Merge branch 'hw/tutorial-favor-switch-over-checkout'

[git] / refs / packed-backend.c

#include "../cache.h"
#include "../config.h"
#include "../refs.h"
#include "refs-internal.h"
#include "packed-backend.h"
#include "../iterator.h"
#include "../lockfile.h"
#include "../chdir-notify.h"

enum mmap_strategy {
        /*
         * Don't use mmap() at all for reading `packed-refs`.
         */
        MMAP_NONE,

        /*
         * Can use mmap() for reading `packed-refs`, but the file must
         * not remain mmapped. This is the usual option on Windows,
         * where you cannot rename a new version of a file onto a file
         * that is currently mmapped.
         */
        MMAP_TEMPORARY,

        /*
         * It is OK to leave the `packed-refs` file mmapped while
         * arbitrary other code is running.
         */
        MMAP_OK
};

#if defined(NO_MMAP)
static enum mmap_strategy mmap_strategy = MMAP_NONE;
#elif defined(MMAP_PREVENTS_DELETE)
static enum mmap_strategy mmap_strategy = MMAP_TEMPORARY;
#else
static enum mmap_strategy mmap_strategy = MMAP_OK;
#endif

struct packed_ref_store;

/*
 * A `snapshot` represents one snapshot of a `packed-refs` file.
 *
 * Normally, this will be a mmapped view of the contents of the
 * `packed-refs` file at the time the snapshot was created. However,
 * if the `packed-refs` file was not sorted, this might point at heap
 * memory holding the contents of the `packed-refs` file with its
 * records sorted by refname.
 *
 * `snapshot` instances are reference counted (via
 * `acquire_snapshot()` and `release_snapshot()`). This is to prevent
 * an instance from disappearing while an iterator is still iterating
 * over it. Instances are garbage collected when their `referrers`
 * count goes to zero.
 *
 * The most recent `snapshot`, if available, is referenced by the
 * `packed_ref_store`. Its freshness is checked whenever
 * `get_snapshot()` is called; if the existing snapshot is obsolete, a
 * new snapshot is taken.
 */
struct snapshot {
        /*
         * A back-pointer to the packed_ref_store with which this
         * snapshot is associated:
         */
        struct packed_ref_store *refs;

        /* Is the `packed-refs` file currently mmapped? */
        int mmapped;

        /*
         * The contents of the `packed-refs` file:
         *
         * - buf -- a pointer to the start of the memory
         * - start -- a pointer to the first byte of actual references
         *   (i.e., after the header line, if one is present)
         * - eof -- a pointer just past the end of the reference
         *   contents
         *
         * If the `packed-refs` file was already sorted, `buf` points
         * at the mmapped contents of the file. If not, it points at
         * heap-allocated memory containing the contents, sorted. If
         * there were no contents (e.g., because the file didn't
         * exist), `buf`, `start`, and `eof` are all NULL.
         */
        char *buf, *start, *eof;

        /*
         * What is the peeled state of the `packed-refs` file that
         * this snapshot represents? (This is usually determined from
         * the file's header.)
         */
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled;

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

        /*
         * The metadata of the `packed-refs` file from which this
         * snapshot was created, used to tell if the file has been
         * replaced since we read it.
         */
        struct stat_validity validity;
};

/*
 * A `ref_store` representing references stored in a `packed-refs`
 * file. It implements the `ref_store` interface, though it has some
 * limitations:
 *
 * - It cannot store symbolic references.
 *
 * - It cannot store reflogs.
 *
 * - It does not support reference renaming (though it could).
 *
 * On the other hand, it can be locked outside of a reference
 * transaction. In that case, it remains locked even after the
 * transaction is done and the new `packed-refs` file is activated.
 */
struct packed_ref_store {
        struct ref_store base;

        unsigned int store_flags;

        /* The path of the "packed-refs" file: */
        char *path;

        /*
         * A snapshot of the values read from the `packed-refs` file,
         * if it might still be current; otherwise, NULL.
         */
        struct snapshot *snapshot;

        /*
         * Lock used for the "packed-refs" file. Note that this (and
         * thus the enclosing `packed_ref_store`) must not be freed.
         */
        struct lock_file lock;

        /*
         * Temporary file used when rewriting new contents to the
         * "packed-refs" file. Note that this (and thus the enclosing
         * `packed_ref_store`) must not be freed.
         */
        struct tempfile *tempfile;
};

/*
 * Increment the reference count of `*snapshot`.
 */
static void acquire_snapshot(struct snapshot *snapshot)
{
        snapshot->referrers++;
}

/*
 * If the buffer in `snapshot` is active, then either munmap the
 * memory and close the file, or free the memory. Then set the buffer
 * pointers to NULL.
 */
static void clear_snapshot_buffer(struct snapshot *snapshot)
{
        if (snapshot->mmapped) {
                if (munmap(snapshot->buf, snapshot->eof - snapshot->buf))
                        die_errno("error ummapping packed-refs file %s",
                                  snapshot->refs->path);
                snapshot->mmapped = 0;
        } else {
                free(snapshot->buf);
        }
        snapshot->buf = snapshot->start = snapshot->eof = NULL;
}

