load_subtree(): combine some common code

[git] / notes.c

#include "cache.h"
#include "config.h"
#include "notes.h"
#include "blob.h"
#include "tree.h"
#include "utf8.h"
#include "strbuf.h"
#include "tree-walk.h"
#include "string-list.h"
#include "refs.h"

/*
 * Use a non-balancing simple 16-tree structure with struct int_node as
 * internal nodes, and struct leaf_node as leaf nodes. Each int_node has a
 * 16-array of pointers to its children.
 * The bottom 2 bits of each pointer is used to identify the pointer type
 * - ptr & 3 == 0 - NULL pointer, assert(ptr == NULL)
 * - ptr & 3 == 1 - pointer to next internal node - cast to struct int_node *
 * - ptr & 3 == 2 - pointer to note entry - cast to struct leaf_node *
 * - ptr & 3 == 3 - pointer to subtree entry - cast to struct leaf_node *
 *
 * The root node is a statically allocated struct int_node.
 */
struct int_node {
        void *a[16];
};

/*
 * Leaf nodes come in two variants, note entries and subtree entries,
 * distinguished by the LSb of the leaf node pointer (see above).
 * As a note entry, the key is the SHA1 of the referenced object, and the
 * value is the SHA1 of the note object.
 * As a subtree entry, the key is the prefix SHA1 (w/trailing NULs) of the
 * referenced object, using the last byte of the key to store the length of
 * the prefix. The value is the SHA1 of the tree object containing the notes
 * subtree.
 */
struct leaf_node {
        struct object_id key_oid;
        struct object_id val_oid;
};

/*
 * A notes tree may contain entries that are not notes, and that do not follow
 * the naming conventions of notes. There are typically none/few of these, but
 * we still need to keep track of them. Keep a simple linked list sorted alpha-
 * betically on the non-note path. The list is populated when parsing tree
 * objects in load_subtree(), and the non-notes are correctly written back into
 * the tree objects produced by write_notes_tree().
 */
struct non_note {
        struct non_note *next; /* grounded (last->next == NULL) */
        char *path;
        unsigned int mode;
        struct object_id oid;
};

#define PTR_TYPE_NULL     0
#define PTR_TYPE_INTERNAL 1
#define PTR_TYPE_NOTE     2
#define PTR_TYPE_SUBTREE  3

#define GET_PTR_TYPE(ptr)       ((uintptr_t) (ptr) & 3)
#define CLR_PTR_TYPE(ptr)       ((void *) ((uintptr_t) (ptr) & ~3))
#define SET_PTR_TYPE(ptr, type) ((void *) ((uintptr_t) (ptr) | (type)))

#define GET_NIBBLE(n, sha1) ((((sha1)[(n) >> 1]) >> ((~(n) & 0x01) << 2)) & 0x0f)

#define KEY_INDEX (GIT_SHA1_RAWSZ - 1)
#define FANOUT_PATH_SEPARATORS ((GIT_SHA1_HEXSZ / 2) - 1)
#define SUBTREE_SHA1_PREFIXCMP(key_sha1, subtree_sha1) \
        (memcmp(key_sha1, subtree_sha1, subtree_sha1[KEY_INDEX]))

struct notes_tree default_notes_tree;

static struct string_list display_notes_refs = STRING_LIST_INIT_NODUP;
static struct notes_tree **display_notes_trees;

static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
                struct int_node *node, unsigned int n);

/*
 * Search the tree until the appropriate location for the given key is found:
 * 1. Start at the root node, with n = 0
 * 2. If a[0] at the current level is a matching subtree entry, unpack that
 *    subtree entry and remove it; restart search at the current level.
 * 3. Use the nth nibble of the key as an index into a:
 *    - If a[n] is an int_node, recurse from #2 into that node and increment n
 *    - If a matching subtree entry, unpack that subtree entry (and remove it);
 *      restart search at the current level.
 *    - Otherwise, we have found one of the following:
 *      - a subtree entry which does not match the key
 *      - a note entry which may or may not match the key
 *      - an unused leaf node (NULL)
 *      In any case, set *tree and *n, and return pointer to the tree location.
 */
static void **note_tree_search(struct notes_tree *t, struct int_node **tree,
                unsigned char *n, const unsigned char *key_sha1)
{
        struct leaf_node *l;
        unsigned char i;
        void *p = (*tree)->a[0];

