (ref_entry->flag & REF_DIR) is zero.
 */
struct ref_value {
        /*
         * The name of the object to which this reference resolves
         * (which may be a tag object).  If REF_ISBROKEN, this is
         * null.  If REF_ISSYMREF, then this is the name of the object
         * referred to by the last reference in the symlink chain.
         */
        unsigned char sha1[20];

        /*
         * If REF_KNOWS_PEELED, then this field holds the peeled value
         * of this reference, or null if the reference is known not to
         * be peelable.  See the documentation for peel_ref() for an
         * exact definition of "peelable".
         */
        unsigned char peeled[20];
};

struct ref_cache;

/*
 * Information used (along with the information in ref_entry) to
 * describe a level in the hierarchy of references.  This data
 * structure only occurs embedded in a union in struct ref_entry, and
 * only when (ref_entry.flag & REF_DIR) is set.  In that case,
 * (ref_entry.flag & REF_INCOMPLETE) determines whether the references
 * in the directory have already been read:
 *
 *     (ref_entry.flag & REF_INCOMPLETE) unset -- a directory of loose
 *         or packed references, already read.
 *
 *     (ref_entry.flag & REF_INCOMPLETE) set -- a directory of loose
 *         references that hasn't been read yet (nor has any of its
 *         subdirectories).
 *
 * Entries within a directory are stored within a growable array of
 * pointers to ref_entries (entries, nr, alloc).  Entries 0 <= i <
 * sorted are sorted by their component name in strcmp() order and the
 * remaining entries are unsorted.
 *
 * Loose references are read lazily, one directory at a time.  When a
 * directory of loose references is read, then all of the references
 * in that directory are stored, and REF_INCOMPLETE stubs are created
 * for any subdirectories, but the subdirectories themselves are not
 * read.  The reading is triggered by get_ref_dir().
 */
struct ref_dir {
        int nr, alloc;

        /*
         * Entries with index 0 <= i < sorted are sorted by name.  New
         * entries are appended to the list unsorted, and are sorted
         * only when required; thus we avoid the need to sort the list
         * after the addition of every reference.
         */
        int sorted;

        /* A pointer to the ref_cache that contains this ref_dir. */
        struct ref_cache *ref_cache;

        struct ref_entry **entries;
};

/*
 * Bit values for ref_entry::flag.  REF_ISSYMREF=0x01,
 * REF_ISPACKED=0x02, REF_ISBROKEN=0x04 and REF_BAD_NAME=0x08 are
 * public values; see refs.h.
 */

/*
 * The field ref_entry->u.value.peeled of this value entry contains
 * the correct peeled value for the reference, which might be
 * null_sha1 if the reference is not a tag or if it is broken.
 */
#define REF_KNOWS_PEELED 0x10

/* ref_entry represents a directory of references */
#define REF_DIR 0x20

/*
 * Entry has not yet been read from disk (used only for REF_DIR
 * entries representing loose references)
 */
#define REF_INCOMPLETE 0x40

/*
 * A ref_entry represents either a reference or a "subdirectory" of
 * references.
 *
 * Each directory in the reference namespace is represented by a
 * ref_entry with (flags & REF_DIR) set and containing a subdir member
 * that holds the entries in that directory that have been read so
 * far.  If (flags & REF_INCOMPLETE) is set, then the directory and
 * its subdirectories haven't been read yet.  REF_INCOMPLETE is only
 * used for loose reference directories.
 *
 * References are represented by a ref_entry with (flags & REF_DIR)
 * unset and a value member that describes the reference's value.  The
 * flag member is at the ref_entry level, but it is also needed to
 * interpret the contents of the value field (in other words, a
 * ref_value object is not very much use without the enclosing
 * ref_entry).
 *
 * Reference names cannot end with slash and directories' names are
 * always stored with a trailing slash (except for the top-level
 * directory, which is always denoted by "").  This has two nice
 * consequences: (1) when the entries in each subdir are sorted
 * lexicographically by name (as they usually are), the references in
 * a whole tree can be generated in lexicographic order by traversing
 * the tree in left-to-right, depth-first order; (2) the names of
 * references and subdirectories cannot conflict, and therefore the
 * presence of an empty subdirectory does not block the creation of a
 * similarly-named reference.  (The fact that reference names with the
 * same leading components can conflict *with each other* is a
 * separate issue that is regulated by verify_refname_available().)
 *
 * Please note that the name field contains the fully-qualified
 * reference (or subdirectory) name.  Space could be saved by only
 * storing the relative names.  But that would require the full names
 * to be generated on the fly when iterating in do_for_each_ref(), and
 * would break callback functions, who have always been able to assume
 * that the name strings that they are passed will not be freed during
 * the iteration.
 */
struct ref_entry {
        unsigned char flag; /* ISSYMREF? ISPACKED? */
        union {
                struct ref_value value; /* if not (flags&REF_DIR) */
                struct ref_dir subdir; /* if (flags&REF_DIR) */
        } u;
        /*
         * The full name of the reference (e.g., "refs/heads/master")
         * or the full name of the directory with a trailing slash
         * (e.g., "refs/heads/"):
         */
        char name[FLEX_ARRAY];
};

static void read_loose_refs(const char *dirname, struct ref_dir *dir);

static struct ref_dir *get_ref_dir(struct ref_entry *entry)
{
        struct ref_dir *dir;
        assert(entry->flag & REF_DIR);
        dir = &entry->u.subdir;
        if (entry->flag & REF_INCOMPLETE) {
                read_loose_refs(entry->name, dir);
                entry->flag &= ~REF_INCOMPLETE;
        }
        return dir;
}

/*
 * Check if a refname is safe.
 * For refs that start with "refs/" we consider it safe as long they do
 * not try to resolve to outside of refs/.
 *
 * For all other refs we only consider them safe iff they only contain
 * upper case characters and '_' (like "HEAD" AND "MERGE_HEAD", and not like
 * "config").
 */
static int refname_is_safe(const char *refname)
{
        if (starts_with(refname, "refs/")) {
                char *buf;
                int result;

                buf = xmalloc(strlen(refname) + 1);
                /*
                 * Does the refname try to escape refs/?
                 * For example: refs/foo/../bar is safe but refs/foo/../../bar
                 * is not.
                 */
                result = !normalize_path_copy(buf, refname + strlen("refs/"));
                free(buf);
                return result;
        }
        while (*refname) {
                if (!isupper(*refname) && *refname != '_')
                        return 0;
                refname++;
        }
        return 1;
}

static struct ref_entry *create_ref_entry(const char *refname,
                                          const unsigned char *sha1, int flag,
                                          int check_name)
{
        int len;
        struct ref_entry *ref;

        if (check_name &&
            check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
                die("Reference has invalid format: '%s'", refname);
        if (!check_name && !refname_is_safe(refname))
                die("Reference has invalid name: '%s'", refname);
        len = strlen(refname) + 1;
        ref = xmalloc(sizeof(struct ref_entry) + len);
        hashcpy(ref->u.value.sha1, sha1);
        hashclr(ref->u.value.peeled);
        memcpy(ref->name, refname, len);
        ref->flag = flag;
        return ref;
}

static void clear_ref_dir(struct ref_dir *dir);

static void free_ref_entry(struct ref_entry *entry)
{
        if (entry->flag & REF_DIR) {
                /*
                 * Do not use get_ref_dir() here, as that might
                 * trigger the reading of loose refs.
                 */
                clear_ref_dir(&entry->u.subdir);
        }
        free(entry);
}

/*
 * Add a ref_entry to the end of dir (unsorted).  Entry is always
 * stored directly in dir; no recursion into subdirectories is
 * done.
 */
static void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry)
{
        ALLOC_GROW(dir->entries, dir->nr + 1, dir->alloc);
        dir->entries[dir->nr++] = entry;
        /* optimize for the case that entries are added in order */
        if (dir->nr == 1 ||
            (dir->nr == dir->sorted + 1 &&
             strcmp(dir->entries[dir->nr - 2]->name,
                    dir->entries[dir->nr - 1]->name) < 0))
                dir->sorted = dir->nr;
}

/*
 * Clear and free all entries in dir, recursively.
 */
static void clear_ref_dir(struct ref_dir *dir)
{
        int i;
        for (i = 0; i < dir->nr; i++)
                free_ref_entry(dir->entries[i]);
        free(dir->entries);
        dir->sorted = dir->nr = dir->alloc = 0;
        dir->entries = NULL;
}

/*
 * Create a struct ref_entry object for the specified dirname.
 * dirname is the name of the directory with a trailing slash (e.g.,
 * "refs/heads/") or "" for the top-level directory.
 */
static struct ref_entry *create_dir_entry(struct ref_cache *ref_cache,
                                          const char *dirname, size_t len,
                                          int incomplete)
{
        struct ref_entry *direntry;
        direntry = xcalloc(1, sizeof(struct ref_entry) + len + 1);
        memcpy(direntry->name, dirname, len);
        direntry->name[len] = '\0';
        direntry->u.subdir.ref_cache = ref_cache;
        direntry->flag = REF_DIR | (incomplete ? REF_INCOMPLETE : 0);
        return direntry;
}

static int ref_entry_cmp(const void *a, const void *b)
{
        struct ref_entry *one = *(struct ref_entry **)a;
        struct ref_entry *two = *(struct ref_entry **)b;
        return strcmp(one->name, two->name);
}

static void sort_ref_dir(struct ref_dir *dir);

struct string_slice {
        size_t len;
        const char *str;
};

static int ref_entry_cmp_sslice(const void *key_, const void *ent_)
{
        const struct string_slice *key = key_;
        const struct ref_entry *ent = *(const struct ref_entry * const *)ent_;
        int cmp = strncmp(key->str, ent->name, key->len);
        if (cmp)
                return cmp;
        return '\0' - (unsigned char)ent->name[key->len];
}

/*
 * Return the index of the entry with the given refname from the
 * ref_dir (non-recursively), sorting dir if necessary.  Return -1 if
 * no such entry is found.  dir must already be complete.
 */
static int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len)
{
        struct ref_entry **r;
        struct string_slice key;

        if (refname == NULL || !dir->nr)
                return -1;

        sort_ref_dir(dir);
        key.len = len;
        key.str = refname;
        r = bsearch(&key, dir->entries, dir->nr, sizeof(*dir->entries),
                    ref_entry_cmp_sslice);

        if (r == NULL)
                return -1;

        return r - dir->entries;
}

/*
 * Search for a directory entry directly within dir (without
 * recursing).  Sort dir if necessary.  subdirname must be a directory
 * name (i.e., end in '/').  If mkdir is set, then create the
 * directory if it is missing; otherwise, return NULL if the desired
 * directory cannot be found.  dir must already be complete.
 */
static struct ref_dir *search_for_subdir(struct ref_dir *dir,
                                         const char *subdirname, size_t len,
                                         int mkdir)
{
        int entry_index = search_ref_dir(dir, subdirname, len);
        struct ref_entry *entry;
        if (entry_index == -1) {
                if (!mkdir)
                        return NULL;
                /*
                 * Since dir is complete, the absence of a subdir
                 * means that the subdir really doesn't exist;
                 * therefore, create an empty record for it but mark
                 * the record complete.
                 */
                entry = create_dir_entry(dir->ref_cache, subdirname, len, 0);
                add_entry_to_dir(dir, entry);
        } else {
                entry = dir->entries[entry_index];
        }
        return get_ref_dir(entry);
}

/*
 * If refname is a reference name, find the ref_dir within the dir
 * tree that should hold refname.  If refname is a directory name
 * (i.e., ends in '/'), then return that ref_dir itself.  dir must
 * represent the top-level directory and must already be complete.
 * Sort ref_dirs and recurse into subdirectories as necessary.  If
 * mkdir is set, then create any missing directories; otherwise,
 * return NULL if the desired directory cannot be found.
 */
static struct ref_dir *find_containing_dir(struct ref_dir *dir,
                                           const char *refname, int mkdir)
{
        const char *slash;
        for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
                size_t dirnamelen = slash - refname + 1;
                struct ref_dir *subdir;
                subdir = search_for_subdir(dir, refname, dirnamelen, mkdir);
                if (!subdir) {
                        dir = NULL;
                        break;
                }
                dir = subdir;
        }

        return dir;
}

/*
 * Find the value entry with the given name in dir, sorting ref_dirs
 * and recursing into subdirectories as necessary.  If the name is not
 * found or it corresponds to a directory entry, return NULL.
 */
static struct ref_entry *find_ref(struct ref_dir *dir, const char *refname)
{
        int entry_index;
        struct ref_entry *entry;
        dir = find_containing_dir(dir, refname, 0);
        if (!dir)
                return NULL;
        entry_index = search_ref_dir(dir, refname, strlen(refname));
        if (entry_index == -1)
                return NULL;
        entry = dir->entries[entry_index];
        return (entry->flag & REF_DIR) ? NULL : entry;
}

/*
 * Remove the entry with the given name from dir, recursing into
 * subdirectories as necessary.  If refname is the name of a directory
 * (i.e., ends with '/'), then remove the directory and its contents.
 * If the removal was successful, return the number of entries
 * remaining in the directory entry that contained the deleted entry.
 * If the name was not found, return -1.  Please note that this
 * function only deletes the entry from the cache; it does not delete
 * it from the filesystem or ensure that other cache entries (which
 * might be symbolic references to the removed entry) are updated.
 * Nor does it remove any containing dir entries that might be made
 * empty by the removal.  dir must represent the top-level directory
 * and must already be complete.
 */
static int remove_entry(struct ref_dir *dir, const char *refname)
{
        int refname_len = strlen(refname);
        int entry_index;
        struct ref_entry *entry;
        int is_dir = refname[refname_len - 1] == '/';
        if (is_dir) {
                /*
                 * refname represents a reference directory.  Remove
                 * the trailing slash; otherwise we will get the
                 * directory *representing* refname rather than the
                 * one *containing* it.
                 */
                char *dirname = xmemdupz(refname, refname_len - 1);
                dir = find_containing_dir(dir, dirname, 0);
                free(dirname);
        } else {
                dir = find_containing_dir(dir, refname, 0);
        }
        if (!dir)
                return -1;
        entry_index = search_ref_dir(dir, refname, refname_len);
        if (entry_index == -1)
                return -1;
        entry = dir->entries[entry_index];

        memmove(&dir->entries[entry_index],
                &dir->entries[entry_index + 1],
                (dir->nr - entry_index - 1) * sizeof(*dir->entries)
                );
        dir->nr--;
        if (dir->sorted > entry_index)
                dir->sorted--;
        free_ref_entry(entry);
        return dir->nr;
}

/*
 * Add a ref_entry to the ref_dir (unsorted), recursing into
 * subdirectories as necessary.  dir must represent the top-level
 * directory.  Return 0 on success.
 */
static int add_ref(struct ref_dir *dir, struct ref_entry *ref)
{
        dir = find_containing_dir(dir, ref->name, 1);
        if (!dir)
                return -1;
        add_entry_to_dir(dir, ref);
        return 0;
}

/*
 * Emit a warning and return true iff ref1 and ref2 have the same name
 * and the same sha1.  Die if they have the same name but different
 * sha1s.
 */
static int is_dup_ref(const struct ref_entry *ref1, const struct ref_entry *ref2)
{
        if (strcmp(ref1->name, ref2->name))
                return 0;