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

struct ref_store *packed_ref_store_create(const char *path,
                                          unsigned int store_flags)
{
        struct packed_ref_store *refs = xcalloc(1, sizeof(*refs));
        struct ref_store *ref_store = (struct ref_store *)refs;

        base_ref_store_init(ref_store, &refs_be_packed);
        refs->store_flags = store_flags;

        refs->path = xstrdup(path);
        chdir_notify_reparent("packed-refs", &refs->path);

        return ref_store;
}

/*
 * Downcast `ref_store` to `packed_ref_store`. Die if `ref_store` is
 * not a `packed_ref_store`. Also die if `packed_ref_store` doesn't
 * support at least the flags specified in `required_flags`. `caller`
 * is used in any necessary error messages.
 */
static struct packed_ref_store *packed_downcast(struct ref_store *ref_store,
                                                unsigned int required_flags,
                                                const char *caller)
{
        struct packed_ref_store *refs;

        if (ref_store->be != &refs_be_packed)
                BUG("ref_store is type \"%s\" not \"packed\" in %s",
                    ref_store->be->name, caller);

        refs = (struct packed_ref_store *)ref_store;

        if ((refs->store_flags & required_flags) != required_flags)
                BUG("unallowed operation (%s), requires %x, has %x\n",
                    caller, required_flags, refs->store_flags);

        return refs;
}

static void clear_snapshot(struct packed_ref_store *refs)
{
        if (refs->snapshot) {
                struct snapshot *snapshot = refs->snapshot;

                refs->snapshot = NULL;
                release_snapshot(snapshot);
        }
}

static NORETURN void die_unterminated_line(const char *path,
                                           const char *p, size_t len)
{
        if (len < 80)
                die("unterminated line in %s: %.*s", path, (int)len, p);
        else
                die("unterminated line in %s: %.75s...", path, p);
}

static NORETURN void die_invalid_line(const char *path,
                                      const char *p, size_t len)
{
        const char *eol = memchr(p, '\n', len);

        if (!eol)
                die_unterminated_line(path, p, len);
        else if (eol - p < 80)
                die("unexpected line in %s: %.*s", path, (int)(eol - p), p);
        else
                die("unexpected line in %s: %.75s...", path, p);

}

struct snapshot_record {
        const char *start;
        size_t len;
};

static int cmp_packed_ref_records(const void *v1, const void *v2)
{
        const struct snapshot_record *e1 = v1, *e2 = v2;
        const char *r1 = e1->start + the_hash_algo->hexsz + 1;
        const char *r2 = e2->start + the_hash_algo->hexsz + 1;

        while (1) {
                if (*r1 == '\n')
                        return *r2 == '\n' ? 0 : -1;
                if (*r1 != *r2) {
                        if (*r2 == '\n')
                                return 1;
                        else
                                return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
                }
                r1++;
                r2++;
        }
}

/*
 * Compare a snapshot record at `rec` to the specified NUL-terminated
 * refname.
 */
static int cmp_record_to_refname(const char *rec, const char *refname)
{
        const char *r1 = rec + the_hash_algo->hexsz + 1;
        const char *r2 = refname;

        while (1) {
                if (*r1 == '\n')
                        return *r2 ? -1 : 0;
                if (!*r2)
                        return 1;
                if (*r1 != *r2)
                        return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
                r1++;
                r2++;
        }
}

/*
 * `snapshot->buf` is not known to be sorted. Check whether it is, and
 * if not, sort it into new memory and munmap/free the old storage.
 */
static void sort_snapshot(struct snapshot *snapshot)
{
        struct snapshot_record *records = NULL;
        size_t alloc = 0, nr = 0;
        int sorted = 1;
        const char *pos, *eof, *eol;
        size_t len, i;
        char *new_buffer, *dst;

        pos = snapshot->start;
        eof = snapshot->eof;

        if (pos == eof)
                return;

        len = eof - pos;

        /*
         * Initialize records based on a crude estimate of the number
         * of references in the file (we'll grow it below if needed):
         */
        ALLOC_GROW(records, len / 80 + 20, alloc);

        while (pos < eof) {
                eol = memchr(pos, '\n', eof - pos);
                if (!eol)
                        /* The safety check should prevent this. */
                        BUG("unterminated line found in packed-refs");
                if (eol - pos < the_hash_algo->hexsz + 2)
                        die_invalid_line(snapshot->refs->path,
                                         pos, eof - pos);
                eol++;
                if (eol < eof && *eol == '^') {
                        /*
                         * Keep any peeled line together with its
                         * reference:
                         */
                        const char *peeled_start = eol;

                        eol = memchr(peeled_start, '\n', eof - peeled_start);
                        if (!eol)
                                /* The safety check should prevent this. */
                                BUG("unterminated peeled line found in packed-refs");
                        eol++;
                }

                ALLOC_GROW(records, nr + 1, alloc);
                records[nr].start = pos;
                records[nr].len = eol - pos;
                nr++;

                if (sorted &&
                    nr > 1 &&
                    cmp_packed_ref_records(&records[nr - 2],
                                           &records[nr - 1]) >= 0)
                        sorted = 0;

                pos = eol;
        }

        if (sorted)
                goto cleanup;

        /* We need to sort the memory. First we sort the records array: */
        QSORT(records, nr, cmp_packed_ref_records);

        /*
         * Allocate a new chunk of memory, and copy the old memory to
         * the new in the order indicated by `records` (not bothering
         * with the header line):
         */
        new_buffer = xmalloc(len);
        for (dst = new_buffer, i = 0; i < nr; i++) {
                memcpy(dst, records[i].start, records[i].len);
                dst += records[i].len;
        }

        /*
         * Now munmap the old buffer and use the sorted buffer in its
         * place:
         */
        clear_snapshot_buffer(snapshot);
        snapshot->buf = snapshot->start = new_buffer;
        snapshot->eof = new_buffer + len;

cleanup:
        free(records);
}

/*
 * Return a pointer to the start of the record that contains the
 * character `*p` (which must be within the buffer). If no other
 * record start is found, return `buf`.
 */
static const char *find_start_of_record(const char *buf, const char *p)
{
        while (p > buf && (p[-1] != '\n' || p[0] == '^'))
                p--;
        return p;
}

/*
 * Return a pointer to the start of the record following the record
 * that contains `*p`. If none is found before `end`, return `end`.
 */
static const char *find_end_of_record(const char *p, const char *end)
{
        while (++p < end && (p[-1] != '\n' || p[0] == '^'))
                ;
        return p;
}