        if (GET_PTR_TYPE(p) == PTR_TYPE_SUBTREE) {
                l = (struct leaf_node *) CLR_PTR_TYPE(p);
                if (!SUBTREE_SHA1_PREFIXCMP(key_sha1, l->key_oid.hash)) {
                        /* unpack tree and resume search */
                        (*tree)->a[0] = NULL;
                        load_subtree(t, l, *tree, *n);
                        free(l);
                        return note_tree_search(t, tree, n, key_sha1);
                }
        }

        i = GET_NIBBLE(*n, key_sha1);
        p = (*tree)->a[i];
        switch (GET_PTR_TYPE(p)) {
        case PTR_TYPE_INTERNAL:
                *tree = CLR_PTR_TYPE(p);
                (*n)++;
                return note_tree_search(t, tree, n, key_sha1);
        case PTR_TYPE_SUBTREE:
                l = (struct leaf_node *) CLR_PTR_TYPE(p);
                if (!SUBTREE_SHA1_PREFIXCMP(key_sha1, l->key_oid.hash)) {
                        /* unpack tree and resume search */
                        (*tree)->a[i] = NULL;
                        load_subtree(t, l, *tree, *n);
                        free(l);
                        return note_tree_search(t, tree, n, key_sha1);
                }
                /* fall through */
        default:
                return &((*tree)->a[i]);
        }
}

/*
 * To find a leaf_node:
 * Search to the tree location appropriate for the given key:
 * If a note entry with matching key, return the note entry, else return NULL.
 */
static struct leaf_node *note_tree_find(struct notes_tree *t,
                struct int_node *tree, unsigned char n,
                const unsigned char *key_sha1)
{
        void **p = note_tree_search(t, &tree, &n, key_sha1);
        if (GET_PTR_TYPE(*p) == PTR_TYPE_NOTE) {
                struct leaf_node *l = (struct leaf_node *) CLR_PTR_TYPE(*p);
                if (!hashcmp(key_sha1, l->key_oid.hash))
                        return l;
        }
        return NULL;
}

/*
 * How to consolidate an int_node:
 * If there are > 1 non-NULL entries, give up and return non-zero.
 * Otherwise replace the int_node at the given index in the given parent node
 * with the only NOTE entry (or a NULL entry if no entries) from the given
 * tree, and return 0.
 */
static int note_tree_consolidate(struct int_node *tree,
        struct int_node *parent, unsigned char index)
{
        unsigned int i;
        void *p = NULL;

        assert(tree && parent);
        assert(CLR_PTR_TYPE(parent->a[index]) == tree);

        for (i = 0; i < 16; i++) {
                if (GET_PTR_TYPE(tree->a[i]) != PTR_TYPE_NULL) {
                        if (p) /* more than one entry */
                                return -2;
                        p = tree->a[i];
                }
        }

        if (p && (GET_PTR_TYPE(p) != PTR_TYPE_NOTE))
                return -2;
        /* replace tree with p in parent[index] */
        parent->a[index] = p;
        free(tree);
        return 0;
}

/*
 * To remove a leaf_node:
 * Search to the tree location appropriate for the given leaf_node's key:
 * - If location does not hold a matching entry, abort and do nothing.
 * - Copy the matching entry's value into the given entry.
 * - Replace the matching leaf_node with a NULL entry (and free the leaf_node).
 * - Consolidate int_nodes repeatedly, while walking up the tree towards root.
 */
static void note_tree_remove(struct notes_tree *t,
                struct int_node *tree, unsigned char n,
                struct leaf_node *entry)
{
        struct leaf_node *l;
        struct int_node *parent_stack[GIT_SHA1_RAWSZ];
        unsigned char i, j;
        void **p = note_tree_search(t, &tree, &n, entry->key_oid.hash);

        assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
        if (GET_PTR_TYPE(*p) != PTR_TYPE_NOTE)
                return; /* type mismatch, nothing to remove */
        l = (struct leaf_node *) CLR_PTR_TYPE(*p);
        if (oidcmp(&l->key_oid, &entry->key_oid))
                return; /* key mismatch, nothing to remove */

        /* we have found a matching entry */
        oidcpy(&entry->val_oid, &l->val_oid);
        free(l);
        *p = SET_PTR_TYPE(NULL, PTR_TYPE_NULL);