        /* Duplicate name; make sure that they don't conflict: */

        if ((ref1->flag & REF_DIR) || (ref2->flag & REF_DIR))
                /* This is impossible by construction */
                die("Reference directory conflict: %s", ref1->name);

        if (hashcmp(ref1->u.value.sha1, ref2->u.value.sha1))
                die("Duplicated ref, and SHA1s don't match: %s", ref1->name);

        warning("Duplicated ref: %s", ref1->name);
        return 1;
}

/*
 * Sort the entries in dir non-recursively (if they are not already
 * sorted) and remove any duplicate entries.
 */
static void sort_ref_dir(struct ref_dir *dir)
{
        int i, j;
        struct ref_entry *last = NULL;

        /*
         * This check also prevents passing a zero-length array to qsort(),
         * which is a problem on some platforms.
         */
        if (dir->sorted == dir->nr)
                return;

        qsort(dir->entries, dir->nr, sizeof(*dir->entries), ref_entry_cmp);

        /* Remove any duplicates: */
        for (i = 0, j = 0; j < dir->nr; j++) {
                struct ref_entry *entry = dir->entries[j];
                if (last && is_dup_ref(last, entry))
                        free_ref_entry(entry);
                else
                        last = dir->entries[i++] = entry;
        }
        dir->sorted = dir->nr = i;
}

/* Include broken references in a do_for_each_ref*() iteration: */
#define DO_FOR_EACH_INCLUDE_BROKEN 0x01

/*
 * Return true iff the reference described by entry can be resolved to
 * an object in the database.  Emit a warning if the referred-to
 * object does not exist.
 */
static int ref_resolves_to_object(struct ref_entry *entry)
{
        if (entry->flag & REF_ISBROKEN)
                return 0;
        if (!has_sha1_file(entry->u.value.sha1)) {
                error("%s does not point to a valid object!", entry->name);
                return 0;
        }
        return 1;
}

/*
 * current_ref is a performance hack: when iterating over references
 * using the for_each_ref*() functions, current_ref is set to the
 * current reference's entry before calling the callback function.  If
 * the callback function calls peel_ref(), then peel_ref() first
 * checks whether the reference to be peeled is the current reference
 * (it usually is) and if so, returns that reference's peeled version
 * if it is available.  This avoids a refname lookup in a common case.
 */
static struct ref_entry *current_ref;

typedef int each_ref_entry_fn(struct ref_entry *entry, void *cb_data);

struct ref_entry_cb {
        const char *base;
        int trim;
        int flags;
        each_ref_fn *fn;
        void *cb_data;
};

/*
 * Handle one reference in a do_for_each_ref*()-style iteration,
 * calling an each_ref_fn for each entry.
 */
static int do_one_ref(struct ref_entry *entry, void *cb_data)
{
        struct ref_entry_cb *data = cb_data;
        struct ref_entry *old_current_ref;
        int retval;

        if (!starts_with(entry->name, data->base))
                return 0;

        if (!(data->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
              !ref_resolves_to_object(entry))
                return 0;

        /* Store the old value, in case this is a recursive call: */
        old_current_ref = current_ref;
        current_ref = entry;
        retval = data->fn(entry->name + data->trim, entry->u.value.sha1,
                          entry->flag, data->cb_data);
        current_ref = old_current_ref;
        return retval;
}

/*
 * Call fn for each reference in dir that has index in the range
 * offset <= index < dir->nr.  Recurse into subdirectories that are in
 * that index range, sorting them before iterating.  This function
 * does not sort dir itself; it should be sorted beforehand.  fn is
 * called for all references, including broken ones.
 */
static int do_for_each_entry_in_dir(struct ref_dir *dir, int offset,
                                    each_ref_entry_fn fn, void *cb_data)
{
        int i;
        assert(dir->sorted == dir->nr);
        for (i = offset; i < dir->nr; i++) {
                struct ref_entry *entry = dir->entries[i];
                int retval;
                if (entry->flag & REF_DIR) {
                        struct ref_dir *subdir = get_ref_dir(entry);
                        sort_ref_dir(subdir);
                        retval = do_for_each_entry_in_dir(subdir, 0, fn, cb_data);
                } else {
                        retval = fn(entry, cb_data);
                }
                if (retval)
                        return retval;
        }
        return 0;
}

/*
 * Call fn for each reference in the union of dir1 and dir2, in order
 * by refname.  Recurse into subdirectories.  If a value entry appears
 * in both dir1 and dir2, then only process the version that is in
 * dir2.  The input dirs must already be sorted, but subdirs will be
 * sorted as needed.  fn is called for all references, including
 * broken ones.
 */
static int do_for_each_entry_in_dirs(struct ref_dir *dir1,
                                     struct ref_dir *dir2,
                                     each_ref_entry_fn fn, void *cb_data)
{
        int retval;
        int i1 = 0, i2 = 0;

        assert(dir1->sorted == dir1->nr);
        assert(dir2->sorted == dir2->nr);
        while (1) {
                struct ref_entry *e1, *e2;
                int cmp;
                if (i1 == dir1->nr) {
                        return do_for_each_entry_in_dir(dir2, i2, fn, cb_data);
                }
                if (i2 == dir2->nr) {
                        return do_for_each_entry_in_dir(dir1, i1, fn, cb_data);
                }
                e1 = dir1->entries[i1];
                e2 = dir2->entries[i2];
                cmp = strcmp(e1->name, e2->name);
                if (cmp == 0) {
                        if ((e1->flag & REF_DIR) && (e2->flag & REF_DIR)) {
                                /* Both are directories; descend them in parallel. */
                                struct ref_dir *subdir1 = get_ref_dir(e1);
                                struct ref_dir *subdir2 = get_ref_dir(e2);
                                sort_ref_dir(subdir1);
                                sort_ref_dir(subdir2);
                                retval = do_for_each_entry_in_dirs(
                                                subdir1, subdir2, fn, cb_data);
                                i1++;
                                i2++;
                        } else if (!(e1->flag & REF_DIR) && !(e2->flag & REF_DIR)) {
                                /* Both are references; ignore the one from dir1. */
                                retval = fn(e2, cb_data);
                                i1++;
                                i2++;
                        } else {
                                die("conflict between reference and directory: %s",
                                    e1->name);
                        }
                } else {
                        struct ref_entry *e;
                        if (cmp < 0) {
                                e = e1;
                                i1++;
                        } else {
                                e = e2;
                                i2++;
                        }
                        if (e->flag & REF_DIR) {
                                struct ref_dir *subdir = get_ref_dir(e);
                                sort_ref_dir(subdir);
                                retval = do_for_each_entry_in_dir(
                                                subdir, 0, fn, cb_data);
                        } else {
                                retval = fn(e, cb_data);
                        }
                }
                if (retval)
                        return retval;
        }
}

/*
 * Load all of the refs from the dir into our in-memory cache. The hard work
 * of loading loose refs is done by get_ref_dir(), so we just need to recurse
 * through all of the sub-directories. We do not even need to care about
 * sorting, as traversal order does not matter to us.
 */
static void prime_ref_dir(struct ref_dir *dir)
{
        int i;
        for (i = 0; i < dir->nr; i++) {
                struct ref_entry *entry = dir->entries[i];
                if (entry->flag & REF_DIR)
                        prime_ref_dir(get_ref_dir(entry));
        }
}

struct nonmatching_ref_data {
        const struct string_list *skip;
        const char *conflicting_refname;
};

static int nonmatching_ref_fn(struct ref_entry *entry, void *vdata)
{
        struct nonmatching_ref_data *data = vdata;

        if (data->skip && string_list_has_string(data->skip, entry->name))
                return 0;

        data->conflicting_refname = entry->name;
        return 1;
}

/*
 * Return 0 if a reference named refname could be created without
 * conflicting with the name of an existing reference in dir.
 * Otherwise, return a negative value and write an explanation to err.
 * If extras is non-NULL, it is a list of additional refnames with
 * which refname is not allowed to conflict. If skip is non-NULL,
 * ignore potential conflicts with refs in skip (e.g., because they
 * are scheduled for deletion in the same operation). Behavior is
 * undefined if the same name is listed in both extras and skip.
 *
 * Two reference names conflict if one of them exactly matches the
 * leading components of the other; e.g., "refs/foo/bar" conflicts
 * with both "refs/foo" and with "refs/foo/bar/baz" but not with
 * "refs/foo/bar" or "refs/foo/barbados".
 *
 * extras and skip must be sorted.
 */
static int verify_refname_available(const char *refname,
                                    const struct string_list *extras,
                                    const struct string_list *skip,
                                    struct ref_dir *dir,
                                    struct strbuf *err)
{
        const char *slash;
        int pos;
        struct strbuf dirname = STRBUF_INIT;
        int ret = -1;

        /*
         * For the sake of comments in this function, suppose that
         * refname is "refs/foo/bar".
         */

        assert(err);

        strbuf_grow(&dirname, strlen(refname) + 1);
        for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
                /* Expand dirname to the new prefix, not including the trailing slash: */
                strbuf_add(&dirname, refname + dirname.len, slash - refname - dirname.len);

                /*
                 * We are still at a leading dir of the refname (e.g.,
                 * "refs/foo"; if there is a reference with that name,
                 * it is a conflict, *unless* it is in skip.
                 */
                if (dir) {
                        pos = search_ref_dir(dir, dirname.buf, dirname.len);
                        if (pos >= 0 &&
                            (!skip || !string_list_has_string(skip, dirname.buf))) {
                                /*
                                 * We found a reference whose name is
                                 * a proper prefix of refname; e.g.,
                                 * "refs/foo", and is not in skip.
                                 */
                                strbuf_addf(err, "'%s' exists; cannot create '%s'",
                                            dirname.buf, refname);
                                goto cleanup;
                        }
                }

                if (extras && string_list_has_string(extras, dirname.buf) &&
                    (!skip || !string_list_has_string(skip, dirname.buf))) {
                        strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
                                    refname, dirname.buf);
                        goto cleanup;
                }

                /*
                 * Otherwise, we can try to continue our search with
                 * the next component. So try to look up the
                 * directory, e.g., "refs/foo/". If we come up empty,
                 * we know there is nothing under this whole prefix,
                 * but even in that case we still have to continue the
                 * search for conflicts with extras.
                 */
                strbuf_addch(&dirname, '/');
                if (dir) {
                        pos = search_ref_dir(dir, dirname.buf, dirname.len);
                        if (pos < 0) {
                                /*
                                 * There was no directory "refs/foo/",
                                 * so there is nothing under this
                                 * whole prefix. So there is no need
                                 * to continue looking for conflicting
                                 * references. But we need to continue
                                 * looking for conflicting extras.
                                 */
                                dir = NULL;
                        } else {
                                dir = get_ref_dir(dir->entries[pos]);
                        }
                }
        }

        /*
         * We are at the leaf of our refname (e.g., "refs/foo/bar").
         * There is no point in searching for a reference with that
         * name, because a refname isn't considered to conflict with
         * itself. But we still need to check for references whose
         * names are in the "refs/foo/bar/" namespace, because they
         * *do* conflict.
         */
        strbuf_addstr(&dirname, refname + dirname.len);
        strbuf_addch(&dirname, '/');

        if (dir) {
                pos = search_ref_dir(dir, dirname.buf, dirname.len);

                if (pos >= 0) {
                        /*
                         * We found a directory named "$refname/"
                         * (e.g., "refs/foo/bar/"). It is a problem
                         * iff it contains any ref that is not in
                         * "skip".
                         */
                        struct nonmatching_ref_data data;

                        data.skip = skip;
                        data.conflicting_refname = NULL;
                        dir = get_ref_dir(dir->entries[pos]);
                        sort_ref_dir(dir);
                        if (do_for_each_entry_in_dir(dir, 0, nonmatching_ref_fn, &data)) {
                                strbuf_addf(err, "'%s' exists; cannot create '%s'",
                                            data.conflicting_refname, refname);
                                goto cleanup;
                        }
                }
        }

        if (extras) {
                /*
                 * Check for entries in extras that start with
                 * "$refname/". We do that by looking for the place
                 * where "$refname/" would be inserted in extras. If
                 * there is an entry at that position that starts with
                 * "$refname/" and is not in skip, then we have a
                 * conflict.
                 */
                for (pos = string_list_find_insert_index(extras, dirname.buf, 0);
                     pos < extras->nr; pos++) {
                        const char *extra_refname = extras->items[pos].string;

                        if (!starts_with(extra_refname, dirname.buf))
                                break;

                        if (!skip || !string_list_has_string(skip, extra_refname)) {
                                strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
                                            refname, extra_refname);
                                goto cleanup;
                        }
                }
        }

        /* No conflicts were found */
        ret = 0;

cleanup:
        strbuf_release(&dirname);
        return ret;
}

struct packed_ref_cache {
        struct ref_entry *root;

        /*
         * Count of references to the data structure in this instance,
         * including the pointer from ref_cache::packed if any.  The
         * data will not be freed as long as the reference count is
         * nonzero.
         */
        unsigned int referrers;

        /*
         * Iff the packed-refs file associated with this instance is
         * currently locked for writing, this points at the associated
         * lock (which is owned by somebody else).  The referrer count
         * is also incremented when the file is locked and decremented
         * when it is unlocked.
         */
        struct lock_file *lock;

        /* The metadata from when this packed-refs cache was read */
        struct stat_validity validity;
};

/*
 * Future: need to be in "struct repository"
 * when doing a full libification.
 */
static struct ref_cache {
        struct ref_cache *next;
        struct ref_entry *loose;
        struct packed_ref_cache *packed;
        /*
         * The submodule name, or "" for the main repo.  We allocate
         * length 1 rather than FLEX_ARRAY so that the main ref_cache
         * is initialized correctly.
         */
        char name[1];
} ref_cache, *submodule_ref_caches;

/* Lock used for the main packed-refs file: */
static struct lock_file packlock;

/*
 * Increment the reference count of *packed_refs.
 */
static void acquire_packed_ref_cache(struct packed_ref_cache *packed_refs)
{
        packed_refs->referrers++;
}

/*
 * Decrease the reference count of *packed_refs.  If it goes to zero,
 * free *packed_refs and return true; otherwise return false.
 */
static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
{
        if (!--packed_refs->referrers) {
                free_ref_entry(packed_refs->root);
                stat_validity_clear(&packed_refs->validity);
                free(packed_refs);
                return 1;
        } else {
                return 0;
        }
}

static void clear_packed_ref_cache(struct ref_cache *refs)
{
        if (refs->packed) {
                struct packed_ref_cache *packed_refs = refs->packed;

                if (packed_refs->lock)
                        die("internal error: packed-ref cache cleared while locked");
                refs->packed = NULL;
                release_packed_ref_cache(packed_refs);
        }
}

static void clear_loose_ref_cache(struct ref_cache *refs)
{
        if (refs->loose) {
                free_ref_entry(refs->loose);
                refs->loose = NULL;
        }
}

static struct ref_cache *create_ref_cache(const char *submodule)
{
        int len;
        struct ref_cache *refs;
        if (!submodule)
                submodule = "";
        len = strlen(submodule) + 1;
        refs = xcalloc(1, sizeof(struct ref_cache) + len);
        memcpy(refs->name, submodule, len);
        return refs;
}

/*
 * Return a pointer to a ref_cache for the specified submodule. For
 * the main repository, use submodule==NULL. The returned structure
 * will be allocated and initialized but not necessarily populated; it
 * should not be freed.
 */
static struct ref_cache *get_ref_cache(const char *submodule)
{
        struct ref_cache *refs;

        if (!submodule || !*submodule)
                return &ref_cache;

        for (refs = submodule_ref_caches; refs; refs = refs->next)
                if (!strcmp(submodule, refs->name))
                        return refs;

        refs = create_ref_cache(submodule);
        refs->next = submodule_ref_caches;
        submodule_ref_caches = refs;
        return refs;
}