/*
 * We want to be able to compare mmapped reference records quickly,
 * without totally parsing them. We can do so because the records are
 * LF-terminated, and the refname should start exactly (GIT_SHA1_HEXSZ
 * + 1) bytes past the beginning of the record.
 *
 * But what if the `packed-refs` file contains garbage? We're willing
 * to tolerate not detecting the problem, as long as we don't produce
 * totally garbled output (we can't afford to check the integrity of
 * the whole file during every Git invocation). But we do want to be
 * sure that we never read past the end of the buffer in memory and
 * perform an illegal memory access.
 *
 * Guarantee that minimum level of safety by verifying that the last
 * record in the file is LF-terminated, and that it has at least
 * (GIT_SHA1_HEXSZ + 1) characters before the LF. Die if either of
 * these checks fails.
 */
static void verify_buffer_safe(struct snapshot *snapshot)
{
        const char *start = snapshot->start;
        const char *eof = snapshot->eof;
        const char *last_line;

        if (start == eof)
                return;

        last_line = find_start_of_record(start, eof - 1);
        if (*(eof - 1) != '\n' || eof - last_line < the_hash_algo->hexsz + 2)
                die_invalid_line(snapshot->refs->path,
                                 last_line, eof - last_line);
}

#define SMALL_FILE_SIZE (32*1024)

/*
 * Depending on `mmap_strategy`, either mmap or read the contents of
 * the `packed-refs` file into the snapshot. Return 1 if the file
 * existed and was read, or 0 if the file was absent or empty. Die on
 * errors.
 */
static int load_contents(struct snapshot *snapshot)
{
        int fd;
        struct stat st;
        size_t size;
        ssize_t bytes_read;

        fd = open(snapshot->refs->path, O_RDONLY);
        if (fd < 0) {
                if (errno == ENOENT) {
                        /*
                         * This is OK; it just means that no
                         * "packed-refs" file has been written yet,
                         * which is equivalent to it being empty,
                         * which is its state when initialized with
                         * zeros.
                         */
                        return 0;
                } else {
                        die_errno("couldn't read %s", snapshot->refs->path);
                }
        }

        stat_validity_update(&snapshot->validity, fd);

        if (fstat(fd, &st) < 0)
                die_errno("couldn't stat %s", snapshot->refs->path);
        size = xsize_t(st.st_size);

        if (!size) {
                close(fd);
                return 0;
        } else if (mmap_strategy == MMAP_NONE || size <= SMALL_FILE_SIZE) {
                snapshot->buf = xmalloc(size);
                bytes_read = read_in_full(fd, snapshot->buf, size);
                if (bytes_read < 0 || bytes_read != size)
                        die_errno("couldn't read %s", snapshot->refs->path);
                snapshot->mmapped = 0;
        } else {
                snapshot->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
                snapshot->mmapped = 1;
        }
        close(fd);

        snapshot->start = snapshot->buf;
        snapshot->eof = snapshot->buf + size;

        return 1;
}

/*
 * Find the place in `snapshot->buf` where the start of the record for
 * `refname` starts. If `mustexist` is true and the reference doesn't
 * exist, then return NULL. If `mustexist` is false and the reference
 * doesn't exist, then return the point where that reference would be
 * inserted, or `snapshot->eof` (which might be NULL) if it would be
 * inserted at the end of the file. In the latter mode, `refname`
 * doesn't have to be a proper reference name; for example, one could
 * search for "refs/replace/" to find the start of any replace
 * references.
 *
 * The record is sought using a binary search, so `snapshot->buf` must
 * be sorted.
 */
static const char *find_reference_location(struct snapshot *snapshot,
                                           const char *refname, int mustexist)
{
        /*
         * This is not *quite* a garden-variety binary search, because
         * the data we're searching is made up of records, and we
         * always need to find the beginning of a record to do a
         * comparison. A "record" here is one line for the reference
         * itself and zero or one peel lines that start with '^'. Our
         * loop invariant is described in the next two comments.
         */

        /*
         * A pointer to the character at the start of a record whose
         * preceding records all have reference names that come
         * *before* `refname`.
         */
        const char *lo = snapshot->start;

        /*
         * A pointer to a the first character of a record whose
         * reference name comes *after* `refname`.
         */
        const char *hi = snapshot->eof;

        while (lo != hi) {
                const char *mid, *rec;
                int cmp;

                mid = lo + (hi - lo) / 2;
                rec = find_start_of_record(lo, mid);
                cmp = cmp_record_to_refname(rec, refname);
                if (cmp < 0) {
                        lo = find_end_of_record(mid, hi);
                } else if (cmp > 0) {
                        hi = rec;
                } else {
                        return rec;
                }
        }

        if (mustexist)
                return NULL;
        else
                return lo;
}

/*
 * Create a newly-allocated `snapshot` of the `packed-refs` file in
 * its current state and return it. The return value will already have
 * its reference count incremented.
 *
 * A comment line of the form "# pack-refs with: " may contain zero or
 * more traits. We interpret the traits as follows:
 *
 *   Neither `peeled` nor `fully-peeled`:
 *
 *      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).
 *
 *   `sorted`:
 *
 *      The references in this file are known to be sorted by refname.
 */
static struct snapshot *create_snapshot(struct packed_ref_store *refs)
{
        struct snapshot *snapshot = xcalloc(1, sizeof(*snapshot));
        int sorted = 0;

        snapshot->refs = refs;
        acquire_snapshot(snapshot);
        snapshot->peeled = PEELED_NONE;

        if (!load_contents(snapshot))
                return snapshot;