        /* consolidate this tree level, and parent levels, if possible */
        if (!n)
                return; /* cannot consolidate top level */
        /* first, build stack of ancestors between root and current node */
        parent_stack[0] = t->root;
        for (i = 0; i < n; i++) {
                j = GET_NIBBLE(i, entry->key_oid.hash);
                parent_stack[i + 1] = CLR_PTR_TYPE(parent_stack[i]->a[j]);
        }
        assert(i == n && parent_stack[i] == tree);
        /* next, unwind stack until note_tree_consolidate() is done */
        while (i > 0 &&
               !note_tree_consolidate(parent_stack[i], parent_stack[i - 1],
                                      GET_NIBBLE(i - 1, entry->key_oid.hash)))
                i--;
}

/*
 * To insert a leaf_node:
 * Search to the tree location appropriate for the given leaf_node's key:
 * - If location is unused (NULL), store the tweaked pointer directly there
 * - If location holds a note entry that matches the note-to-be-inserted, then
 *   combine the two notes (by calling the given combine_notes function).
 * - If location holds a note entry that matches the subtree-to-be-inserted,
 *   then unpack the subtree-to-be-inserted into the location.
 * - If location holds a matching subtree entry, unpack the subtree at that
 *   location, and restart the insert operation from that level.
 * - Else, create a new int_node, holding both the node-at-location and the
 *   node-to-be-inserted, and store the new int_node into the location.
 */
static int note_tree_insert(struct notes_tree *t, struct int_node *tree,
                unsigned char n, struct leaf_node *entry, unsigned char type,
                combine_notes_fn combine_notes)
{
        struct int_node *new_node;
        struct leaf_node *l;
        void **p = note_tree_search(t, &tree, &n, entry->key_oid.hash);
        int ret = 0;

        assert(GET_PTR_TYPE(entry) == 0); /* no type bits set */
        l = (struct leaf_node *) CLR_PTR_TYPE(*p);
        switch (GET_PTR_TYPE(*p)) {
        case PTR_TYPE_NULL:
                assert(!*p);
                if (is_null_oid(&entry->val_oid))
                        free(entry);
                else
                        *p = SET_PTR_TYPE(entry, type);
                return 0;
        case PTR_TYPE_NOTE:
                switch (type) {
                case PTR_TYPE_NOTE:
                        if (!oidcmp(&l->key_oid, &entry->key_oid)) {
                                /* skip concatenation if l == entry */
                                if (!oidcmp(&l->val_oid, &entry->val_oid))
                                        return 0;

                                ret = combine_notes(l->val_oid.hash,
                                                    entry->val_oid.hash);
                                if (!ret && is_null_oid(&l->val_oid))
                                        note_tree_remove(t, tree, n, entry);
                                free(entry);
                                return ret;
                        }
                        break;
                case PTR_TYPE_SUBTREE:
                        if (!SUBTREE_SHA1_PREFIXCMP(l->key_oid.hash,
                                                    entry->key_oid.hash)) {
                                /* unpack 'entry' */
                                load_subtree(t, entry, tree, n);
                                free(entry);
                                return 0;
                        }
                        break;
                }
                break;
        case PTR_TYPE_SUBTREE:
                if (!SUBTREE_SHA1_PREFIXCMP(entry->key_oid.hash, l->key_oid.hash)) {
                        /* unpack 'l' and restart insert */
                        *p = NULL;
                        load_subtree(t, l, tree, n);
                        free(l);
                        return note_tree_insert(t, tree, n, entry, type,
                                                combine_notes);
                }
                break;
        }

        /* non-matching leaf_node */
        assert(GET_PTR_TYPE(*p) == PTR_TYPE_NOTE ||
               GET_PTR_TYPE(*p) == PTR_TYPE_SUBTREE);
        if (is_null_oid(&entry->val_oid)) { /* skip insertion of empty note */
                free(entry);
                return 0;
        }
        new_node = (struct int_node *) xcalloc(1, sizeof(struct int_node));
        ret = note_tree_insert(t, new_node, n + 1, l, GET_PTR_TYPE(*p),
                               combine_notes);
        if (ret)
                return ret;
        *p = SET_PTR_TYPE(new_node, PTR_TYPE_INTERNAL);
        return note_tree_insert(t, new_node, n + 1, entry, type, combine_notes);
}