/* The length of a peeled reference line in packed-refs, including EOL: */
#define PEELED_LINE_LENGTH 42

/*
 * The packed-refs header line that we write out.  Perhaps other
 * traits will be added later.  The trailing space is required.
 */
static const char PACKED_REFS_HEADER[] =
        "# pack-refs with: peeled fully-peeled \n";

/*
 * Parse one line from a packed-refs file.  Write the SHA1 to sha1.
 * Return a pointer to the refname within the line (null-terminated),
 * or NULL if there was a problem.
 */
static const char *parse_ref_line(struct strbuf *line, unsigned char *sha1)
{
        const char *ref;

        /*
         * 42: the answer to everything.
         *
         * In this case, it happens to be the answer to
         *  40 (length of sha1 hex representation)
         *  +1 (space in between hex and name)
         *  +1 (newline at the end of the line)
         */
        if (line->len <= 42)
                return NULL;

        if (get_sha1_hex(line->buf, sha1) < 0)
                return NULL;
        if (!isspace(line->buf[40]))
                return NULL;

        ref = line->buf + 41;
        if (isspace(*ref))
                return NULL;

        if (line->buf[line->len - 1] != '\n')
                return NULL;
        line->buf[--line->len] = 0;

        return ref;
}

/*
 * Read f, which is a packed-refs file, into dir.
 *
 * A comment line of the form "# pack-refs with: " may contain zero or
 * more traits. We interpret the traits as follows:
 *
 *   No traits:
 *
 *      Probably no references are peeled. But if the file contains a
 *      peeled value for a reference, we will use it.
 *
 *   peeled:
 *
 *      References under "refs/tags/", if they *can* be peeled, *are*
 *      peeled in this file. References outside of "refs/tags/" are
 *      probably not peeled even if they could have been, but if we find
 *      a peeled value for such a reference we will use it.
 *
 *   fully-peeled:
 *
 *      All references in the file that can be peeled are peeled.
 *      Inversely (and this is more important), any references in the
 *      file for which no peeled value is recorded is not peelable. This
 *      trait should typically be written alongside "peeled" for
 *      compatibility with older clients, but we do not require it
 *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
 */
static void read_packed_refs(FILE *f, struct ref_dir *dir)
{
        struct ref_entry *last = NULL;
        struct strbuf line = STRBUF_INIT;
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;

        while (strbuf_getwholeline(&line, f, '\n') != EOF) {
                unsigned char sha1[20];
                const char *refname;
                const char *traits;

                if (skip_prefix(line.buf, "# pack-refs with:", &traits)) {
                        if (strstr(traits, " fully-peeled "))
                                peeled = PEELED_FULLY;
                        else if (strstr(traits, " peeled "))
                                peeled = PEELED_TAGS;
                        /* perhaps other traits later as well */
                        continue;
                }

                refname = parse_ref_line(&line, sha1);
                if (refname) {
                        int flag = REF_ISPACKED;

                        if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                                hashclr(sha1);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        last = create_ref_entry(refname, sha1, flag, 0);
                        if (peeled == PEELED_FULLY ||
                            (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
                                last->flag |= REF_KNOWS_PEELED;
                        add_ref(dir, last);
                        continue;
                }
                if (last &&
                    line.buf[0] == '^' &&
                    line.len == PEELED_LINE_LENGTH &&
                    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
                    !get_sha1_hex(line.buf + 1, sha1)) {
                        hashcpy(last->u.value.peeled, sha1);
                        /*
                         * Regardless of what the file header said,
                         * we definitely know the value of *this*
                         * reference:
                         */
                        last->flag |= REF_KNOWS_PEELED;
                }
        }

        strbuf_release(&line);
}

/*
 * Get the packed_ref_cache for the specified ref_cache, creating it
 * if necessary.
 */
static struct packed_ref_cache *get_packed_ref_cache(struct ref_cache *refs)
{
        const char *packed_refs_file;

        if (*refs->name)
                packed_refs_file = git_path_submodule(refs->name, "packed-refs");
        else
                packed_refs_file = git_path("packed-refs");

        if (refs->packed &&
            !stat_validity_check(&refs->packed->validity, packed_refs_file))
                clear_packed_ref_cache(refs);

        if (!refs->packed) {
                FILE *f;

                refs->packed = xcalloc(1, sizeof(*refs->packed));
                acquire_packed_ref_cache(refs->packed);
                refs->packed->root = create_dir_entry(refs, "", 0, 0);
                f = fopen(packed_refs_file, "r");
                if (f) {
                        stat_validity_update(&refs->packed->validity, fileno(f));
                        read_packed_refs(f, get_ref_dir(refs->packed->root));
                        fclose(f);
                }
        }
        return refs->packed;
}

static struct ref_dir *get_packed_ref_dir(struct packed_ref_cache *packed_ref_cache)
{
        return get_ref_dir(packed_ref_cache->root);
}

static struct ref_dir *get_packed_refs(struct ref_cache *refs)
{
        return get_packed_ref_dir(get_packed_ref_cache(refs));
}

void add_packed_ref(const char *refname, const unsigned char *sha1)
{
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);

        if (!packed_ref_cache->lock)
                die("internal error: packed refs not locked");
        add_ref(get_packed_ref_dir(packed_ref_cache),
                create_ref_entry(refname, sha1, REF_ISPACKED, 1));
}

/*
 * Read the loose references from the namespace dirname into dir
 * (without recursing).  dirname must end with '/'.  dir must be the
 * directory entry corresponding to dirname.
 */
static void read_loose_refs(const char *dirname, struct ref_dir *dir)
{
        struct ref_cache *refs = dir->ref_cache;
        DIR *d;
        const char *path;
        struct dirent *de;
        int dirnamelen = strlen(dirname);
        struct strbuf refname;

        if (*refs->name)
                path = git_path_submodule(refs->name, "%s", dirname);
        else
                path = git_path("%s", dirname);

        d = opendir(path);
        if (!d)
                return;

        strbuf_init(&refname, dirnamelen + 257);
        strbuf_add(&refname, dirname, dirnamelen);

        while ((de = readdir(d)) != NULL) {
                unsigned char sha1[20];
                struct stat st;
                int flag;
                const char *refdir;

                if (de->d_name[0] == '.')
                        continue;
                if (ends_with(de->d_name, ".lock"))
                        continue;
                strbuf_addstr(&refname, de->d_name);
                refdir = *refs->name
                        ? git_path_submodule(refs->name, "%s", refname.buf)
                        : git_path("%s", refname.buf);
                if (stat(refdir, &st) < 0) {
                        ; /* silently ignore */
                } else if (S_ISDIR(st.st_mode)) {
                        strbuf_addch(&refname, '/');
                        add_entry_to_dir(dir,
                                         create_dir_entry(refs, refname.buf,
                                                          refname.len, 1));
                } else {
                        if (*refs->name) {
                                hashclr(sha1);
                                flag = 0;
                                if (resolve_gitlink_ref(refs->name, refname.buf, sha1) < 0) {
                                        hashclr(sha1);
                                        flag |= REF_ISBROKEN;
                                }
                        } else if (read_ref_full(refname.buf,
                                                 RESOLVE_REF_READING,
                                                 sha1, &flag)) {
                                hashclr(sha1);
                                flag |= REF_ISBROKEN;
                        }
                        if (check_refname_format(refname.buf,
                                                 REFNAME_ALLOW_ONELEVEL)) {
                                hashclr(sha1);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        add_entry_to_dir(dir,
                                         create_ref_entry(refname.buf, sha1, flag, 0));
                }
                strbuf_setlen(&refname, dirnamelen);
        }
        strbuf_release(&refname);
        closedir(d);
}

static struct ref_dir *get_loose_refs(struct ref_cache *refs)
{
        if (!refs->loose) {
                /*
                 * Mark the top-level directory complete because we
                 * are about to read the only subdirectory that can
                 * hold references:
                 */
                refs->loose = create_dir_entry(refs, "", 0, 0);
                /*
                 * Create an incomplete entry for "refs/":
                 */
                add_entry_to_dir(get_ref_dir(refs->loose),
                                 create_dir_entry(refs, "refs/", 5, 1));
        }
        return get_ref_dir(refs->loose);
}

/* We allow "recursive" symbolic refs. Only within reason, though */
#define MAXDEPTH 5
#define MAXREFLEN (1024)

/*
 * Called by resolve_gitlink_ref_recursive() after it failed to read
 * from the loose refs in ref_cache refs. Find <refname> in the
 * packed-refs file for the submodule.
 */
static int resolve_gitlink_packed_ref(struct ref_cache *refs,
                                      const char *refname, unsigned char *sha1)
{
        struct ref_entry *ref;
        struct ref_dir *dir = get_packed_refs(refs);

        ref = find_ref(dir, refname);
        if (ref == NULL)
                return -1;

        hashcpy(sha1, ref->u.value.sha1);
        return 0;
}

static int resolve_gitlink_ref_recursive(struct ref_cache *refs,
                                         const char *refname, unsigned char *sha1,
                                         int recursion)
{
        int fd, len;
        char buffer[128], *p;
        char *path;

        if (recursion > MAXDEPTH || strlen(refname) > MAXREFLEN)
                return -1;
        path = *refs->name
                ? git_path_submodule(refs->name, "%s", refname)
                : git_path("%s", refname);
        fd = open(path, O_RDONLY);
        if (fd < 0)
                return resolve_gitlink_packed_ref(refs, refname, sha1);

        len = read(fd, buffer, sizeof(buffer)-1);
        close(fd);
        if (len < 0)
                return -1;
        while (len && isspace(buffer[len-1]))
                len--;
        buffer[len] = 0;

        /* Was it a detached head or an old-fashioned symlink? */
        if (!get_sha1_hex(buffer, sha1))
                return 0;

        /* Symref? */
        if (strncmp(buffer, "ref:", 4))
                return -1;
        p = buffer + 4;
        while (isspace(*p))
                p++;

        return resolve_gitlink_ref_recursive(refs, p, sha1, recursion+1);
}

int resolve_gitlink_ref(const char *path, const char *refname, unsigned char *sha1)
{
        int len = strlen(path), retval;
        char *submodule;
        struct ref_cache *refs;

        while (len && path[len-1] == '/')
                len--;
        if (!len)
                return -1;
        submodule = xstrndup(path, len);
        refs = get_ref_cache(submodule);
        free(submodule);

        retval = resolve_gitlink_ref_recursive(refs, refname, sha1, 0);
        return retval;
}

/*
 * Return the ref_entry for the given refname from the packed
 * references.  If it does not exist, return NULL.
 */
static struct ref_entry *get_packed_ref(const char *refname)
{
        return find_ref(get_packed_refs(&ref_cache), refname);
}

/*
 * A loose ref file doesn't exist; check for a packed ref.  The
 * options are forwarded from resolve_safe_unsafe().
 */
static int resolve_missing_loose_ref(const char *refname,
                                     int resolve_flags,
                                     unsigned char *sha1,
                                     int *flags)
{
        struct ref_entry *entry;

        /*
         * The loose reference file does not exist; check for a packed
         * reference.
         */
        entry = get_packed_ref(refname);
        if (entry) {
                hashcpy(sha1, entry->u.value.sha1);
                if (flags)
                        *flags |= REF_ISPACKED;
                return 0;
        }
        /* The reference is not a packed reference, either. */
        if (resolve_flags & RESOLVE_REF_READING) {
                errno = ENOENT;
                return -1;
        } else {
                hashclr(sha1);
                return 0;
        }
}

/* This function needs to return a meaningful errno on failure */
const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned char *sha1, int *flags)
{
        int depth = MAXDEPTH;
        ssize_t len;
        char buffer[256];
        static char refname_buffer[256];
        int bad_name = 0;

        if (flags)
                *flags = 0;

        if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                if (flags)
                        *flags |= REF_BAD_NAME;

                if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
                    !refname_is_safe(refname)) {
                        errno = EINVAL;
                        return NULL;
                }
                /*
                 * dwim_ref() uses REF_ISBROKEN to distinguish between
                 * missing refs and refs that were present but invalid,
                 * to complain about the latter to stderr.
                 *
                 * We don't know whether the ref exists, so don't set
                 * REF_ISBROKEN yet.
                 */
                bad_name = 1;
        }
        for (;;) {
                char path[PATH_MAX];
                struct stat st;
                char *buf;
                int fd;

                if (--depth < 0) {
                        errno = ELOOP;
                        return NULL;
                }

                git_snpath(path, sizeof(path), "%s", refname);

                /*
                 * We might have to loop back here to avoid a race
                 * condition: first we lstat() the file, then we try
                 * to read it as a link or as a file.  But if somebody
                 * changes the type of the file (file <-> directory
                 * <-> symlink) between the lstat() and reading, then
                 * we don't want to report that as an error but rather
                 * try again starting with the lstat().
                 */
        stat_ref:
                if (lstat(path, &st) < 0) {
                        if (errno != ENOENT)
                                return NULL;
                        if (resolve_missing_loose_ref(refname, resolve_flags,
                                                      sha1, flags))
                                return NULL;
                        if (bad_name) {
                                hashclr(sha1);
                                if (flags)
                                        *flags |= REF_ISBROKEN;
                        }
                        return refname;
                }

                /* Follow "normalized" - ie "refs/.." symlinks by hand */
                if (S_ISLNK(st.st_mode)) {
                        len = readlink(path, buffer, sizeof(buffer)-1);
                        if (len < 0) {
                                if (errno == ENOENT || errno == EINVAL)
                                        /* inconsistent with lstat; retry */
                                        goto stat_ref;
                                else
                                        return NULL;
                        }
                        buffer[len] = 0;
                        if (starts_with(buffer, "refs/") &&
                                        !check_refname_format(buffer, 0)) {
                                strcpy(refname_buffer, buffer);
                                refname = refname_buffer;
                                if (flags)
                                        *flags |= REF_ISSYMREF;
                                if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
                                        hashclr(sha1);
                                        return refname;
                                }
                                continue;
                        }
                }

                /* Is it a directory? */
                if (S_ISDIR(st.st_mode)) {
                        errno = EISDIR;
                        return NULL;
                }

                /*
                 * Anything else, just open it and try to use it as
                 * a ref
                 */
                fd = open(path, O_RDONLY);
                if (fd < 0) {
                        if (errno == ENOENT)
                                /* inconsistent with lstat; retry */
                                goto stat_ref;
                        else
                                return NULL;
                }
                len = read_in_full(fd, buffer, sizeof(buffer)-1);
                if (len < 0) {
                        int save_errno = errno;
                        close(fd);
                        errno = save_errno;
                        return NULL;
                }
                close(fd);
                while (len && isspace(buffer[len-1]))
                        len--;
                buffer[len] = '\0';