        /* If the file has a header line, process it: */
        if (snapshot->buf < snapshot->eof && *snapshot->buf == '#') {
                char *tmp, *p, *eol;
                struct string_list traits = STRING_LIST_INIT_NODUP;

                eol = memchr(snapshot->buf, '\n',
                             snapshot->eof - snapshot->buf);
                if (!eol)
                        die_unterminated_line(refs->path,
                                              snapshot->buf,
                                              snapshot->eof - snapshot->buf);

                tmp = xmemdupz(snapshot->buf, eol - snapshot->buf);

                if (!skip_prefix(tmp, "# pack-refs with:", (const char **)&p))
                        die_invalid_line(refs->path,
                                         snapshot->buf,
                                         snapshot->eof - snapshot->buf);

                string_list_split_in_place(&traits, p, ' ', -1);

                if (unsorted_string_list_has_string(&traits, "fully-peeled"))
                        snapshot->peeled = PEELED_FULLY;
                else if (unsorted_string_list_has_string(&traits, "peeled"))
                        snapshot->peeled = PEELED_TAGS;

                sorted = unsorted_string_list_has_string(&traits, "sorted");

                /* perhaps other traits later as well */

                /* The "+ 1" is for the LF character. */
                snapshot->start = eol + 1;

                string_list_clear(&traits, 0);
                free(tmp);
        }

        verify_buffer_safe(snapshot);

        if (!sorted) {
                sort_snapshot(snapshot);

                /*
                 * Reordering the records might have moved a short one
                 * to the end of the buffer, so verify the buffer's
                 * safety again:
                 */
                verify_buffer_safe(snapshot);
        }

        if (mmap_strategy != MMAP_OK && snapshot->mmapped) {
                /*
                 * We don't want to leave the file mmapped, so we are
                 * forced to make a copy now:
                 */
                size_t size = snapshot->eof - snapshot->start;
                char *buf_copy = xmalloc(size);

                memcpy(buf_copy, snapshot->start, size);
                clear_snapshot_buffer(snapshot);
                snapshot->buf = snapshot->start = buf_copy;
                snapshot->eof = buf_copy + size;
        }

        return snapshot;
}

/*
 * Check that `refs->snapshot` (if present) still reflects the
 * contents of the `packed-refs` file. If not, clear the snapshot.
 */
static void validate_snapshot(struct packed_ref_store *refs)
{
        if (refs->snapshot &&
            !stat_validity_check(&refs->snapshot->validity, refs->path))
                clear_snapshot(refs);
}

/*
 * Get the `snapshot` for the specified packed_ref_store, creating and
 * populating it if it hasn't been read before or if the file has been
 * changed (according to its `validity` field) since it was last read.
 * On the other hand, if we hold the lock, then assume that the file
 * hasn't been changed out from under us, so skip the extra `stat()`
 * call in `stat_validity_check()`. This function does *not* increase
 * the snapshot's reference count on behalf of the caller.
 */
static struct snapshot *get_snapshot(struct packed_ref_store *refs)
{
        if (!is_lock_file_locked(&refs->lock))
                validate_snapshot(refs);

        if (!refs->snapshot)
                refs->snapshot = create_snapshot(refs);

        return refs->snapshot;
}

static int packed_read_raw_ref(struct ref_store *ref_store,
                               const char *refname, struct object_id *oid,
                               struct strbuf *referent, unsigned int *type)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
        struct snapshot *snapshot = get_snapshot(refs);
        const char *rec;

        *type = 0;

        rec = find_reference_location(snapshot, refname, 1);

        if (!rec) {
                /* refname is not a packed reference. */
                errno = ENOENT;
                return -1;
        }

        if (get_oid_hex(rec, oid))
                die_invalid_line(refs->path, rec, snapshot->eof - rec);

        *type = REF_ISPACKED;
        return 0;
}

/*
 * This value is set in `base.flags` if the peeled value of the
 * current reference is known. In that case, `peeled` contains the
 * correct peeled value for the reference, which might be `null_oid`
 * if the reference is not a tag or if it is broken.
 */
#define REF_KNOWS_PEELED 0x40

/*
 * An iterator over a snapshot of a `packed-refs` file.
 */
struct packed_ref_iterator {
        struct ref_iterator base;

        struct snapshot *snapshot;

        /* The current position in the snapshot's buffer: */
        const char *pos;

        /* The end of the part of the buffer that will be iterated over: */
        const char *eof;

        /* Scratch space for current values: */
        struct object_id oid, peeled;
        struct strbuf refname_buf;

        unsigned int flags;
};

/*
 * Move the iterator to the next record in the snapshot, without
 * respect for whether the record is actually required by the current
 * iteration. Adjust the fields in `iter` and return `ITER_OK` or
 * `ITER_DONE`. This function does not free the iterator in the case
 * of `ITER_DONE`.
 */
static int next_record(struct packed_ref_iterator *iter)
{
        const char *p = iter->pos, *eol;

        strbuf_reset(&iter->refname_buf);

        if (iter->pos == iter->eof)
                return ITER_DONE;

        iter->base.flags = REF_ISPACKED;

        if (iter->eof - p < the_hash_algo->hexsz + 2 ||
            parse_oid_hex(p, &iter->oid, &p) ||
            !isspace(*p++))
                die_invalid_line(iter->snapshot->refs->path,
                                 iter->pos, iter->eof - iter->pos);

        eol = memchr(p, '\n', iter->eof - p);
        if (!eol)
                die_unterminated_line(iter->snapshot->refs->path,
                                      iter->pos, iter->eof - iter->pos);

        strbuf_add(&iter->refname_buf, p, eol - p);
        iter->base.refname = iter->refname_buf.buf;

        if (check_refname_format(iter->base.refname, REFNAME_ALLOW_ONELEVEL)) {
                if (!refname_is_safe(iter->base.refname))
                        die("packed refname is dangerous: %s",
                            iter->base.refname);
                oidclr(&iter->oid);
                iter->base.flags |= REF_BAD_NAME | REF_ISBROKEN;
        }
        if (iter->snapshot->peeled == PEELED_FULLY ||
            (iter->snapshot->peeled == PEELED_TAGS &&
             starts_with(iter->base.refname, "refs/tags/")))
                iter->base.flags |= REF_KNOWS_PEELED;

        iter->pos = eol + 1;

        if (iter->pos < iter->eof && *iter->pos == '^') {
                p = iter->pos + 1;
                if (iter->eof - p < the_hash_algo->hexsz + 1 ||
                    parse_oid_hex(p, &iter->peeled, &p) ||
                    *p++ != '\n')
                        die_invalid_line(iter->snapshot->refs->path,
                                         iter->pos, iter->eof - iter->pos);
                iter->pos = p;