/* Free the entire notes data contained in the given tree */
static void note_tree_free(struct int_node *tree)
{
        unsigned int i;
        for (i = 0; i < 16; i++) {
                void *p = tree->a[i];
                switch (GET_PTR_TYPE(p)) {
                case PTR_TYPE_INTERNAL:
                        note_tree_free(CLR_PTR_TYPE(p));
                        /* fall through */
                case PTR_TYPE_NOTE:
                case PTR_TYPE_SUBTREE:
                        free(CLR_PTR_TYPE(p));
                }
        }
}

/*
 * Convert a partial SHA1 hex string to the corresponding partial SHA1 value.
 * - hex      - Partial SHA1 segment in ASCII hex format
 * - hex_len  - Length of above segment. Must be multiple of 2 between 0 and 40
 * - oid      - Partial SHA1 value is written here
 * - oid_len  - Max #bytes to store in sha1, Must be >= hex_len / 2, and < 20
 * Return 0 on success or -1 on error (invalid arguments or input not
 * in hex format). Pad oid with NULs up to oid_len.
 */
static int get_oid_hex_segment(const char *hex, unsigned int hex_len,
                unsigned char *oid, unsigned int oid_len)
{
        unsigned int i, len = hex_len >> 1;
        if (hex_len % 2 != 0 || len > oid_len)
                return -1;
        for (i = 0; i < len; i++) {
                unsigned int val = (hexval(hex[0]) << 4) | hexval(hex[1]);
                if (val & ~0xff)
                        return -1;
                *oid++ = val;
                hex += 2;
        }
        for (; i < oid_len; i++)
                *oid++ = 0;
        return 0;
}

static int non_note_cmp(const struct non_note *a, const struct non_note *b)
{
        return strcmp(a->path, b->path);
}

/* note: takes ownership of path string */
static void add_non_note(struct notes_tree *t, char *path,
                unsigned int mode, const unsigned char *sha1)
{
        struct non_note *p = t->prev_non_note, *n;
        n = (struct non_note *) xmalloc(sizeof(struct non_note));
        n->next = NULL;
        n->path = path;
        n->mode = mode;
        hashcpy(n->oid.hash, sha1);
        t->prev_non_note = n;

        if (!t->first_non_note) {
                t->first_non_note = n;
                return;
        }

        if (non_note_cmp(p, n) < 0)
                ; /* do nothing  */
        else if (non_note_cmp(t->first_non_note, n) <= 0)
                p = t->first_non_note;
        else {
                /* n sorts before t->first_non_note */
                n->next = t->first_non_note;
                t->first_non_note = n;
                return;
        }

        /* n sorts equal or after p */
        while (p->next && non_note_cmp(p->next, n) <= 0)
                p = p->next;

        if (non_note_cmp(p, n) == 0) { /* n ~= p; overwrite p with n */
                assert(strcmp(p->path, n->path) == 0);
                p->mode = n->mode;
                oidcpy(&p->oid, &n->oid);
                free(n);
                t->prev_non_note = p;
                return;
        }

        /* n sorts between p and p->next */
        n->next = p->next;
        p->next = n;
}

static void load_subtree(struct notes_tree *t, struct leaf_node *subtree,
                struct int_node *node, unsigned int n)
{
        struct object_id object_oid;
        unsigned int prefix_len;
        void *buf;
        struct tree_desc desc;
        struct name_entry entry;

        buf = fill_tree_descriptor(&desc, subtree->val_oid.hash);
        if (!buf)
                die("Could not read %s for notes-index",
                     oid_to_hex(&subtree->val_oid));

        prefix_len = subtree->key_oid.hash[KEY_INDEX];
        assert(prefix_len * 2 >= n);
        memcpy(object_oid.hash, subtree->key_oid.hash, prefix_len);
        while (tree_entry(&desc, &entry)) {
                unsigned char type;
                struct leaf_node *l;
                int path_len = strlen(entry.path);

                if (path_len == 2 * (GIT_SHA1_RAWSZ - prefix_len)) {
                        /* This is potentially the remainder of the SHA-1 */

                        if (!S_ISREG(entry.mode))
                                /* notes must be blobs */
                                goto handle_non_note;

                        if (get_oid_hex_segment(entry.path, path_len,
                                                object_oid.hash + prefix_len,
                                                GIT_SHA1_RAWSZ - prefix_len))
                                goto handle_non_note; /* entry.path is not a SHA1 */

                        type = PTR_TYPE_NOTE;
                } else if (path_len == 2) {
                        /* This is potentially an internal node */