                /*
                 * Is it a symbolic ref?
                 */
                if (!starts_with(buffer, "ref:")) {
                        /*
                         * Please note that FETCH_HEAD has a second
                         * line containing other data.
                         */
                        if (get_sha1_hex(buffer, sha1) ||
                            (buffer[40] != '\0' && !isspace(buffer[40]))) {
                                if (flags)
                                        *flags |= REF_ISBROKEN;
                                errno = EINVAL;
                                return NULL;
                        }
                        if (bad_name) {
                                hashclr(sha1);
                                if (flags)
                                        *flags |= REF_ISBROKEN;
                        }
                        return refname;
                }
                if (flags)
                        *flags |= REF_ISSYMREF;
                buf = buffer + 4;
                while (isspace(*buf))
                        buf++;
                refname = strcpy(refname_buffer, buf);
                if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
                        hashclr(sha1);
                        return refname;
                }
                if (check_refname_format(buf, REFNAME_ALLOW_ONELEVEL)) {
                        if (flags)
                                *flags |= REF_ISBROKEN;

                        if (!(resolve_flags & RESOLVE_REF_ALLOW_BAD_NAME) ||
                            !refname_is_safe(buf)) {
                                errno = EINVAL;
                                return NULL;
                        }
                        bad_name = 1;
                }
        }
}

char *resolve_refdup(const char *ref, int resolve_flags, unsigned char *sha1, int *flags)
{
        return xstrdup_or_null(resolve_ref_unsafe(ref, resolve_flags, sha1, flags));
}

/* The argument to filter_refs */
struct ref_filter {
        const char *pattern;
        each_ref_fn *fn;
        void *cb_data;
};

int read_ref_full(const char *refname, int resolve_flags, unsigned char *sha1, int *flags)
{
        if (resolve_ref_unsafe(refname, resolve_flags, sha1, flags))
                return 0;
        return -1;
}

int read_ref(const char *refname, unsigned char *sha1)
{
        return read_ref_full(refname, RESOLVE_REF_READING, sha1, NULL);
}

int ref_exists(const char *refname)
{
        unsigned char sha1[20];
        return !!resolve_ref_unsafe(refname, RESOLVE_REF_READING, sha1, NULL);
}

static int filter_refs(const char *refname, const unsigned char *sha1, int flags,
                       void *data)
{
        struct ref_filter *filter = (struct ref_filter *)data;
        if (wildmatch(filter->pattern, refname, 0, NULL))
                return 0;
        return filter->fn(refname, sha1, flags, filter->cb_data);
}

enum peel_status {
        /* object was peeled successfully: */
        PEEL_PEELED = 0,

        /*
         * object cannot be peeled because the named object (or an
         * object referred to by a tag in the peel chain), does not
         * exist.
         */
        PEEL_INVALID = -1,

        /* object cannot be peeled because it is not a tag: */
        PEEL_NON_TAG = -2,

        /* ref_entry contains no peeled value because it is a symref: */
        PEEL_IS_SYMREF = -3,

        /*
         * ref_entry cannot be peeled because it is broken (i.e., the
         * symbolic reference cannot even be resolved to an object
         * name):
         */
        PEEL_BROKEN = -4
};

/*
 * Peel the named object; i.e., if the object is a tag, resolve the
 * tag recursively until a non-tag is found.  If successful, store the
 * result to sha1 and return PEEL_PEELED.  If the object is not a tag
 * or is not valid, return PEEL_NON_TAG or PEEL_INVALID, respectively,
 * and leave sha1 unchanged.
 */
static enum peel_status peel_object(const unsigned char *name, unsigned char *sha1)
{
        struct object *o = lookup_unknown_object(name);

        if (o->type == OBJ_NONE) {
                int type = sha1_object_info(name, NULL);
                if (type < 0 || !object_as_type(o, type, 0))
                        return PEEL_INVALID;
        }

        if (o->type != OBJ_TAG)
                return PEEL_NON_TAG;

        o = deref_tag_noverify(o);
        if (!o)
                return PEEL_INVALID;

        hashcpy(sha1, o->sha1);
        return PEEL_PEELED;
}

/*
 * Peel the entry (if possible) and return its new peel_status.  If
 * repeel is true, re-peel the entry even if there is an old peeled
 * value that is already stored in it.
 *
 * It is OK to call this function with a packed reference entry that
 * might be stale and might even refer to an object that has since
 * been garbage-collected.  In such a case, if the entry has
 * REF_KNOWS_PEELED then leave the status unchanged and return
 * PEEL_PEELED or PEEL_NON_TAG; otherwise, return PEEL_INVALID.
 */
static enum peel_status peel_entry(struct ref_entry *entry, int repeel)
{
        enum peel_status status;

        if (entry->flag & REF_KNOWS_PEELED) {
                if (repeel) {
                        entry->flag &= ~REF_KNOWS_PEELED;
                        hashclr(entry->u.value.peeled);
                } else {
                        return is_null_sha1(entry->u.value.peeled) ?
                                PEEL_NON_TAG : PEEL_PEELED;
                }
        }
        if (entry->flag & REF_ISBROKEN)
                return PEEL_BROKEN;
        if (entry->flag & REF_ISSYMREF)
                return PEEL_IS_SYMREF;

        status = peel_object(entry->u.value.sha1, entry->u.value.peeled);
        if (status == PEEL_PEELED || status == PEEL_NON_TAG)
                entry->flag |= REF_KNOWS_PEELED;
        return status;
}

int peel_ref(const char *refname, unsigned char *sha1)
{
        int flag;
        unsigned char base[20];

        if (current_ref && (current_ref->name == refname
                            || !strcmp(current_ref->name, refname))) {
                if (peel_entry(current_ref, 0))
                        return -1;
                hashcpy(sha1, current_ref->u.value.peeled);
                return 0;
        }

        if (read_ref_full(refname, RESOLVE_REF_READING, base, &flag))
                return -1;

        /*
         * If the reference is packed, read its ref_entry from the
         * cache in the hope that we already know its peeled value.
         * We only try this optimization on packed references because
         * (a) forcing the filling of the loose reference cache could
         * be expensive and (b) loose references anyway usually do not
         * have REF_KNOWS_PEELED.
         */
        if (flag & REF_ISPACKED) {
                struct ref_entry *r = get_packed_ref(refname);
                if (r) {
                        if (peel_entry(r, 0))
                                return -1;
                        hashcpy(sha1, r->u.value.peeled);
                        return 0;
                }
        }

        return peel_object(base, sha1);
}

struct warn_if_dangling_data {
        FILE *fp;
        const char *refname;
        const struct string_list *refnames;
        const char *msg_fmt;
};

static int warn_if_dangling_symref(const char *refname, const unsigned char *sha1,
                                   int flags, void *cb_data)
{
        struct warn_if_dangling_data *d = cb_data;
        const char *resolves_to;
        unsigned char junk[20];

        if (!(flags & REF_ISSYMREF))
                return 0;

        resolves_to = resolve_ref_unsafe(refname, 0, junk, NULL);
        if (!resolves_to
            || (d->refname
                ? strcmp(resolves_to, d->refname)
                : !string_list_has_string(d->refnames, resolves_to))) {
                return 0;
        }

        fprintf(d->fp, d->msg_fmt, refname);
        fputc('\n', d->fp);
        return 0;
}

void warn_dangling_symref(FILE *fp, const char *msg_fmt, const char *refname)
{
        struct warn_if_dangling_data data;

        data.fp = fp;
        data.refname = refname;
        data.refnames = NULL;
        data.msg_fmt = msg_fmt;
        for_each_rawref(warn_if_dangling_symref, &data);
}

void warn_dangling_symrefs(FILE *fp, const char *msg_fmt, const struct string_list *refnames)
{
        struct warn_if_dangling_data data;

        data.fp = fp;
        data.refname = NULL;
        data.refnames = refnames;
        data.msg_fmt = msg_fmt;
        for_each_rawref(warn_if_dangling_symref, &data);
}

/*
 * Call fn for each reference in the specified ref_cache, omitting
 * references not in the containing_dir of base.  fn is called for all
 * references, including broken ones.  If fn ever returns a non-zero
 * value, stop the iteration and return that value; otherwise, return
 * 0.
 */
static int do_for_each_entry(struct ref_cache *refs, const char *base,
                             each_ref_entry_fn fn, void *cb_data)
{
        struct packed_ref_cache *packed_ref_cache;
        struct ref_dir *loose_dir;
        struct ref_dir *packed_dir;
        int retval = 0;

        /*
         * We must make sure that all loose refs are read before accessing the
         * packed-refs file; this avoids a race condition in which loose refs
         * are migrated to the packed-refs file by a simultaneous process, but
         * our in-memory view is from before the migration. get_packed_ref_cache()
         * takes care of making sure our view is up to date with what is on
         * disk.
         */
        loose_dir = get_loose_refs(refs);
        if (base && *base) {
                loose_dir = find_containing_dir(loose_dir, base, 0);
        }
        if (loose_dir)
                prime_ref_dir(loose_dir);

        packed_ref_cache = get_packed_ref_cache(refs);
        acquire_packed_ref_cache(packed_ref_cache);
        packed_dir = get_packed_ref_dir(packed_ref_cache);
        if (base && *base) {
                packed_dir = find_containing_dir(packed_dir, base, 0);
        }

        if (packed_dir && loose_dir) {
                sort_ref_dir(packed_dir);
                sort_ref_dir(loose_dir);
                retval = do_for_each_entry_in_dirs(
                                packed_dir, loose_dir, fn, cb_data);
        } else if (packed_dir) {
                sort_ref_dir(packed_dir);
                retval = do_for_each_entry_in_dir(
                                packed_dir, 0, fn, cb_data);
        } else if (loose_dir) {
                sort_ref_dir(loose_dir);
                retval = do_for_each_entry_in_dir(
                                loose_dir, 0, fn, cb_data);
        }

        release_packed_ref_cache(packed_ref_cache);
        return retval;
}

/*
 * Call fn for each reference in the specified ref_cache for which the
 * refname begins with base.  If trim is non-zero, then trim that many
 * characters off the beginning of each refname before passing the
 * refname to fn.  flags can be DO_FOR_EACH_INCLUDE_BROKEN to include
 * broken references in the iteration.  If fn ever returns a non-zero
 * value, stop the iteration and return that value; otherwise, return
 * 0.
 */
static int do_for_each_ref(struct ref_cache *refs, const char *base,
                           each_ref_fn fn, int trim, int flags, void *cb_data)
{
        struct ref_entry_cb data;
        data.base = base;
        data.trim = trim;
        data.flags = flags;
        data.fn = fn;
        data.cb_data = cb_data;

        if (ref_paranoia < 0)
                ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
        if (ref_paranoia)
                data.flags |= DO_FOR_EACH_INCLUDE_BROKEN;

        return do_for_each_entry(refs, base, do_one_ref, &data);
}

static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
{
        unsigned char sha1[20];
        int flag;

        if (submodule) {
                if (resolve_gitlink_ref(submodule, "HEAD", sha1) == 0)
                        return fn("HEAD", sha1, 0, cb_data);

                return 0;
        }

        if (!read_ref_full("HEAD", RESOLVE_REF_READING, sha1, &flag))
                return fn("HEAD", sha1, flag, cb_data);

        return 0;
}

int head_ref(each_ref_fn fn, void *cb_data)
{
        return do_head_ref(NULL, fn, cb_data);
}

int head_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
{
        return do_head_ref(submodule, fn, cb_data);
}

int for_each_ref(each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(&ref_cache, "", fn, 0, 0, cb_data);
}

int for_each_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(get_ref_cache(submodule), "", fn, 0, 0, cb_data);
}

int for_each_ref_in(const char *prefix, each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(&ref_cache, prefix, fn, strlen(prefix), 0, cb_data);
}

int for_each_ref_in_submodule(const char *submodule, const char *prefix,
                each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(get_ref_cache(submodule), prefix, fn, strlen(prefix), 0, cb_data);
}

int for_each_tag_ref(each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in("refs/tags/", fn, cb_data);
}

int for_each_tag_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in_submodule(submodule, "refs/tags/", fn, cb_data);
}

int for_each_branch_ref(each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in("refs/heads/", fn, cb_data);
}

int for_each_branch_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in_submodule(submodule, "refs/heads/", fn, cb_data);
}

int for_each_remote_ref(each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in("refs/remotes/", fn, cb_data);
}

int for_each_remote_ref_submodule(const char *submodule, each_ref_fn fn, void *cb_data)
{
        return for_each_ref_in_submodule(submodule, "refs/remotes/", fn, cb_data);
}

int for_each_replace_ref(each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(&ref_cache, "refs/replace/", fn, 13, 0, cb_data);
}

int head_ref_namespaced(each_ref_fn fn, void *cb_data)
{
        struct strbuf buf = STRBUF_INIT;
        int ret = 0;
        unsigned char sha1[20];
        int flag;

        strbuf_addf(&buf, "%sHEAD", get_git_namespace());
        if (!read_ref_full(buf.buf, RESOLVE_REF_READING, sha1, &flag))
                ret = fn(buf.buf, sha1, flag, cb_data);
        strbuf_release(&buf);

        return ret;
}

int for_each_namespaced_ref(each_ref_fn fn, void *cb_data)
{
        struct strbuf buf = STRBUF_INIT;
        int ret;
        strbuf_addf(&buf, "%srefs/", get_git_namespace());
        ret = do_for_each_ref(&ref_cache, buf.buf, fn, 0, 0, cb_data);
        strbuf_release(&buf);
        return ret;
}

int for_each_glob_ref_in(each_ref_fn fn, const char *pattern,
        const char *prefix, void *cb_data)
{
        struct strbuf real_pattern = STRBUF_INIT;
        struct ref_filter filter;
        int ret;

        if (!prefix && !starts_with(pattern, "refs/"))
                strbuf_addstr(&real_pattern, "refs/");
        else if (prefix)
                strbuf_addstr(&real_pattern, prefix);
        strbuf_addstr(&real_pattern, pattern);

        if (!has_glob_specials(pattern)) {
                /* Append implied '/' '*' if not present. */
                if (real_pattern.buf[real_pattern.len - 1] != '/')
                        strbuf_addch(&real_pattern, '/');
                /* No need to check for '*', there is none. */
                strbuf_addch(&real_pattern, '*');
        }

        filter.pattern = real_pattern.buf;
        filter.fn = fn;
        filter.cb_data = cb_data;
        ret = for_each_ref(filter_refs, &filter);

        strbuf_release(&real_pattern);
        return ret;
}

int for_each_glob_ref(each_ref_fn fn, const char *pattern, void *cb_data)
{
        return for_each_glob_ref_in(fn, pattern, NULL, cb_data);
}

int for_each_rawref(each_ref_fn fn, void *cb_data)
{
        return do_for_each_ref(&ref_cache, "", fn, 0,
                               DO_FOR_EACH_INCLUDE_BROKEN, cb_data);
}

const char *prettify_refname(const char *name)
{
        return name + (
                starts_with(name, "refs/heads/") ? 11 :
                starts_with(name, "refs/tags/") ? 10 :
                starts_with(name, "refs/remotes/") ? 13 :
                0);
}

static const char *ref_rev_parse_rules[] = {
        "%.*s",
        "refs/%.*s",
        "refs/tags/%.*s",
        "refs/heads/%.*s",
        "refs/remotes/%.*s",
        "refs/remotes/%.*s/HEAD",
        NULL
};

int refname_match(const char *abbrev_name, const char *full_name)
{
        const char **p;
        const int abbrev_name_len = strlen(abbrev_name);

        for (p = ref_rev_parse_rules; *p; p++) {
                if (!strcmp(full_name, mkpath(*p, abbrev_name_len, abbrev_name))) {
                        return 1;
                }
        }

        return 0;
}

static void unlock_ref(struct ref_lock *lock)
{
        /* Do not free lock->lk -- atexit() still looks at them */
        if (lock->lk)
                rollback_lock_file(lock->lk);
        free(lock->ref_name);
        free(lock->orig_ref_name);
        free(lock);
}