                /*
                 * Regardless of what the file header said, we
                 * definitely know the value of *this* reference. But
                 * we suppress it if the reference is broken:
                 */
                if ((iter->base.flags & REF_ISBROKEN)) {
                        oidclr(&iter->peeled);
                        iter->base.flags &= ~REF_KNOWS_PEELED;
                } else {
                        iter->base.flags |= REF_KNOWS_PEELED;
                }
        } else {
                oidclr(&iter->peeled);
        }

        return ITER_OK;
}

static int packed_ref_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct packed_ref_iterator *iter =
                (struct packed_ref_iterator *)ref_iterator;
        int ok;

        while ((ok = next_record(iter)) == ITER_OK) {
                if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
                    ref_type(iter->base.refname) != REF_TYPE_PER_WORKTREE)
                        continue;

                if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
                    !ref_resolves_to_object(iter->base.refname, &iter->oid,
                                            iter->flags))
                        continue;

                return ITER_OK;
        }

        if (ref_iterator_abort(ref_iterator) != ITER_DONE)
                ok = ITER_ERROR;

        return ok;
}

static int packed_ref_iterator_peel(struct ref_iterator *ref_iterator,
                                   struct object_id *peeled)
{
        struct packed_ref_iterator *iter =
                (struct packed_ref_iterator *)ref_iterator;

        if ((iter->base.flags & REF_KNOWS_PEELED)) {
                oidcpy(peeled, &iter->peeled);
                return is_null_oid(&iter->peeled) ? -1 : 0;
        } else if ((iter->base.flags & (REF_ISBROKEN | REF_ISSYMREF))) {
                return -1;
        } else {
                return !!peel_object(&iter->oid, peeled);
        }
}

static int packed_ref_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct packed_ref_iterator *iter =
                (struct packed_ref_iterator *)ref_iterator;
        int ok = ITER_DONE;

        strbuf_release(&iter->refname_buf);
        release_snapshot(iter->snapshot);
        base_ref_iterator_free(ref_iterator);
        return ok;
}

static struct ref_iterator_vtable packed_ref_iterator_vtable = {
        packed_ref_iterator_advance,
        packed_ref_iterator_peel,
        packed_ref_iterator_abort
};

static struct ref_iterator *packed_ref_iterator_begin(
                struct ref_store *ref_store,
                const char *prefix, unsigned int flags)
{
        struct packed_ref_store *refs;
        struct snapshot *snapshot;
        const char *start;
        struct packed_ref_iterator *iter;
        struct ref_iterator *ref_iterator;
        unsigned int required_flags = REF_STORE_READ;

        if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
                required_flags |= REF_STORE_ODB;
        refs = packed_downcast(ref_store, required_flags, "ref_iterator_begin");

        /*
         * Note that `get_snapshot()` internally checks whether the
         * snapshot is up to date with what is on disk, and re-reads
         * it if not.
         */
        snapshot = get_snapshot(refs);

        if (prefix && *prefix)
                start = find_reference_location(snapshot, prefix, 0);
        else
                start = snapshot->start;

        if (start == snapshot->eof)
                return empty_ref_iterator_begin();

        iter = xcalloc(1, sizeof(*iter));
        ref_iterator = &iter->base;
        base_ref_iterator_init(ref_iterator, &packed_ref_iterator_vtable, 1);

        iter->snapshot = snapshot;
        acquire_snapshot(snapshot);

        iter->pos = start;
        iter->eof = snapshot->eof;
        strbuf_init(&iter->refname_buf, 0);

        iter->base.oid = &iter->oid;

        iter->flags = flags;

        if (prefix && *prefix)
                /* Stop iteration after we've gone *past* prefix: */
                ref_iterator = prefix_ref_iterator_begin(ref_iterator, prefix, 0);

        return ref_iterator;
}

/*
 * 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. On
 * error, return a nonzero value and leave errno set at the value left
 * by the failing call to `fprintf()`.
 */
static int write_packed_entry(FILE *fh, const char *refname,
                              const struct object_id *oid,
                              const struct object_id *peeled)
{
        if (fprintf(fh, "%s %s\n", oid_to_hex(oid), refname) < 0 ||
            (peeled && fprintf(fh, "^%s\n", oid_to_hex(peeled)) < 0))
                return -1;

        return 0;
}

int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_WRITE | REF_STORE_MAIN,
                                "packed_refs_lock");
        static int timeout_configured = 0;
        static int timeout_value = 1000;

        if (!timeout_configured) {
                git_config_get_int("core.packedrefstimeout", &timeout_value);
                timeout_configured = 1;
        }

        /*
         * Note that we close the lockfile immediately because we
         * don't write new content to it, but rather to a separate
         * tempfile.
         */
        if (hold_lock_file_for_update_timeout(
                            &refs->lock,
                            refs->path,
                            flags, timeout_value) < 0) {
                unable_to_lock_message(refs->path, errno, err);
                return -1;
        }

        if (close_lock_file_gently(&refs->lock)) {
                strbuf_addf(err, "unable to close %s: %s", refs->path, strerror(errno));
                rollback_lock_file(&refs->lock);
                return -1;
        }