                        if (!S_ISDIR(entry.mode))
                                /* internal nodes must be trees */
                                goto handle_non_note;

                        if (get_oid_hex_segment(entry.path, 2,
                                                object_oid.hash + prefix_len,
                                                GIT_SHA1_RAWSZ - prefix_len))
                                goto handle_non_note; /* entry.path is not a SHA1 */

                        object_oid.hash[KEY_INDEX] = (unsigned char) (prefix_len + 1);

                        type = PTR_TYPE_SUBTREE;
                } else {
                        /* This can't be part of a note */
                        goto handle_non_note;
                }

                l = xcalloc(1, sizeof(*l));
                oidcpy(&l->key_oid, &object_oid);
                oidcpy(&l->val_oid, entry.oid);
                if (note_tree_insert(t, node, n, l, type,
                                     combine_notes_concatenate))
                        die("Failed to load %s %s into notes tree "
                            "from %s",
                            type == PTR_TYPE_NOTE ? "note" : "subtree",
                            oid_to_hex(&l->key_oid), t->ref);

                continue;

handle_non_note:
                /*
                 * Determine full path for this non-note entry. The
                 * filename is already found in entry.path, but the
                 * directory part of the path must be deduced from the
                 * subtree containing this entry based on our
                 * knowledge that the overall notes tree follows a
                 * strict byte-based progressive fanout structure
                 * (i.e. using 2/38, 2/2/36, etc. fanouts).
                 */
                {
                        struct strbuf non_note_path = STRBUF_INIT;
                        const char *q = oid_to_hex(&subtree->key_oid);
                        int i;
                        for (i = 0; i < prefix_len; i++) {
                                strbuf_addch(&non_note_path, *q++);
                                strbuf_addch(&non_note_path, *q++);
                                strbuf_addch(&non_note_path, '/');
                        }
                        strbuf_addstr(&non_note_path, entry.path);
                        add_non_note(t, strbuf_detach(&non_note_path, NULL),
                                     entry.mode, entry.oid->hash);
                }
        }
        free(buf);
}

/*
 * Determine optimal on-disk fanout for this part of the notes tree
 *
 * Given a (sub)tree and the level in the internal tree structure, determine
 * whether or not the given existing fanout should be expanded for this
 * (sub)tree.
 *
 * Values of the 'fanout' variable:
 * - 0: No fanout (all notes are stored directly in the root notes tree)
 * - 1: 2/38 fanout
 * - 2: 2/2/36 fanout
 * - 3: 2/2/2/34 fanout
 * etc.
 */
static unsigned char determine_fanout(struct int_node *tree, unsigned char n,
                unsigned char fanout)
{
        /*
         * The following is a simple heuristic that works well in practice:
         * For each even-numbered 16-tree level (remember that each on-disk
         * fanout level corresponds to _two_ 16-tree levels), peek at all 16
         * entries at that tree level. If all of them are either int_nodes or
         * subtree entries, then there are likely plenty of notes below this
         * level, so we return an incremented fanout.
         */
        unsigned int i;
        if ((n % 2) || (n > 2 * fanout))
                return fanout;
        for (i = 0; i < 16; i++) {
                switch (GET_PTR_TYPE(tree->a[i])) {
                case PTR_TYPE_SUBTREE:
                case PTR_TYPE_INTERNAL:
                        continue;
                default:
                        return fanout;
                }
        }
        return fanout + 1;
}

/* hex SHA1 + 19 * '/' + NUL */
#define FANOUT_PATH_MAX GIT_SHA1_HEXSZ + FANOUT_PATH_SEPARATORS + 1

static void construct_path_with_fanout(const unsigned char *sha1,
                unsigned char fanout, char *path)
{
        unsigned int i = 0, j = 0;
        const char *hex_sha1 = sha1_to_hex(sha1);
        assert(fanout < GIT_SHA1_RAWSZ);
        while (fanout) {
                path[i++] = hex_sha1[j++];
                path[i++] = hex_sha1[j++];
                path[i++] = '/';
                fanout--;
        }
        xsnprintf(path + i, FANOUT_PATH_MAX - i, "%s", hex_sha1 + j);
}

static int for_each_note_helper(struct notes_tree *t, struct int_node *tree,
                unsigned char n, unsigned char fanout, int flags,
                each_note_fn fn, void *cb_data)
{
        unsigned int i;
        void *p;
        int ret = 0;
        struct leaf_node *l;
        static char path[FANOUT_PATH_MAX];