/* This function should make sure errno is meaningful on error */
static struct ref_lock *verify_lock(struct ref_lock *lock,
        const unsigned char *old_sha1, int mustexist)
{
        if (read_ref_full(lock->ref_name,
                          mustexist ? RESOLVE_REF_READING : 0,
                          lock->old_sha1, NULL)) {
                int save_errno = errno;
                error("Can't verify ref %s", lock->ref_name);
                unlock_ref(lock);
                errno = save_errno;
                return NULL;
        }
        if (hashcmp(lock->old_sha1, old_sha1)) {
                error("Ref %s is at %s but expected %s", lock->ref_name,
                        sha1_to_hex(lock->old_sha1), sha1_to_hex(old_sha1));
                unlock_ref(lock);
                errno = EBUSY;
                return NULL;
        }
        return lock;
}

static int remove_empty_directories(const char *file)
{
        /* we want to create a file but there is a directory there;
         * if that is an empty directory (or a directory that contains
         * only empty directories), remove them.
         */
        struct strbuf path;
        int result, save_errno;

        strbuf_init(&path, 20);
        strbuf_addstr(&path, file);

        result = remove_dir_recursively(&path, REMOVE_DIR_EMPTY_ONLY);
        save_errno = errno;

        strbuf_release(&path);
        errno = save_errno;

        return result;
}

/*
 * *string and *len will only be substituted, and *string returned (for
 * later free()ing) if the string passed in is a magic short-hand form
 * to name a branch.
 */
static char *substitute_branch_name(const char **string, int *len)
{
        struct strbuf buf = STRBUF_INIT;
        int ret = interpret_branch_name(*string, *len, &buf);

        if (ret == *len) {
                size_t size;
                *string = strbuf_detach(&buf, &size);
                *len = size;
                return (char *)*string;
        }

        return NULL;
}

int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref)
{
        char *last_branch = substitute_branch_name(&str, &len);
        const char **p, *r;
        int refs_found = 0;

        *ref = NULL;
        for (p = ref_rev_parse_rules; *p; p++) {
                char fullref[PATH_MAX];
                unsigned char sha1_from_ref[20];
                unsigned char *this_result;
                int flag;

                this_result = refs_found ? sha1_from_ref : sha1;
                mksnpath(fullref, sizeof(fullref), *p, len, str);
                r = resolve_ref_unsafe(fullref, RESOLVE_REF_READING,
                                       this_result, &flag);
                if (r) {
                        if (!refs_found++)
                                *ref = xstrdup(r);
                        if (!warn_ambiguous_refs)
                                break;
                } else if ((flag & REF_ISSYMREF) && strcmp(fullref, "HEAD")) {
                        warning("ignoring dangling symref %s.", fullref);
                } else if ((flag & REF_ISBROKEN) && strchr(fullref, '/')) {
                        warning("ignoring broken ref %s.", fullref);
                }
        }
        free(last_branch);
        return refs_found;
}

int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
{
        char *last_branch = substitute_branch_name(&str, &len);
        const char **p;
        int logs_found = 0;

        *log = NULL;
        for (p = ref_rev_parse_rules; *p; p++) {
                unsigned char hash[20];
                char path[PATH_MAX];
                const char *ref, *it;

                mksnpath(path, sizeof(path), *p, len, str);
                ref = resolve_ref_unsafe(path, RESOLVE_REF_READING,
                                         hash, NULL);
                if (!ref)
                        continue;
                if (reflog_exists(path))
                        it = path;
                else if (strcmp(ref, path) && reflog_exists(ref))
                        it = ref;
                else
                        continue;
                if (!logs_found++) {
                        *log = xstrdup(it);
                        hashcpy(sha1, hash);
                }
                if (!warn_ambiguous_refs)
                        break;
        }
        free(last_branch);
        return logs_found;
}

/*
 * Locks a ref returning the lock on success and NULL on failure.
 * On failure errno is set to something meaningful.
 */
static struct ref_lock *lock_ref_sha1_basic(const char *refname,
                                            const unsigned char *old_sha1,
                                            const struct string_list *extras,
                                            const struct string_list *skip,
                                            unsigned int flags, int *type_p)
{
        char *ref_file;
        const char *orig_refname = refname;
        struct ref_lock *lock;
        int last_errno = 0;
        int type, lflags;
        int mustexist = (old_sha1 && !is_null_sha1(old_sha1));
        int resolve_flags = 0;
        int attempts_remaining = 3;
        struct strbuf err = STRBUF_INIT;

        lock = xcalloc(1, sizeof(struct ref_lock));
        lock->lock_fd = -1;

        if (mustexist)
                resolve_flags |= RESOLVE_REF_READING;
        if (flags & REF_DELETING) {
                resolve_flags |= RESOLVE_REF_ALLOW_BAD_NAME;
                if (flags & REF_NODEREF)
                        resolve_flags |= RESOLVE_REF_NO_RECURSE;
        }

        refname = resolve_ref_unsafe(refname, resolve_flags,
                                     lock->old_sha1, &type);
        if (!refname && errno == EISDIR) {
                /* we are trying to lock foo but we used to
                 * have foo/bar which now does not exist;
                 * it is normal for the empty directory 'foo'
                 * to remain.
                 */
                ref_file = git_path("%s", orig_refname);
                if (remove_empty_directories(ref_file)) {
                        last_errno = errno;
                        error("there are still refs under '%s'", orig_refname);
                        goto error_return;
                }
                refname = resolve_ref_unsafe(orig_refname, resolve_flags,
                                             lock->old_sha1, &type);
        }
        if (type_p)
            *type_p = type;
        if (!refname) {
                last_errno = errno;
                error("unable to resolve reference %s: %s",
                        orig_refname, strerror(errno));
                goto error_return;
        }
        /*
         * If the ref did not exist and we are creating it, make sure
         * there is no existing packed ref whose name begins with our
         * refname, nor a packed ref whose name is a proper prefix of
         * our refname.
         */
        if (is_null_sha1(lock->old_sha1) &&
            verify_refname_available(refname, extras, skip,
                                     get_packed_refs(&ref_cache), &err)) {
                error("%s", err.buf);
                last_errno = ENOTDIR;
                goto error_return;
        }

        lock->lk = xcalloc(1, sizeof(struct lock_file));

        lflags = 0;
        if (flags & REF_NODEREF) {
                refname = orig_refname;
                lflags |= LOCK_NO_DEREF;
        }
        lock->ref_name = xstrdup(refname);
        lock->orig_ref_name = xstrdup(orig_refname);
        ref_file = git_path("%s", refname);

 retry:
        switch (safe_create_leading_directories(ref_file)) {
        case SCLD_OK:
                break; /* success */
        case SCLD_VANISHED:
                if (--attempts_remaining > 0)
                        goto retry;
                /* fall through */
        default:
                last_errno = errno;
                error("unable to create directory for %s", ref_file);
                goto error_return;
        }

        lock->lock_fd = hold_lock_file_for_update(lock->lk, ref_file, lflags);
        if (lock->lock_fd < 0) {
                last_errno = errno;
                if (errno == ENOENT && --attempts_remaining > 0)
                        /*
                         * Maybe somebody just deleted one of the
                         * directories leading to ref_file.  Try
                         * again:
                         */
                        goto retry;
                else {
                        unable_to_lock_message(ref_file, errno, &err);
                        error("%s", err.buf);
                        goto error_return;
                }
        }
        return old_sha1 ? verify_lock(lock, old_sha1, mustexist) : lock;

 error_return:
        unlock_ref(lock);
        strbuf_release(&err);
        errno = last_errno;
        return NULL;
}

/*
 * Write an entry to the packed-refs file for the specified refname.
 * If peeled is non-NULL, write it as the entry's peeled value.
 */
static void write_packed_entry(FILE *fh, char *refname, unsigned char *sha1,
                               unsigned char *peeled)
{
        fprintf_or_die(fh, "%s %s\n", sha1_to_hex(sha1), refname);
        if (peeled)
                fprintf_or_die(fh, "^%s\n", sha1_to_hex(peeled));
}

/*
 * An each_ref_entry_fn that writes the entry to a packed-refs file.
 */
static int write_packed_entry_fn(struct ref_entry *entry, void *cb_data)
{
        enum peel_status peel_status = peel_entry(entry, 0);

        if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
                error("internal error: %s is not a valid packed reference!",
                      entry->name);
        write_packed_entry(cb_data, entry->name, entry->u.value.sha1,
                           peel_status == PEEL_PEELED ?
                           entry->u.value.peeled : NULL);
        return 0;
}

/* This should return a meaningful errno on failure */
int lock_packed_refs(int flags)
{
        struct packed_ref_cache *packed_ref_cache;

        if (hold_lock_file_for_update(&packlock, git_path("packed-refs"), flags) < 0)
                return -1;
        /*
         * Get the current packed-refs while holding the lock.  If the
         * packed-refs file has been modified since we last read it,
         * this will automatically invalidate the cache and re-read
         * the packed-refs file.
         */
        packed_ref_cache = get_packed_ref_cache(&ref_cache);
        packed_ref_cache->lock = &packlock;
        /* Increment the reference count to prevent it from being freed: */
        acquire_packed_ref_cache(packed_ref_cache);
        return 0;
}

/*
 * Commit the packed refs changes.
 * On error we must make sure that errno contains a meaningful value.
 */
int commit_packed_refs(void)
{
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);
        int error = 0;
        int save_errno = 0;
        FILE *out;

        if (!packed_ref_cache->lock)
                die("internal error: packed-refs not locked");

        out = fdopen_lock_file(packed_ref_cache->lock, "w");
        if (!out)
                die_errno("unable to fdopen packed-refs descriptor");

        fprintf_or_die(out, "%s", PACKED_REFS_HEADER);
        do_for_each_entry_in_dir(get_packed_ref_dir(packed_ref_cache),
                                 0, write_packed_entry_fn, out);

        if (commit_lock_file(packed_ref_cache->lock)) {
                save_errno = errno;
                error = -1;
        }
        packed_ref_cache->lock = NULL;
        release_packed_ref_cache(packed_ref_cache);
        errno = save_errno;
        return error;
}

void rollback_packed_refs(void)
{
        struct packed_ref_cache *packed_ref_cache =
                get_packed_ref_cache(&ref_cache);

        if (!packed_ref_cache->lock)
                die("internal error: packed-refs not locked");
        rollback_lock_file(packed_ref_cache->lock);
        packed_ref_cache->lock = NULL;
        release_packed_ref_cache(packed_ref_cache);
        clear_packed_ref_cache(&ref_cache);
}

struct ref_to_prune {
        struct ref_to_prune *next;
        unsigned char sha1[20];
        char name[FLEX_ARRAY];
};

struct pack_refs_cb_data {
        unsigned int flags;
        struct ref_dir *packed_refs;
        struct ref_to_prune *ref_to_prune;
};

/*
 * An each_ref_entry_fn that is run over loose references only.  If
 * the loose reference can be packed, add an entry in the packed ref
 * cache.  If the reference should be pruned, also add it to
 * ref_to_prune in the pack_refs_cb_data.
 */
static int pack_if_possible_fn(struct ref_entry *entry, void *cb_data)
{
        struct pack_refs_cb_data *cb = cb_data;
        enum peel_status peel_status;
        struct ref_entry *packed_entry;
        int is_tag_ref = starts_with(entry->name, "refs/tags/");

        /* ALWAYS pack tags */
        if (!(cb->flags & PACK_REFS_ALL) && !is_tag_ref)
                return 0;

        /* Do not pack symbolic or broken refs: */
        if ((entry->flag & REF_ISSYMREF) || !ref_resolves_to_object(entry))
                return 0;

        /* Add a packed ref cache entry equivalent to the loose entry. */
        peel_status = peel_entry(entry, 1);
        if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
                die("internal error peeling reference %s (%s)",
                    entry->name, sha1_to_hex(entry->u.value.sha1));
        packed_entry = find_ref(cb->packed_refs, entry->name);
        if (packed_entry) {
                /* Overwrite existing packed entry with info from loose entry */
                packed_entry->flag = REF_ISPACKED | REF_KNOWS_PEELED;
                hashcpy(packed_entry->u.value.sha1, entry->u.value.sha1);
        } else {
                packed_entry = create_ref_entry(entry->name, entry->u.value.sha1,
                                                REF_ISPACKED | REF_KNOWS_PEELED, 0);
                add_ref(cb->packed_refs, packed_entry);
        }
        hashcpy(packed_entry->u.value.peeled, entry->u.value.peeled);

        /* Schedule the loose reference for pruning if requested. */
        if ((cb->flags & PACK_REFS_PRUNE)) {
                int namelen = strlen(entry->name) + 1;
                struct ref_to_prune *n = xcalloc(1, sizeof(*n) + namelen);
                hashcpy(n->sha1, entry->u.value.sha1);
                strcpy(n->name, entry->name);
                n->next = cb->ref_to_prune;
                cb->ref_to_prune = n;
        }
        return 0;
}

/*
 * Remove empty parents, but spare refs/ and immediate subdirs.
 * Note: munges *name.
 */
static void try_remove_empty_parents(char *name)
{
        char *p, *q;
        int i;
        p = name;
        for (i = 0; i < 2; i++) { /* refs/{heads,tags,...}/ */
                while (*p && *p != '/')
                        p++;
                /* tolerate duplicate slashes; see check_refname_format() */
                while (*p == '/')
                        p++;
        }
        for (q = p; *q; q++)
                ;
        while (1) {
                while (q > p && *q != '/')
                        q--;
                while (q > p && *(q-1) == '/')
                        q--;
                if (q == p)
                        break;
                *q = '\0';
                if (rmdir(git_path("%s", name)))
                        break;
        }
}

/* make sure nobody touched the ref, and unlink */
static void prune_ref(struct ref_to_prune *r)
{
        struct ref_transaction *transaction;
        struct strbuf err = STRBUF_INIT;

        if (check_refname_format(r->name, 0))
                return;

        transaction = ref_transaction_begin(&err);
        if (!transaction ||
            ref_transaction_delete(transaction, r->name, r->sha1,
                                   REF_ISPRUNING, NULL, &err) ||
            ref_transaction_commit(transaction, &err)) {
                ref_transaction_free(transaction);
                error("%s", err.buf);
                strbuf_release(&err);
                return;
        }
        ref_transaction_free(transaction);
        strbuf_release(&err);
        try_remove_empty_parents(r->name);
}

static void prune_refs(struct ref_to_prune *r)
{
        while (r) {
                prune_ref(r);
                r = r->next;
        }
}

int pack_refs(unsigned int flags)
{
        struct pack_refs_cb_data cbdata;

        memset(&cbdata, 0, sizeof(cbdata));
        cbdata.flags = flags;

        lock_packed_refs(LOCK_DIE_ON_ERROR);
        cbdata.packed_refs = get_packed_refs(&ref_cache);

        do_for_each_entry_in_dir(get_loose_refs(&ref_cache), 0,
                                 pack_if_possible_fn, &cbdata);

        if (commit_packed_refs())
                die_errno("unable to overwrite old ref-pack file");

        prune_refs(cbdata.ref_to_prune);
        return 0;
}

int repack_without_refs(struct string_list *refnames, struct strbuf *err)
{
        struct ref_dir *packed;
        struct string_list_item *refname;
        int ret, needs_repacking = 0, removed = 0;

        assert(err);