        /*
         * There is a stat-validity problem might cause `update-ref -d`
         * lost the newly commit of a ref, because a new `packed-refs`
         * file might has the same on-disk file attributes such as
         * timestamp, file size and inode value, but has a changed
         * ref value.
         *
         * This could happen with a very small chance when
         * `update-ref -d` is called and at the same time another
         * `pack-refs --all` process is running.
         *
         * Now that we hold the `packed-refs` lock, it is important
         * to make sure we could read the latest version of
         * `packed-refs` file no matter we have just mmap it or not.
         * So what need to do is clear the snapshot if we hold it
         * already.
         */
        clear_snapshot(refs);

        /*
         * Now make sure that the packed-refs file as it exists in the
         * locked state is loaded into the snapshot:
         */
        get_snapshot(refs);
        return 0;
}

void packed_refs_unlock(struct ref_store *ref_store)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE,
                        "packed_refs_unlock");

        if (!is_lock_file_locked(&refs->lock))
                BUG("packed_refs_unlock() called when not locked");
        rollback_lock_file(&refs->lock);
}

int packed_refs_is_locked(struct ref_store *ref_store)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE,
                        "packed_refs_is_locked");

        return is_lock_file_locked(&refs->lock);
}

/*
 * The packed-refs header line that we write out. Perhaps other traits
 * will be added later.
 *
 * Note that earlier versions of Git used to parse these traits by
 * looking for " trait " in the line. For this reason, the space after
 * the colon and the trailing space are required.
 */
static const char PACKED_REFS_HEADER[] =
        "# pack-refs with: peeled fully-peeled sorted \n";

static int packed_init_db(struct ref_store *ref_store, struct strbuf *err)
{
        /* Nothing to do. */
        return 0;
}

/*
 * Write the packed refs from the current snapshot to the packed-refs
 * tempfile, incorporating any changes from `updates`. `updates` must
 * be a sorted string list whose keys are the refnames and whose util
 * values are `struct ref_update *`. On error, rollback the tempfile,
 * write an error message to `err`, and return a nonzero value.
 *
 * The packfile must be locked before calling this function and will
 * remain locked when it is done.
 */
static int write_with_updates(struct packed_ref_store *refs,
                              struct string_list *updates,
                              struct strbuf *err)
{
        struct ref_iterator *iter = NULL;
        size_t i;
        int ok;
        FILE *out;
        struct strbuf sb = STRBUF_INIT;
        char *packed_refs_path;

        if (!is_lock_file_locked(&refs->lock))
                BUG("write_with_updates() called while unlocked");

        /*
         * If packed-refs is a symlink, we want to overwrite the
         * symlinked-to file, not the symlink itself. Also, put the
         * staging file next to it:
         */
        packed_refs_path = get_locked_file_path(&refs->lock);
        strbuf_addf(&sb, "%s.new", packed_refs_path);
        free(packed_refs_path);
        refs->tempfile = create_tempfile(sb.buf);
        if (!refs->tempfile) {
                strbuf_addf(err, "unable to create file %s: %s",
                            sb.buf, strerror(errno));
                strbuf_release(&sb);
                return -1;
        }
        strbuf_release(&sb);

        out = fdopen_tempfile(refs->tempfile, "w");
        if (!out) {
                strbuf_addf(err, "unable to fdopen packed-refs tempfile: %s",
                            strerror(errno));
                goto error;
        }

        if (fprintf(out, "%s", PACKED_REFS_HEADER) < 0)
                goto write_error;

        /*
         * We iterate in parallel through the current list of refs and
         * the list of updates, processing an entry from at least one
         * of the lists each time through the loop. When the current
         * list of refs is exhausted, set iter to NULL. When the list
         * of updates is exhausted, leave i set to updates->nr.
         */
        iter = packed_ref_iterator_begin(&refs->base, "",
                                         DO_FOR_EACH_INCLUDE_BROKEN);
        if ((ok = ref_iterator_advance(iter)) != ITER_OK)
                iter = NULL;

        i = 0;

        while (iter || i < updates->nr) {
                struct ref_update *update = NULL;
                int cmp;

                if (i >= updates->nr) {
                        cmp = -1;
                } else {
                        update = updates->items[i].util;

                        if (!iter)
                                cmp = +1;
                        else
                                cmp = strcmp(iter->refname, update->refname);
                }

                if (!cmp) {
                        /*
                         * There is both an old value and an update
                         * for this reference. Check the old value if
                         * necessary:
                         */
                        if ((update->flags & REF_HAVE_OLD)) {
                                if (is_null_oid(&update->old_oid)) {
                                        strbuf_addf(err, "cannot update ref '%s': "
                                                    "reference already exists",
                                                    update->refname);
                                        goto error;
                                } else if (!oideq(&update->old_oid, iter->oid)) {
                                        strbuf_addf(err, "cannot update ref '%s': "
                                                    "is at %s but expected %s",
                                                    update->refname,
                                                    oid_to_hex(iter->oid),
                                                    oid_to_hex(&update->old_oid));
                                        goto error;
                                }
                        }

                        /* Now figure out what to use for the new value: */
                        if ((update->flags & REF_HAVE_NEW)) {
                                /*
                                 * The update takes precedence. Skip
                                 * the iterator over the unneeded
                                 * value.
                                 */
                                if ((ok = ref_iterator_advance(iter)) != ITER_OK)
                                        iter = NULL;
                                cmp = +1;
                        } else {
                                /*
                                 * The update doesn't actually want to
                                 * change anything. We're done with it.
                                 */
                                i++;
                                cmp = -1;
                        }
                } else if (cmp > 0) {
                        /*
                         * There is no old value but there is an
                         * update for this reference. Make sure that
                         * the update didn't expect an existing value:
                         */
                        if ((update->flags & REF_HAVE_OLD) &&
                            !is_null_oid(&update->old_oid)) {
                                strbuf_addf(err, "cannot update ref '%s': "
                                            "reference is missing but expected %s",
                                            update->refname,
                                            oid_to_hex(&update->old_oid));
                                goto error;
                        }
                }

                if (cmp < 0) {
                        /* Pass the old reference through. */

                        struct object_id peeled;
                        int peel_error = ref_iterator_peel(iter, &peeled);