        fanout = determine_fanout(tree, n, fanout);
        for (i = 0; i < 16; i++) {
redo:
                p = tree->a[i];
                switch (GET_PTR_TYPE(p)) {
                case PTR_TYPE_INTERNAL:
                        /* recurse into int_node */
                        ret = for_each_note_helper(t, CLR_PTR_TYPE(p), n + 1,
                                fanout, flags, fn, cb_data);
                        break;
                case PTR_TYPE_SUBTREE:
                        l = (struct leaf_node *) CLR_PTR_TYPE(p);
                        /*
                         * Subtree entries in the note tree represent parts of
                         * the note tree that have not yet been explored. There
                         * is a direct relationship between subtree entries at
                         * level 'n' in the tree, and the 'fanout' variable:
                         * Subtree entries at level 'n <= 2 * fanout' should be
                         * preserved, since they correspond exactly to a fanout
                         * directory in the on-disk structure. However, subtree
                         * entries at level 'n > 2 * fanout' should NOT be
                         * preserved, but rather consolidated into the above
                         * notes tree level. We achieve this by unconditionally
                         * unpacking subtree entries that exist below the
                         * threshold level at 'n = 2 * fanout'.
                         */
                        if (n <= 2 * fanout &&
                            flags & FOR_EACH_NOTE_YIELD_SUBTREES) {
                                /* invoke callback with subtree */
                                unsigned int path_len =
                                        l->key_oid.hash[KEY_INDEX] * 2 + fanout;
                                assert(path_len < FANOUT_PATH_MAX - 1);
                                construct_path_with_fanout(l->key_oid.hash,
                                                           fanout,
                                                           path);
                                /* Create trailing slash, if needed */
                                if (path[path_len - 1] != '/')
                                        path[path_len++] = '/';
                                path[path_len] = '\0';
                                ret = fn(&l->key_oid, &l->val_oid,
                                         path,
                                         cb_data);
                        }
                        if (n > fanout * 2 ||
                            !(flags & FOR_EACH_NOTE_DONT_UNPACK_SUBTREES)) {
                                /* unpack subtree and resume traversal */
                                tree->a[i] = NULL;
                                load_subtree(t, l, tree, n);
                                free(l);
                                goto redo;
                        }
                        break;
                case PTR_TYPE_NOTE:
                        l = (struct leaf_node *) CLR_PTR_TYPE(p);
                        construct_path_with_fanout(l->key_oid.hash, fanout,
                                                   path);
                        ret = fn(&l->key_oid, &l->val_oid, path,
                                 cb_data);
                        break;
                }
                if (ret)
                        return ret;
        }
        return 0;
}

struct tree_write_stack {
        struct tree_write_stack *next;
        struct strbuf buf;
        char path[2]; /* path to subtree in next, if any */
};

static inline int matches_tree_write_stack(struct tree_write_stack *tws,
                const char *full_path)
{
        return  full_path[0] == tws->path[0] &&
                full_path[1] == tws->path[1] &&
                full_path[2] == '/';
}

static void write_tree_entry(struct strbuf *buf, unsigned int mode,
                const char *path, unsigned int path_len, const
                unsigned char *sha1)
{
        strbuf_addf(buf, "%o %.*s%c", mode, path_len, path, '\0');
        strbuf_add(buf, sha1, GIT_SHA1_RAWSZ);
}

static void tree_write_stack_init_subtree(struct tree_write_stack *tws,
                const char *path)
{
        struct tree_write_stack *n;
        assert(!tws->next);
        assert(tws->path[0] == '\0' && tws->path[1] == '\0');
        n = (struct tree_write_stack *)
                xmalloc(sizeof(struct tree_write_stack));
        n->next = NULL;
        strbuf_init(&n->buf, 256 * (32 + GIT_SHA1_HEXSZ)); /* assume 256 entries per tree */
        n->path[0] = n->path[1] = '\0';
        tws->next = n;
        tws->path[0] = path[0];
        tws->path[1] = path[1];
}

static int tree_write_stack_finish_subtree(struct tree_write_stack *tws)
{
        int ret;
        struct tree_write_stack *n = tws->next;
        struct object_id s;
        if (n) {
                ret = tree_write_stack_finish_subtree(n);
                if (ret)
                        return ret;
                ret = write_sha1_file(n->buf.buf, n->buf.len, tree_type, s.hash);
                if (ret)
                        return ret;
                strbuf_release(&n->buf);
                free(n);
                tws->next = NULL;
                write_tree_entry(&tws->buf, 040000, tws->path, 2, s.hash);
                tws->path[0] = tws->path[1] = '\0';
        }
        return 0;
}

static int write_each_note_helper(struct tree_write_stack *tws,
                const char *path, unsigned int mode,
                const struct object_id *oid)
{
        size_t path_len = strlen(path);
        unsigned int n = 0;
        int ret;