        /* Look for a packed ref */
        for_each_string_list_item(refname, refnames) {
                if (get_packed_ref(refname->string)) {
                        needs_repacking = 1;
                        break;
                }
        }

        /* Avoid locking if we have nothing to do */
        if (!needs_repacking)
                return 0; /* no refname exists in packed refs */

        if (lock_packed_refs(0)) {
                unable_to_lock_message(git_path("packed-refs"), errno, err);
                return -1;
        }
        packed = get_packed_refs(&ref_cache);

        /* Remove refnames from the cache */
        for_each_string_list_item(refname, refnames)
                if (remove_entry(packed, refname->string) != -1)
                        removed = 1;
        if (!removed) {
                /*
                 * All packed entries disappeared while we were
                 * acquiring the lock.
                 */
                rollback_packed_refs();
                return 0;
        }

        /* Write what remains */
        ret = commit_packed_refs();
        if (ret)
                strbuf_addf(err, "unable to overwrite old ref-pack file: %s",
                            strerror(errno));
        return ret;
}

static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
{
        assert(err);

        if (!(flag & REF_ISPACKED) || flag & REF_ISSYMREF) {
                /*
                 * loose.  The loose file name is the same as the
                 * lockfile name, minus ".lock":
                 */
                char *loose_filename = get_locked_file_path(lock->lk);
                int res = unlink_or_msg(loose_filename, err);
                free(loose_filename);
                if (res)
                        return 1;
        }
        return 0;
}

int delete_ref(const char *refname, const unsigned char *sha1, unsigned int flags)
{
        struct ref_transaction *transaction;
        struct strbuf err = STRBUF_INIT;

        transaction = ref_transaction_begin(&err);
        if (!transaction ||
            ref_transaction_delete(transaction, refname,
                                   (sha1 && !is_null_sha1(sha1)) ? sha1 : NULL,
                                   flags, NULL, &err) ||
            ref_transaction_commit(transaction, &err)) {
                error("%s", err.buf);
                ref_transaction_free(transaction);
                strbuf_release(&err);
                return 1;
        }
        ref_transaction_free(transaction);
        strbuf_release(&err);
        return 0;
}

/*
 * People using contrib's git-new-workdir have .git/logs/refs ->
 * /some/other/path/.git/logs/refs, and that may live on another device.
 *
 * IOW, to avoid cross device rename errors, the temporary renamed log must
 * live into logs/refs.
 */
#define TMP_RENAMED_LOG  "logs/refs/.tmp-renamed-log"

static int rename_tmp_log(const char *newrefname)
{
        int attempts_remaining = 4;

 retry:
        switch (safe_create_leading_directories(git_path("logs/%s", newrefname))) {
        case SCLD_OK:
                break; /* success */
        case SCLD_VANISHED:
                if (--attempts_remaining > 0)
                        goto retry;
                /* fall through */
        default:
                error("unable to create directory for %s", newrefname);
                return -1;
        }

        if (rename(git_path(TMP_RENAMED_LOG), git_path("logs/%s", newrefname))) {
                if ((errno==EISDIR || errno==ENOTDIR) && --attempts_remaining > 0) {
                        /*
                         * rename(a, b) when b is an existing
                         * directory ought to result in ISDIR, but
                         * Solaris 5.8 gives ENOTDIR.  Sheesh.
                         */
                        if (remove_empty_directories(git_path("logs/%s", newrefname))) {
                                error("Directory not empty: logs/%s", newrefname);
                                return -1;
                        }
                        goto retry;
                } else if (errno == ENOENT && --attempts_remaining > 0) {
                        /*
                         * Maybe another process just deleted one of
                         * the directories in the path to newrefname.
                         * Try again from the beginning.
                         */
                        goto retry;
                } else {
                        error("unable to move logfile "TMP_RENAMED_LOG" to logs/%s: %s",
                                newrefname, strerror(errno));
                        return -1;
                }
        }
        return 0;
}

static int rename_ref_available(const char *oldname, const char *newname)
{
        struct string_list skip = STRING_LIST_INIT_NODUP;
        struct strbuf err = STRBUF_INIT;
        int ret;

        string_list_insert(&skip, oldname);
        ret = !verify_refname_available(newname, NULL, &skip,
                                        get_packed_refs(&ref_cache), &err)
                && !verify_refname_available(newname, NULL, &skip,
                                             get_loose_refs(&ref_cache), &err);
        if (!ret)
                error("%s", err.buf);

        string_list_clear(&skip, 0);
        strbuf_release(&err);
        return ret;
}

static int write_ref_sha1(struct ref_lock *lock, const unsigned char *sha1,
                          const char *logmsg);

int rename_ref(const char *oldrefname, const char *newrefname, const char *logmsg)
{
        unsigned char sha1[20], orig_sha1[20];
        int flag = 0, logmoved = 0;
        struct ref_lock *lock;
        struct stat loginfo;
        int log = !lstat(git_path("logs/%s", oldrefname), &loginfo);
        const char *symref = NULL;

        if (log && S_ISLNK(loginfo.st_mode))
                return error("reflog for %s is a symlink", oldrefname);

        symref = resolve_ref_unsafe(oldrefname, RESOLVE_REF_READING,
                                    orig_sha1, &flag);
        if (flag & REF_ISSYMREF)
                return error("refname %s is a symbolic ref, renaming it is not supported",
                        oldrefname);
        if (!symref)
                return error("refname %s not found", oldrefname);

        if (!rename_ref_available(oldrefname, newrefname))
                return 1;

        if (log && rename(git_path("logs/%s", oldrefname), git_path(TMP_RENAMED_LOG)))
                return error("unable to move logfile logs/%s to "TMP_RENAMED_LOG": %s",
                        oldrefname, strerror(errno));

        if (delete_ref(oldrefname, orig_sha1, REF_NODEREF)) {
                error("unable to delete old %s", oldrefname);
                goto rollback;
        }

        if (!read_ref_full(newrefname, RESOLVE_REF_READING, sha1, NULL) &&
            delete_ref(newrefname, sha1, REF_NODEREF)) {
                if (errno==EISDIR) {
                        if (remove_empty_directories(git_path("%s", newrefname))) {
                                error("Directory not empty: %s", newrefname);
                                goto rollback;
                        }
                } else {
                        error("unable to delete existing %s", newrefname);
                        goto rollback;
                }
        }

        if (log && rename_tmp_log(newrefname))
                goto rollback;

        logmoved = log;

        lock = lock_ref_sha1_basic(newrefname, NULL, NULL, NULL, 0, NULL);
        if (!lock) {
                error("unable to lock %s for update", newrefname);
                goto rollback;
        }
        hashcpy(lock->old_sha1, orig_sha1);
        if (write_ref_sha1(lock, orig_sha1, logmsg)) {
                error("unable to write current sha1 into %s", newrefname);
                goto rollback;
        }

        return 0;

 rollback:
        lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, NULL, 0, NULL);
        if (!lock) {
                error("unable to lock %s for rollback", oldrefname);
                goto rollbacklog;
        }

        flag = log_all_ref_updates;
        log_all_ref_updates = 0;
        if (write_ref_sha1(lock, orig_sha1, NULL))
                error("unable to write current sha1 into %s", oldrefname);
        log_all_ref_updates = flag;

 rollbacklog:
        if (logmoved && rename(git_path("logs/%s", newrefname), git_path("logs/%s", oldrefname)))
                error("unable to restore logfile %s from %s: %s",
                        oldrefname, newrefname, strerror(errno));
        if (!logmoved && log &&
            rename(git_path(TMP_RENAMED_LOG), git_path("logs/%s", oldrefname)))
                error("unable to restore logfile %s from "TMP_RENAMED_LOG": %s",
                        oldrefname, strerror(errno));

        return 1;
}

static int close_ref(struct ref_lock *lock)
{
        if (close_lock_file(lock->lk))
                return -1;
        lock->lock_fd = -1;
        return 0;
}

static int commit_ref(struct ref_lock *lock)
{
        if (commit_lock_file(lock->lk))
                return -1;
        lock->lock_fd = -1;
        return 0;
}

/*
 * copy the reflog message msg to buf, which has been allocated sufficiently
 * large, while cleaning up the whitespaces.  Especially, convert LF to space,
 * because reflog file is one line per entry.
 */
static int copy_msg(char *buf, const char *msg)
{
        char *cp = buf;
        char c;
        int wasspace = 1;

        *cp++ = '\t';
        while ((c = *msg++)) {
                if (wasspace && isspace(c))
                        continue;
                wasspace = isspace(c);
                if (wasspace)
                        c = ' ';
                *cp++ = c;
        }
        while (buf < cp && isspace(cp[-1]))
                cp--;
        *cp++ = '\n';
        return cp - buf;
}

/* This function must set a meaningful errno on failure */
int log_ref_setup(const char *refname, char *logfile, int bufsize)
{
        int logfd, oflags = O_APPEND | O_WRONLY;

        git_snpath(logfile, bufsize, "logs/%s", refname);
        if (log_all_ref_updates &&
            (starts_with(refname, "refs/heads/") ||
             starts_with(refname, "refs/remotes/") ||
             starts_with(refname, "refs/notes/") ||
             !strcmp(refname, "HEAD"))) {
                if (safe_create_leading_directories(logfile) < 0) {
                        int save_errno = errno;
                        error("unable to create directory for %s", logfile);
                        errno = save_errno;
                        return -1;
                }
                oflags |= O_CREAT;
        }

        logfd = open(logfile, oflags, 0666);
        if (logfd < 0) {
                if (!(oflags & O_CREAT) && (errno == ENOENT || errno == EISDIR))
                        return 0;

                if (errno == EISDIR) {
                        if (remove_empty_directories(logfile)) {
                                int save_errno = errno;
                                error("There are still logs under '%s'",
                                      logfile);
                                errno = save_errno;
                                return -1;
                        }
                        logfd = open(logfile, oflags, 0666);
                }

                if (logfd < 0) {
                        int save_errno = errno;
                        error("Unable to append to %s: %s", logfile,
                              strerror(errno));
                        errno = save_errno;
                        return -1;
                }
        }

        adjust_shared_perm(logfile);
        close(logfd);
        return 0;
}

static int log_ref_write_fd(int fd, const unsigned char *old_sha1,
                            const unsigned char *new_sha1,
                            const char *committer, const char *msg)
{
        int msglen, written;
        unsigned maxlen, len;
        char *logrec;

        msglen = msg ? strlen(msg) : 0;
        maxlen = strlen(committer) + msglen + 100;
        logrec = xmalloc(maxlen);
        len = sprintf(logrec, "%s %s %s\n",
                      sha1_to_hex(old_sha1),
                      sha1_to_hex(new_sha1),
                      committer);
        if (msglen)
                len += copy_msg(logrec + len - 1, msg) - 1;

        written = len <= maxlen ? write_in_full(fd, logrec, len) : -1;
        free(logrec);
        if (written != len)
                return -1;

        return 0;
}

static int log_ref_write(const char *refname, const unsigned char *old_sha1,
                         const unsigned char *new_sha1, const char *msg)
{
        int logfd, result, oflags = O_APPEND | O_WRONLY;
        char log_file[PATH_MAX];

        if (log_all_ref_updates < 0)
                log_all_ref_updates = !is_bare_repository();

        result = log_ref_setup(refname, log_file, sizeof(log_file));
        if (result)
                return result;

        logfd = open(log_file, oflags);
        if (logfd < 0)
                return 0;
        result = log_ref_write_fd(logfd, old_sha1, new_sha1,
                                  git_committer_info(0), msg);
        if (result) {
                int save_errno = errno;
                close(logfd);
                error("Unable to append to %s", log_file);
                errno = save_errno;
                return -1;
        }
        if (close(logfd)) {
                int save_errno = errno;
                error("Unable to append to %s", log_file);
                errno = save_errno;
                return -1;
        }
        return 0;
}

int is_branch(const char *refname)
{
        return !strcmp(refname, "HEAD") || starts_with(refname, "refs/heads/");
}

/*
 * Write sha1 into the ref specified by the lock. Make sure that errno
 * is sane on error.
 */
static int write_ref_sha1(struct ref_lock *lock,
        const unsigned char *sha1, const char *logmsg)
{
        static char term = '\n';
        struct object *o;

        o = parse_object(sha1);
        if (!o) {
                error("Trying to write ref %s with nonexistent object %s",
                        lock->ref_name, sha1_to_hex(sha1));
                unlock_ref(lock);
                errno = EINVAL;
                return -1;
        }
        if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
                error("Trying to write non-commit object %s to branch %s",
                        sha1_to_hex(sha1), lock->ref_name);
                unlock_ref(lock);
                errno = EINVAL;
                return -1;
        }
        if (write_in_full(lock->lock_fd, sha1_to_hex(sha1), 40) != 40 ||
            write_in_full(lock->lock_fd, &term, 1) != 1 ||
            close_ref(lock) < 0) {
                int save_errno = errno;
                error("Couldn't write %s", lock->lk->filename.buf);
                unlock_ref(lock);
                errno = save_errno;
                return -1;
        }
        clear_loose_ref_cache(&ref_cache);
        if (log_ref_write(lock->ref_name, lock->old_sha1, sha1, logmsg) < 0 ||
            (strcmp(lock->ref_name, lock->orig_ref_name) &&
             log_ref_write(lock->orig_ref_name, lock->old_sha1, sha1, logmsg) < 0)) {
                unlock_ref(lock);
                return -1;
        }
        if (strcmp(lock->orig_ref_name, "HEAD") != 0) {
                /*
                 * Special hack: If a branch is updated directly and HEAD
                 * points to it (may happen on the remote side of a push
                 * for example) then logically the HEAD reflog should be
                 * updated too.
                 * A generic solution implies reverse symref information,
                 * but finding all symrefs pointing to the given branch
                 * would be rather costly for this rare event (the direct
                 * update of a branch) to be worth it.  So let's cheat and
                 * check with HEAD only which should cover 99% of all usage
                 * scenarios (even 100% of the default ones).
                 */
                unsigned char head_sha1[20];
                int head_flag;
                const char *head_ref;
                head_ref = resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
                                              head_sha1, &head_flag);
                if (head_ref && (head_flag & REF_ISSYMREF) &&
                    !strcmp(head_ref, lock->ref_name))
                        log_ref_write("HEAD", lock->old_sha1, sha1, logmsg);
        }
        if (commit_ref(lock)) {
                error("Couldn't set %s", lock->ref_name);
                unlock_ref(lock);
                return -1;
        }
        unlock_ref(lock);
        return 0;
}

int create_symref(const char *ref_target, const char *refs_heads_master,
                  const char *logmsg)
{
        const char *lockpath;
        char ref[1000];
        int fd, len, written;
        char *git_HEAD = git_pathdup("%s", ref_target);
        unsigned char old_sha1[20], new_sha1[20];

        if (logmsg && read_ref(ref_target, old_sha1))
                hashclr(old_sha1);

        if (safe_create_leading_directories(git_HEAD) < 0)
                return error("unable to create directory for %s", git_HEAD);