                        if (write_packed_entry(out, iter->refname,
                                               iter->oid,
                                               peel_error ? NULL : &peeled))
                                goto write_error;

                        if ((ok = ref_iterator_advance(iter)) != ITER_OK)
                                iter = NULL;
                } else if (is_null_oid(&update->new_oid)) {
                        /*
                         * The update wants to delete the reference,
                         * and the reference either didn't exist or we
                         * have already skipped it. So we're done with
                         * the update (and don't have to write
                         * anything).
                         */
                        i++;
                } else {
                        struct object_id peeled;
                        int peel_error = peel_object(&update->new_oid,
                                                     &peeled);

                        if (write_packed_entry(out, update->refname,
                                               &update->new_oid,
                                               peel_error ? NULL : &peeled))
                                goto write_error;

                        i++;
                }
        }

        if (ok != ITER_DONE) {
                strbuf_addstr(err, "unable to write packed-refs file: "
                              "error iterating over old contents");
                goto error;
        }

        if (close_tempfile_gently(refs->tempfile)) {
                strbuf_addf(err, "error closing file %s: %s",
                            get_tempfile_path(refs->tempfile),
                            strerror(errno));
                strbuf_release(&sb);
                delete_tempfile(&refs->tempfile);
                return -1;
        }

        return 0;

write_error:
        strbuf_addf(err, "error writing to %s: %s",
                    get_tempfile_path(refs->tempfile), strerror(errno));

error:
        if (iter)
                ref_iterator_abort(iter);

        delete_tempfile(&refs->tempfile);
        return -1;
}

int is_packed_transaction_needed(struct ref_store *ref_store,
                                 struct ref_transaction *transaction)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ,
                        "is_packed_transaction_needed");
        struct strbuf referent = STRBUF_INIT;
        size_t i;
        int ret;

        if (!is_lock_file_locked(&refs->lock))
                BUG("is_packed_transaction_needed() called while unlocked");

        /*
         * We're only going to bother returning false for the common,
         * trivial case that references are only being deleted, their
         * old values are not being checked, and the old `packed-refs`
         * file doesn't contain any of those reference(s). This gives
         * false positives for some other cases that could
         * theoretically be optimized away:
         *
         * 1. It could be that the old value is being verified without
         *    setting a new value. In this case, we could verify the
         *    old value here and skip the update if it agrees. If it
         *    disagrees, we could either let the update go through
         *    (the actual commit would re-detect and report the
         *    problem), or come up with a way of reporting such an
         *    error to *our* caller.
         *
         * 2. It could be that a new value is being set, but that it
         *    is identical to the current packed value of the
         *    reference.
         *
         * Neither of these cases will come up in the current code,
         * because the only caller of this function passes to it a
         * transaction that only includes `delete` updates with no
         * `old_id`. Even if that ever changes, false positives only
         * cause an optimization to be missed; they do not affect
         * correctness.
         */

        /*
         * Start with the cheap checks that don't require old
         * reference values to be read:
         */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];

                if (update->flags & REF_HAVE_OLD)
                        /* Have to check the old value -> needed. */
                        return 1;

                if ((update->flags & REF_HAVE_NEW) && !is_null_oid(&update->new_oid))
                        /* Have to set a new value -> needed. */
                        return 1;
        }

        /*
         * The transaction isn't checking any old values nor is it
         * setting any nonzero new values, so it still might be able
         * to be skipped. Now do the more expensive check: the update
         * is needed if any of the updates is a delete, and the old
         * `packed-refs` file contains a value for that reference.
         */
        ret = 0;
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                unsigned int type;
                struct object_id oid;

                if (!(update->flags & REF_HAVE_NEW))
                        /*
                         * This reference isn't being deleted -> not
                         * needed.
                         */
                        continue;

                if (!refs_read_raw_ref(ref_store, update->refname,
                                       &oid, &referent, &type) ||
                    errno != ENOENT) {
                        /*
                         * We have to actually delete that reference
                         * -> this transaction is needed.
                         */
                        ret = 1;
                        break;
                }
        }

        strbuf_release(&referent);
        return ret;
}

struct packed_transaction_backend_data {
        /* True iff the transaction owns the packed-refs lock. */
        int own_lock;

        struct string_list updates;
};

static void packed_transaction_cleanup(struct packed_ref_store *refs,
                                       struct ref_transaction *transaction)
{
        struct packed_transaction_backend_data *data = transaction->backend_data;

        if (data) {
                string_list_clear(&data->updates, 0);

                if (is_tempfile_active(refs->tempfile))
                        delete_tempfile(&refs->tempfile);

                if (data->own_lock && is_lock_file_locked(&refs->lock)) {
                        packed_refs_unlock(&refs->base);
                        data->own_lock = 0;
                }

                free(data);
                transaction->backend_data = NULL;
        }

        transaction->state = REF_TRANSACTION_CLOSED;
}

static int packed_transaction_prepare(struct ref_store *ref_store,
                                      struct ref_transaction *transaction,
                                      struct strbuf *err)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
                        "ref_transaction_prepare");
        struct packed_transaction_backend_data *data;
        size_t i;
        int ret = TRANSACTION_GENERIC_ERROR;

        /*
         * Note that we *don't* skip transactions with zero updates,
         * because such a transaction might be executed for the side
         * effect of ensuring that all of the references are peeled or
         * ensuring that the `packed-refs` file is sorted. If the
         * caller wants to optimize away empty transactions, it should
         * do so itself.
         */

        data = xcalloc(1, sizeof(*data));
        string_list_init(&data->updates, 0);

        transaction->backend_data = data;