#ifndef NO_SYMLINK_HEAD
        if (prefer_symlink_refs) {
                unlink(git_HEAD);
                if (!symlink(refs_heads_master, git_HEAD))
                        goto done;
                fprintf(stderr, "no symlink - falling back to symbolic ref\n");
        }
#endif

        len = snprintf(ref, sizeof(ref), "ref: %s\n", refs_heads_master);
        if (sizeof(ref) <= len) {
                error("refname too long: %s", refs_heads_master);
                goto error_free_return;
        }
        lockpath = mkpath("%s.lock", git_HEAD);
        fd = open(lockpath, O_CREAT | O_EXCL | O_WRONLY, 0666);
        if (fd < 0) {
                error("Unable to open %s for writing", lockpath);
                goto error_free_return;
        }
        written = write_in_full(fd, ref, len);
        if (close(fd) != 0 || written != len) {
                error("Unable to write to %s", lockpath);
                goto error_unlink_return;
        }
        if (rename(lockpath, git_HEAD) < 0) {
                error("Unable to create %s", git_HEAD);
                goto error_unlink_return;
        }
        if (adjust_shared_perm(git_HEAD)) {
                error("Unable to fix permissions on %s", lockpath);
        error_unlink_return:
                unlink_or_warn(lockpath);
        error_free_return:
                free(git_HEAD);
                return -1;
        }

#ifndef NO_SYMLINK_HEAD
        done:
#endif
        if (logmsg && !read_ref(refs_heads_master, new_sha1))
                log_ref_write(ref_target, old_sha1, new_sha1, logmsg);

        free(git_HEAD);
        return 0;
}

struct read_ref_at_cb {
        const char *refname;
        unsigned long at_time;
        int cnt;
        int reccnt;
        unsigned char *sha1;
        int found_it;

        unsigned char osha1[20];
        unsigned char nsha1[20];
        int tz;
        unsigned long date;
        char **msg;
        unsigned long *cutoff_time;
        int *cutoff_tz;
        int *cutoff_cnt;
};

static int read_ref_at_ent(unsigned char *osha1, unsigned char *nsha1,
                const char *email, unsigned long timestamp, int tz,
                const char *message, void *cb_data)
{
        struct read_ref_at_cb *cb = cb_data;

        cb->reccnt++;
        cb->tz = tz;
        cb->date = timestamp;

        if (timestamp <= cb->at_time || cb->cnt == 0) {
                if (cb->msg)
                        *cb->msg = xstrdup(message);
                if (cb->cutoff_time)
                        *cb->cutoff_time = timestamp;
                if (cb->cutoff_tz)
                        *cb->cutoff_tz = tz;
                if (cb->cutoff_cnt)
                        *cb->cutoff_cnt = cb->reccnt - 1;
                /*
                 * we have not yet updated cb->[n|o]sha1 so they still
                 * hold the values for the previous record.
                 */
                if (!is_null_sha1(cb->osha1)) {
                        hashcpy(cb->sha1, nsha1);
                        if (hashcmp(cb->osha1, nsha1))
                                warning("Log for ref %s has gap after %s.",
                                        cb->refname, show_date(cb->date, cb->tz, DATE_RFC2822));
                }
                else if (cb->date == cb->at_time)
                        hashcpy(cb->sha1, nsha1);
                else if (hashcmp(nsha1, cb->sha1))
                        warning("Log for ref %s unexpectedly ended on %s.",
                                cb->refname, show_date(cb->date, cb->tz,
                                                   DATE_RFC2822));
                hashcpy(cb->osha1, osha1);
                hashcpy(cb->nsha1, nsha1);
                cb->found_it = 1;
                return 1;
        }
        hashcpy(cb->osha1, osha1);
        hashcpy(cb->nsha1, nsha1);
        if (cb->cnt > 0)
                cb->cnt--;
        return 0;
}

static int read_ref_at_ent_oldest(unsigned char *osha1, unsigned char *nsha1,
                                  const char *email, unsigned long timestamp,
                                  int tz, const char *message, void *cb_data)
{
        struct read_ref_at_cb *cb = cb_data;

        if (cb->msg)
                *cb->msg = xstrdup(message);
        if (cb->cutoff_time)
                *cb->cutoff_time = timestamp;
        if (cb->cutoff_tz)
                *cb->cutoff_tz = tz;
        if (cb->cutoff_cnt)
                *cb->cutoff_cnt = cb->reccnt;
        hashcpy(cb->sha1, osha1);
        if (is_null_sha1(cb->sha1))
                hashcpy(cb->sha1, nsha1);
        /* We just want the first entry */
        return 1;
}

int read_ref_at(const char *refname, unsigned int flags, unsigned long at_time, int cnt,
                unsigned char *sha1, char **msg,
                unsigned long *cutoff_time, int *cutoff_tz, int *cutoff_cnt)
{
        struct read_ref_at_cb cb;

        memset(&cb, 0, sizeof(cb));
        cb.refname = refname;
        cb.at_time = at_time;
        cb.cnt = cnt;
        cb.msg = msg;
        cb.cutoff_time = cutoff_time;
        cb.cutoff_tz = cutoff_tz;
        cb.cutoff_cnt = cutoff_cnt;
        cb.sha1 = sha1;

        for_each_reflog_ent_reverse(refname, read_ref_at_ent, &cb);

        if (!cb.reccnt) {
                if (flags & GET_SHA1_QUIETLY)
                        exit(128);
                else
                        die("Log for %s is empty.", refname);
        }
        if (cb.found_it)
                return 0;

        for_each_reflog_ent(refname, read_ref_at_ent_oldest, &cb);

        return 1;
}

int reflog_exists(const char *refname)
{
        struct stat st;

        return !lstat(git_path("logs/%s", refname), &st) &&
                S_ISREG(st.st_mode);
}

int delete_reflog(const char *refname)
{
        return remove_path(git_path("logs/%s", refname));
}

static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *cb_data)
{
        unsigned char osha1[20], nsha1[20];
        char *email_end, *message;
        unsigned long timestamp;
        int tz;

        /* old SP new SP name <email> SP time TAB msg LF */
        if (sb->len < 83 || sb->buf[sb->len - 1] != '\n' ||
            get_sha1_hex(sb->buf, osha1) || sb->buf[40] != ' ' ||
            get_sha1_hex(sb->buf + 41, nsha1) || sb->buf[81] != ' ' ||
            !(email_end = strchr(sb->buf + 82, '>')) ||
            email_end[1] != ' ' ||
            !(timestamp = strtoul(email_end + 2, &message, 10)) ||
            !message || message[0] != ' ' ||
            (message[1] != '+' && message[1] != '-') ||
            !isdigit(message[2]) || !isdigit(message[3]) ||
            !isdigit(message[4]) || !isdigit(message[5]))
                return 0; /* corrupt? */
        email_end[1] = '\0';
        tz = strtol(message + 1, NULL, 10);
        if (message[6] != '\t')
                message += 6;
        else
                message += 7;
        return fn(osha1, nsha1, sb->buf + 82, timestamp, tz, message, cb_data);
}

static char *find_beginning_of_line(char *bob, char *scan)
{
        while (bob < scan && *(--scan) != '\n')
                ; /* keep scanning backwards */
        /*
         * Return either beginning of the buffer, or LF at the end of
         * the previous line.
         */
        return scan;
}

int for_each_reflog_ent_reverse(const char *refname, each_reflog_ent_fn fn, void *cb_data)
{
        struct strbuf sb = STRBUF_INIT;
        FILE *logfp;
        long pos;
        int ret = 0, at_tail = 1;

        logfp = fopen(git_path("logs/%s", refname), "r");
        if (!logfp)
                return -1;

        /* Jump to the end */
        if (fseek(logfp, 0, SEEK_END) < 0)
                return error("cannot seek back reflog for %s: %s",
                             refname, strerror(errno));
        pos = ftell(logfp);
        while (!ret && 0 < pos) {
                int cnt;
                size_t nread;
                char buf[BUFSIZ];
                char *endp, *scanp;

                /* Fill next block from the end */
                cnt = (sizeof(buf) < pos) ? sizeof(buf) : pos;
                if (fseek(logfp, pos - cnt, SEEK_SET))
                        return error("cannot seek back reflog for %s: %s",
                                     refname, strerror(errno));
                nread = fread(buf, cnt, 1, logfp);
                if (nread != 1)
                        return error("cannot read %d bytes from reflog for %s: %s",
                                     cnt, refname, strerror(errno));
                pos -= cnt;

                scanp = endp = buf + cnt;
                if (at_tail && scanp[-1] == '\n')
                        /* Looking at the final LF at the end of the file */
                        scanp--;
                at_tail = 0;

                while (buf < scanp) {
                        /*
                         * terminating LF of the previous line, or the beginning
                         * of the buffer.
                         */
                        char *bp;

                        bp = find_beginning_of_line(buf, scanp);

                        if (*bp == '\n') {
                                /*
                                 * The newline is the end of the previous line,
                                 * so we know we have complete line starting
                                 * at (bp + 1). Prefix it onto any prior data
                                 * we collected for the line and process it.
                                 */
                                strbuf_splice(&sb, 0, 0, bp + 1, endp - (bp + 1));
                                scanp = bp;
                                endp = bp + 1;
                                ret = show_one_reflog_ent(&sb, fn, cb_data);
                                strbuf_reset(&sb);
                                if (ret)
                                        break;
                        } else if (!pos) {
                                /*
                                 * We are at the start of the buffer, and the
                                 * start of the file; there is no previous
                                 * line, and we have everything for this one.
                                 * Process it, and we can end the loop.
                                 */
                                strbuf_splice(&sb, 0, 0, buf, endp - buf);
                                ret = show_one_reflog_ent(&sb, fn, cb_data);
                                strbuf_reset(&sb);
                                break;
                        }

                        if (bp == buf) {
                                /*
                                 * We are at the start of the buffer, and there
                                 * is more file to read backwards. Which means
                                 * we are in the middle of a line. Note that we
                                 * may get here even if *bp was a newline; that
                                 * just means we are at the exact end of the
                                 * previous line, rather than some spot in the
                                 * middle.
                                 *
                                 * Save away what we have to be combined with
                                 * the data from the next read.
                                 */
                                strbuf_splice(&sb, 0, 0, buf, endp - buf);
                                break;
                        }
                }

        }
        if (!ret && sb.len)
                die("BUG: reverse reflog parser had leftover data");

        fclose(logfp);
        strbuf_release(&sb);
        return ret;
}

int for_each_reflog_ent(const char *refname, each_reflog_ent_fn fn, void *cb_data)
{
        FILE *logfp;
        struct strbuf sb = STRBUF_INIT;
        int ret = 0;

        logfp = fopen(git_path("logs/%s", refname), "r");
        if (!logfp)
                return -1;

        while (!ret && !strbuf_getwholeline(&sb, logfp, '\n'))
                ret = show_one_reflog_ent(&sb, fn, cb_data);
        fclose(logfp);
        strbuf_release(&sb);
        return ret;
}
/*
 * Call fn for each reflog in the namespace indicated by name.  name
 * must be empty or end with '/'.  Name will be used as a scratch
 * space, but its contents will be restored before return.
 */
static int do_for_each_reflog(struct strbuf *name, each_ref_fn fn, void *cb_data)
{
        DIR *d = opendir(git_path("logs/%s", name->buf));
        int retval = 0;
        struct dirent *de;
        int oldlen = name->len;

        if (!d)
                return name->len ? errno : 0;

        while ((de = readdir(d)) != NULL) {
                struct stat st;

                if (de->d_name[0] == '.')
                        continue;
                if (ends_with(de->d_name, ".lock"))
                        continue;
                strbuf_addstr(name, de->d_name);
                if (stat(git_path("logs/%s", name->buf), &st) < 0) {
                        ; /* silently ignore */
                } else {
                        if (S_ISDIR(st.st_mode)) {
                                strbuf_addch(name, '/');
                                retval = do_for_each_reflog(name, fn, cb_data);
                        } else {
                                unsigned char sha1[20];
                                if (read_ref_full(name->buf, 0, sha1, NULL))
                                        retval = error("bad ref for %s", name->buf);
                                else
                                        retval = fn(name->buf, sha1, 0, cb_data);
                        }
                        if (retval)
                                break;
                }
                strbuf_setlen(name, oldlen);
        }
        closedir(d);
        return retval;
}

int for_each_reflog(each_ref_fn fn, void *cb_data)
{
        int retval;
        struct strbuf name;
        strbuf_init(&name, PATH_MAX);
        retval = do_for_each_reflog(&name, fn, cb_data);
        strbuf_release(&name);
        return retval;
}

/**
 * Information needed for a single ref update. Set new_sha1 to the new
 * value or to null_sha1 to delete the ref. To check the old value
 * while the ref is locked, set (flags & REF_HAVE_OLD) and set
 * old_sha1 to the old value, or to null_sha1 to ensure the ref does
 * not exist before update.
 */
struct ref_update {
        /*
         * If (flags & REF_HAVE_NEW), set the reference to this value:
         */
        unsigned char new_sha1[20];
        /*
         * If (flags & REF_HAVE_OLD), check that the reference
         * previously had this value:
         */
        unsigned char old_sha1[20];
        /*
         * One or more of REF_HAVE_NEW, REF_HAVE_OLD, REF_NODEREF,
         * REF_DELETING, and REF_ISPRUNING:
         */
        unsigned int flags;
        struct ref_lock *lock;
        int type;
        char *msg;
        const char refname[FLEX_ARRAY];
};

/*
 * Transaction states.
 * OPEN:   The transaction is in a valid state and can accept new updates.
 *         An OPEN transaction can be committed.
 * CLOSED: A closed transaction is no longer active and no other operations
 *         than free can be used on it in this state.
 *         A transaction can either become closed by successfully committing
 *         an active transaction or if there is a failure while building
 *         the transaction thus rendering it failed/inactive.
 */
enum ref_transaction_state {
        REF_TRANSACTION_OPEN   = 0,
        REF_TRANSACTION_CLOSED = 1
};

/*
 * Data structure for holding a reference transaction, which can
 * consist of checks and updates to multiple references, carried out
 * as atomically as possible.  This structure is opaque to callers.
 */
struct ref_transaction {
        struct ref_update **updates;
        size_t alloc;
        size_t nr;
        enum ref_transaction_state state;
};

struct ref_transaction *ref_transaction_begin(struct strbuf *err)
{
        assert(err);

        return xcalloc(1, sizeof(struct ref_transaction));
}

void ref_transaction_free(struct ref_transaction *transaction)
{
        int i;

        if (!transaction)
                return;

        for (i = 0; i < transaction->nr; i++) {
                free(transaction->updates[i]->msg);
                free(transaction->updates[i]);
        }
        free(transaction->updates);
        free(transaction);
}

static struct ref_update *add_update(struct ref_transaction *transaction,
                                     const char *refname)
{
        size_t len = strlen(refname);
        struct ref_update *update = xcalloc(1, sizeof(*update) + len + 1);

        strcpy((char *)update->refname, refname);
        ALLOC_GROW(transaction->updates, transaction->nr + 1, transaction->alloc);
        transaction->updates[transaction->nr++] = update;
        return update;
}

int ref_transaction_update(struct ref_transaction *transaction,
                           const char *refname,
                           const unsigned char *new_sha1,
                           const unsigned char *old_sha1,
                           unsigned int flags, const char *msg,
                           struct strbuf *err)
{
        struct ref_update *update;

        assert(err);

        if (transaction->state != REF_TRANSACTION_OPEN)
                die("BUG: update called for transaction that is not open");

        if (new_sha1 && !is_null_sha1(new_sha1) &&
            check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                strbuf_addf(err, "refusing to update ref with bad name %s",
                            refname);
                return -1;
        }

        update = add_update(transaction, refname);
        if (new_sha1) {
                hashcpy(update->new_sha1, new_sha1);
                flags |= REF_HAVE_NEW;
        }
        if (old_sha1) {
                hashcpy(update->old_sha1, old_sha1);
                flags |= REF_HAVE_OLD;
        }
        update->flags = flags;
        if (msg)
                update->msg = xstrdup(msg);
        return 0;
}

int ref_transaction_create(struct ref_transaction *transaction,
                           const char *refname,
                           const unsigned char *new_sha1,
                           unsigned int flags, const char *msg,
                           struct strbuf *err)
{
        if (!new_sha1 || is_null_sha1(new_sha1))
                die("BUG: create called without valid new_sha1");
        return ref_transaction_update(transaction, refname, new_sha1,
                                      null_sha1, flags, msg, err);
}

int ref_transaction_delete(struct ref_transaction *transaction,
                           const char *refname,
                           const unsigned char *old_sha1,
                           unsigned int flags, const char *msg,
                           struct strbuf *err)
{
        if (old_sha1 && is_null_sha1(old_sha1))
                die("BUG: delete called with old_sha1 set to zeros");
        return ref_transaction_update(transaction, refname,
                                      null_sha1, old_sha1,
                                      flags, msg, err);
}

int ref_transaction_verify(struct ref_transaction *transaction,
                           const char *refname,
                           const unsigned char *old_sha1,
                           unsigned int flags,
                           struct strbuf *err)
{
        if (!old_sha1)
                die("BUG: verify called with old_sha1 set to NULL");
        return ref_transaction_update(transaction, refname,
                                      NULL, old_sha1,
                                      flags, NULL, err);
}

int update_ref(const char *msg, const char *refname,
               const unsigned char *new_sha1, const unsigned char *old_sha1,
               unsigned int flags, enum action_on_err onerr)
{
        struct ref_transaction *t;
        struct strbuf err = STRBUF_INIT;

        t = ref_transaction_begin(&err);
        if (!t ||
            ref_transaction_update(t, refname, new_sha1, old_sha1,
                                   flags, msg, &err) ||
            ref_transaction_commit(t, &err)) {
                const char *str = "update_ref failed for ref '%s': %s";

                ref_transaction_free(t);
                switch (onerr) {
                case UPDATE_REFS_MSG_ON_ERR:
                        error(str, refname, err.buf);
                        break;
                case UPDATE_REFS_DIE_ON_ERR:
                        die(str, refname, err.buf);
                        break;
                case UPDATE_REFS_QUIET_ON_ERR:
                        break;
                }
                strbuf_release(&err);
                return 1;
        }
        strbuf_release(&err);
        ref_transaction_free(t);
        return 0;
}

static int ref_update_reject_duplicates(struct string_list *refnames,
                                        struct strbuf *err)
{
        int i, n = refnames->nr;

        assert(err);

        for (i = 1; i < n; i++)
                if (!strcmp(refnames->items[i - 1].string, refnames->items[i].string)) {
                        strbuf_addf(err,
                                    "Multiple updates for ref '%s' not allowed.",
                                    refnames->items[i].string);
                        return 1;
                }
        return 0;
}

int ref_transaction_commit(struct ref_transaction *transaction,
                           struct strbuf *err)
{
        int ret = 0, i;
        int n = transaction->nr;
        struct ref_update **updates = transaction->updates;
        struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
        struct string_list_item *ref_to_delete;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;

        assert(err);

        if (transaction->state != REF_TRANSACTION_OPEN)
                die("BUG: commit called for transaction that is not open");

        if (!n) {
                transaction->state = REF_TRANSACTION_CLOSED;
                return 0;
        }

        /* Fail if a refname appears more than once in the transaction: */
        for (i = 0; i < n; i++)
                string_list_append(&affected_refnames, updates[i]->refname);
        string_list_sort(&affected_refnames);
        if (ref_update_reject_duplicates(&affected_refnames, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }

        /* Acquire all locks while verifying old values */
        for (i = 0; i < n; i++) {
                struct ref_update *update = updates[i];
                unsigned int flags = update->flags;

                if ((flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1))
                        flags |= REF_DELETING;
                update->lock = lock_ref_sha1_basic(
                                update->refname,
                                ((update->flags & REF_HAVE_OLD) ?
                                 update->old_sha1 : NULL),
                                &affected_refnames, NULL,
                                flags,
                                &update->type);
                if (!update->lock) {
                        ret = (errno == ENOTDIR)
                                ? TRANSACTION_NAME_CONFLICT
                                : TRANSACTION_GENERIC_ERROR;
                        strbuf_addf(err, "Cannot lock the ref '%s'.",
                                    update->refname);
                        goto cleanup;
                }
        }

        /* Perform updates first so live commits remain referenced */
        for (i = 0; i < n; i++) {
                struct ref_update *update = updates[i];
                int flags = update->flags;

                if ((flags & REF_HAVE_NEW) && !is_null_sha1(update->new_sha1)) {
                        int overwriting_symref = ((update->type & REF_ISSYMREF) &&
                                                  (update->flags & REF_NODEREF));

                        if (!overwriting_symref
                            && !hashcmp(update->lock->old_sha1, update->new_sha1)) {
                                /*
                                 * The reference already has the desired
                                 * value, so we don't need to write it.
                                 */
                                unlock_ref(update->lock);
                                update->lock = NULL;
                        } else if (write_ref_sha1(update->lock, update->new_sha1,
                                                  update->msg)) {
                                update->lock = NULL; /* freed by write_ref_sha1 */
                                strbuf_addf(err, "Cannot update the ref '%s'.",
                                            update->refname);
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
                        } else {
                                /* freed by write_ref_sha1(): */
                                update->lock = NULL;
                        }
                }
        }

        /* Perform deletes now that updates are safely completed */
        for (i = 0; i < n; i++) {
                struct ref_update *update = updates[i];
                int flags = update->flags;

                if ((flags & REF_HAVE_NEW) && is_null_sha1(update->new_sha1)) {
                        if (delete_ref_loose(update->lock, update->type, err)) {
                                ret = TRANSACTION_GENERIC_ERROR;
                                goto cleanup;
                        }

                        if (!(flags & REF_ISPRUNING))
                                string_list_append(&refs_to_delete,
                                                   update->lock->ref_name);
                }
        }

        if (repack_without_refs(&refs_to_delete, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto cleanup;
        }
        for_each_string_list_item(ref_to_delete, &refs_to_delete)
                unlink_or_warn(git_path("logs/%s", ref_to_delete->string));
        clear_loose_ref_cache(&ref_cache);

cleanup:
        transaction->state = REF_TRANSACTION_CLOSED;

        for (i = 0; i < n; i++)
                if (updates[i]->lock)
                        unlock_ref(updates[i]->lock);
        string_list_clear(&refs_to_delete, 0);
        string_list_clear(&affected_refnames, 0);
        return ret;
}

char *shorten_unambiguous_ref(const char *refname, int strict)
{
        int i;
        static char **scanf_fmts;
        static int nr_rules;
        char *short_name;

        if (!nr_rules) {
                /*
                 * Pre-generate scanf formats from ref_rev_parse_rules[].
                 * Generate a format suitable for scanf from a
                 * ref_rev_parse_rules rule by interpolating "%s" at the
                 * location of the "%.*s".
                 */
                size_t total_len = 0;
                size_t offset = 0;

                /* the rule list is NULL terminated, count them first */
                for (nr_rules = 0; ref_rev_parse_rules[nr_rules]; nr_rules++)
                        /* -2 for strlen("%.*s") - strlen("%s"); +1 for NUL */
                        total_len += strlen(ref_rev_parse_rules[nr_rules]) - 2 + 1;

                scanf_fmts = xmalloc(nr_rules * sizeof(char *) + total_len);

                offset = 0;
                for (i = 0; i < nr_rules; i++) {
                        assert(offset < total_len);
                        scanf_fmts[i] = (char *)&scanf_fmts[nr_rules] + offset;
                        offset += snprintf(scanf_fmts[i], total_len - offset,
                                           ref_rev_parse_rules[i], 2, "%s") + 1;
                }
        }

        /* bail out if there are no rules */
        if (!nr_rules)
                return xstrdup(refname);

        /* buffer for scanf result, at most refname must fit */
        short_name = xstrdup(refname);

        /* skip first rule, it will always match */
        for (i = nr_rules - 1; i > 0 ; --i) {
                int j;
                int rules_to_fail = i;
                int short_name_len;

                if (1 != sscanf(refname, scanf_fmts[i], short_name))
                        continue;

                short_name_len = strlen(short_name);

                /*
                 * in strict mode, all (except the matched one) rules
                 * must fail to resolve to a valid non-ambiguous ref
                 */
                if (strict)
                        rules_to_fail = nr_rules;

                /*
                 * check if the short name resolves to a valid ref,
                 * but use only rules prior to the matched one
                 */
                for (j = 0; j < rules_to_fail; j++) {
                        const char *rule = ref_rev_parse_rules[j];
                        char refname[PATH_MAX];

                        /* skip matched rule */
                        if (i == j)
                                continue;

                        /*
                         * the short name is ambiguous, if it resolves
                         * (with this previous rule) to a valid ref
                         * read_ref() returns 0 on success
                         */
                        mksnpath(refname, sizeof(refname),
                                 rule, short_name_len, short_name);
                        if (ref_exists(refname))
                                break;
                }

                /*
                 * short name is non-ambiguous if all previous rules
                 * haven't resolved to a valid ref
                 */
                if (j == rules_to_fail)
                        return short_name;
        }

        free(short_name);
        return xstrdup(refname);
}

static struct string_list *hide_refs;

int parse_hide_refs_config(const char *var, const char *value, const char *section)
{
        if (!strcmp("transfer.hiderefs", var) ||
            /* NEEDSWORK: use parse_config_key() once both are merged */
            (starts_with(var, section) && var[strlen(section)] == '.' &&
             !strcmp(var + strlen(section), ".hiderefs"))) {
                char *ref;
                int len;

                if (!value)
                        return config_error_nonbool(var);
                ref = xstrdup(value);
                len = strlen(ref);
                while (len && ref[len - 1] == '/')
                        ref[--len] = '\0';
                if (!hide_refs) {
                        hide_refs = xcalloc(1, sizeof(*hide_refs));
                        hide_refs->strdup_strings = 1;
                }
                string_list_append(hide_refs, ref);
        }
        return 0;
}

int ref_is_hidden(const char *refname)
{
        struct string_list_item *item;

        if (!hide_refs)
                return 0;
        for_each_string_list_item(item, hide_refs) {
                int len;
                if (!starts_with(refname, item->string))
                        continue;
                len = strlen(item->string);
                if (!refname[len] || refname[len] == '/')
                        return 1;
        }
        return 0;
}

struct expire_reflog_cb {
        unsigned int flags;
        reflog_expiry_should_prune_fn *should_prune_fn;
        void *policy_cb;
        FILE *newlog;
        unsigned char last_kept_sha1[20];
};

static int expire_reflog_ent(unsigned char *osha1, unsigned char *nsha1,
                             const char *email, unsigned long timestamp, int tz,
                             const char *message, void *cb_data)
{
        struct expire_reflog_cb *cb = cb_data;
        struct expire_reflog_policy_cb *policy_cb = cb->policy_cb;

        if (cb->flags & EXPIRE_REFLOGS_REWRITE)
                osha1 = cb->last_kept_sha1;

        if ((*cb->should_prune_fn)(osha1, nsha1, email, timestamp, tz,
                                   message, policy_cb)) {
                if (!cb->newlog)
                        printf("would prune %s", message);
                else if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
                        printf("prune %s", message);
        } else {
                if (cb->newlog) {
                        fprintf(cb->newlog, "%s %s %s %lu %+05d\t%s",
                                sha1_to_hex(osha1), sha1_to_hex(nsha1),
                                email, timestamp, tz, message);
                        hashcpy(cb->last_kept_sha1, nsha1);
                }
                if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
                        printf("keep %s", message);
        }
        return 0;
}

int reflog_expire(const char *refname, const unsigned char *sha1,
                 unsigned int flags,
                 reflog_expiry_prepare_fn prepare_fn,
                 reflog_expiry_should_prune_fn should_prune_fn,
                 reflog_expiry_cleanup_fn cleanup_fn,
                 void *policy_cb_data)
{
        static struct lock_file reflog_lock;
        struct expire_reflog_cb cb;
        struct ref_lock *lock;
        char *log_file;
        int status = 0;
        int type;

        memset(&cb, 0, sizeof(cb));
        cb.flags = flags;
        cb.policy_cb = policy_cb_data;
        cb.should_prune_fn = should_prune_fn;

        /*
         * The reflog file is locked by holding the lock on the
         * reference itself, plus we might need to update the
         * reference if --updateref was specified:
         */
        lock = lock_ref_sha1_basic(refname, sha1, NULL, NULL, 0, &type);
        if (!lock)
                return error("cannot lock ref '%s'", refname);
        if (!reflog_exists(refname)) {
                unlock_ref(lock);
                return 0;
        }

        log_file = git_pathdup("logs/%s", refname);
        if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
                /*
                 * Even though holding $GIT_DIR/logs/$reflog.lock has
                 * no locking implications, we use the lock_file
                 * machinery here anyway because it does a lot of the
                 * work we need, including cleaning up if the program
                 * exits unexpectedly.
                 */
                if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
                        struct strbuf err = STRBUF_INIT;
                        unable_to_lock_message(log_file, errno, &err);
                        error("%s", err.buf);
                        strbuf_release(&err);
                        goto failure;
                }
                cb.newlog = fdopen_lock_file(&reflog_lock, "w");
                if (!cb.newlog) {
                        error("cannot fdopen %s (%s)",
                              reflog_lock.filename.buf, strerror(errno));
                        goto failure;
                }
        }

        (*prepare_fn)(refname, sha1, cb.policy_cb);
        for_each_reflog_ent(refname, expire_reflog_ent, &cb);
        (*cleanup_fn)(cb.policy_cb);

        if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
                /*
                 * It doesn't make sense to adjust a reference pointed
                 * to by a symbolic ref based on expiring entries in
                 * the symbolic reference's reflog. Nor can we update
                 * a reference if there are no remaining reflog
                 * entries.
                 */
                int update = (flags & EXPIRE_REFLOGS_UPDATE_REF) &&
                        !(type & REF_ISSYMREF) &&
                        !is_null_sha1(cb.last_kept_sha1);

                if (close_lock_file(&reflog_lock)) {
                        status |= error("couldn't write %s: %s", log_file,
                                        strerror(errno));
                } else if (update &&
                        (write_in_full(lock->lock_fd,
                                sha1_to_hex(cb.last_kept_sha1), 40) != 40 ||
                         write_str_in_full(lock->lock_fd, "\n") != 1 ||
                         close_ref(lock) < 0)) {
                        status |= error("couldn't write %s",
                                        lock->lk->filename.buf);
                        rollback_lock_file(&reflog_lock);
                } else if (commit_lock_file(&reflog_lock)) {
                        status |= error("unable to commit reflog '%s' (%s)",
                                        log_file, strerror(errno));
                } else if (update && commit_ref(lock)) {
                        status |= error("couldn't set %s", lock->ref_name);
                }
        }
        free(log_file);
        unlock_ref(lock);
        return status;

 failure:
        rollback_lock_file(&reflog_lock);
        free(log_file);
        unlock_ref(lock);
        return -1;
}