        /*
         * Stick the updates in a string list by refname so that we
         * can sort them:
         */
        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *update = transaction->updates[i];
                struct string_list_item *item =
                        string_list_append(&data->updates, update->refname);

                /* Store a pointer to update in item->util: */
                item->util = update;
        }
        string_list_sort(&data->updates);

        if (ref_update_reject_duplicates(&data->updates, err))
                goto failure;

        if (!is_lock_file_locked(&refs->lock)) {
                if (packed_refs_lock(ref_store, 0, err))
                        goto failure;
                data->own_lock = 1;
        }

        if (write_with_updates(refs, &data->updates, err))
                goto failure;

        transaction->state = REF_TRANSACTION_PREPARED;
        return 0;

failure:
        packed_transaction_cleanup(refs, transaction);
        return ret;
}

static int packed_transaction_abort(struct ref_store *ref_store,
                                    struct ref_transaction *transaction,
                                    struct strbuf *err)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
                        "ref_transaction_abort");

        packed_transaction_cleanup(refs, transaction);
        return 0;
}

static int packed_transaction_finish(struct ref_store *ref_store,
                                     struct ref_transaction *transaction,
                                     struct strbuf *err)
{
        struct packed_ref_store *refs = packed_downcast(
                        ref_store,
                        REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
                        "ref_transaction_finish");
        int ret = TRANSACTION_GENERIC_ERROR;
        char *packed_refs_path;

        clear_snapshot(refs);

        packed_refs_path = get_locked_file_path(&refs->lock);
        if (rename_tempfile(&refs->tempfile, packed_refs_path)) {
                strbuf_addf(err, "error replacing %s: %s",
                            refs->path, strerror(errno));
                goto cleanup;
        }

        ret = 0;

cleanup:
        free(packed_refs_path);
        packed_transaction_cleanup(refs, transaction);
        return ret;
}

static int packed_initial_transaction_commit(struct ref_store *ref_store,
                                            struct ref_transaction *transaction,
                                            struct strbuf *err)
{
        return ref_transaction_commit(transaction, err);
}

static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
                             struct string_list *refnames, unsigned int flags)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
        struct strbuf err = STRBUF_INIT;
        struct ref_transaction *transaction;
        struct string_list_item *item;
        int ret;

        (void)refs; /* We need the check above, but don't use the variable */

        if (!refnames->nr)
                return 0;

        /*
         * Since we don't check the references' old_oids, the
         * individual updates can't fail, so we can pack all of the
         * updates into a single transaction.
         */

        transaction = ref_store_transaction_begin(ref_store, &err);
        if (!transaction)
                return -1;

        for_each_string_list_item(item, refnames) {
                if (ref_transaction_delete(transaction, item->string, NULL,
                                           flags, msg, &err)) {
                        warning(_("could not delete reference %s: %s"),
                                item->string, err.buf);
                        strbuf_reset(&err);
                }
        }

        ret = ref_transaction_commit(transaction, &err);

        if (ret) {
                if (refnames->nr == 1)
                        error(_("could not delete reference %s: %s"),
                              refnames->items[0].string, err.buf);
                else
                        error(_("could not delete references: %s"), err.buf);
        }

        ref_transaction_free(transaction);
        strbuf_release(&err);
        return ret;
}

static int packed_pack_refs(struct ref_store *ref_store, unsigned int flags)
{
        /*
         * Packed refs are already packed. It might be that loose refs
         * are packed *into* a packed refs store, but that is done by
         * updating the packed references via a transaction.
         */
        return 0;
}

static int packed_create_symref(struct ref_store *ref_store,
                               const char *refname, const char *target,
                               const char *logmsg)
{
        BUG("packed reference store does not support symrefs");
}

static int packed_rename_ref(struct ref_store *ref_store,
                            const char *oldrefname, const char *newrefname,
                            const char *logmsg)
{
        BUG("packed reference store does not support renaming references");
}

static int packed_copy_ref(struct ref_store *ref_store,
                           const char *oldrefname, const char *newrefname,
                           const char *logmsg)
{
        BUG("packed reference store does not support copying references");
}

static struct ref_iterator *packed_reflog_iterator_begin(struct ref_store *ref_store)
{
        return empty_ref_iterator_begin();
}

static int packed_for_each_reflog_ent(struct ref_store *ref_store,
                                      const char *refname,
                                      each_reflog_ent_fn fn, void *cb_data)
{
        return 0;
}

static int packed_for_each_reflog_ent_reverse(struct ref_store *ref_store,
                                              const char *refname,
                                              each_reflog_ent_fn fn,
                                              void *cb_data)
{
        return 0;
}

static int packed_reflog_exists(struct ref_store *ref_store,
                               const char *refname)
{
        return 0;
}

static int packed_create_reflog(struct ref_store *ref_store,
                               const char *refname, int force_create,
                               struct strbuf *err)
{
        BUG("packed reference store does not support reflogs");
}

static int packed_delete_reflog(struct ref_store *ref_store,
                               const char *refname)
{
        return 0;
}

static int packed_reflog_expire(struct ref_store *ref_store,
                                const char *refname, const struct object_id *oid,
                                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)
{
        return 0;
}

struct ref_storage_be refs_be_packed = {
        NULL,
        "packed",
        packed_ref_store_create,
        packed_init_db,
        packed_transaction_prepare,
        packed_transaction_finish,
        packed_transaction_abort,
        packed_initial_transaction_commit,

        packed_pack_refs,
        packed_create_symref,
        packed_delete_refs,
        packed_rename_ref,
        packed_copy_ref,

        packed_ref_iterator_begin,
        packed_read_raw_ref,

        packed_reflog_iterator_begin,
        packed_for_each_reflog_ent,
        packed_for_each_reflog_ent_reverse,
        packed_reflog_exists,
        packed_create_reflog,
        packed_delete_reflog,
        packed_reflog_expire
};