        /* Determine common part of tree write stack */
        while (tws && 3 * n < path_len &&
               matches_tree_write_stack(tws, path + 3 * n)) {
                n++;
                tws = tws->next;
        }

        /* tws point to last matching tree_write_stack entry */
        ret = tree_write_stack_finish_subtree(tws);
        if (ret)
                return ret;

        /* Start subtrees needed to satisfy path */
        while (3 * n + 2 < path_len && path[3 * n + 2] == '/') {
                tree_write_stack_init_subtree(tws, path + 3 * n);
                n++;
                tws = tws->next;
        }

        /* There should be no more directory components in the given path */
        assert(memchr(path + 3 * n, '/', path_len - (3 * n)) == NULL);

        /* Finally add given entry to the current tree object */
        write_tree_entry(&tws->buf, mode, path + 3 * n, path_len - (3 * n),
                         oid->hash);

        return 0;
}

struct write_each_note_data {
        struct tree_write_stack *root;
        struct non_note *next_non_note;
};

static int write_each_non_note_until(const char *note_path,
                struct write_each_note_data *d)
{
        struct non_note *n = d->next_non_note;
        int cmp = 0, ret;
        while (n && (!note_path || (cmp = strcmp(n->path, note_path)) <= 0)) {
                if (note_path && cmp == 0)
                        ; /* do nothing, prefer note to non-note */
                else {
                        ret = write_each_note_helper(d->root, n->path, n->mode,
                                                     &n->oid);
                        if (ret)
                                return ret;
                }
                n = n->next;
        }
        d->next_non_note = n;
        return 0;
}

static int write_each_note(const struct object_id *object_oid,
                const struct object_id *note_oid, char *note_path,
                void *cb_data)
{
        struct write_each_note_data *d =
                (struct write_each_note_data *) cb_data;
        size_t note_path_len = strlen(note_path);
        unsigned int mode = 0100644;

        if (note_path[note_path_len - 1] == '/') {
                /* subtree entry */
                note_path_len--;
                note_path[note_path_len] = '\0';
                mode = 040000;
        }
        assert(note_path_len <= GIT_SHA1_HEXSZ + FANOUT_PATH_SEPARATORS);

        /* Weave non-note entries into note entries */
        return  write_each_non_note_until(note_path, d) ||
                write_each_note_helper(d->root, note_path, mode, note_oid);
}

struct note_delete_list {
        struct note_delete_list *next;
        const unsigned char *sha1;
};

static int prune_notes_helper(const struct object_id *object_oid,
                const struct object_id *note_oid, char *note_path,
                void *cb_data)
{
        struct note_delete_list **l = (struct note_delete_list **) cb_data;
        struct note_delete_list *n;

        if (has_object_file(object_oid))
                return 0; /* nothing to do for this note */

        /* failed to find object => prune this note */
        n = (struct note_delete_list *) xmalloc(sizeof(*n));
        n->next = *l;
        n->sha1 = object_oid->hash;
        *l = n;
        return 0;
}

int combine_notes_concatenate(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        char *cur_msg = NULL, *new_msg = NULL, *buf;
        unsigned long cur_len, new_len, buf_len;
        enum object_type cur_type, new_type;
        int ret;

        /* read in both note blob objects */
        if (!is_null_sha1(new_sha1))
                new_msg = read_sha1_file(new_sha1, &new_type, &new_len);
        if (!new_msg || !new_len || new_type != OBJ_BLOB) {
                free(new_msg);
                return 0;
        }
        if (!is_null_sha1(cur_sha1))
                cur_msg = read_sha1_file(cur_sha1, &cur_type, &cur_len);
        if (!cur_msg || !cur_len || cur_type != OBJ_BLOB) {
                free(cur_msg);
                free(new_msg);
                hashcpy(cur_sha1, new_sha1);
                return 0;
        }

        /* we will separate the notes by two newlines anyway */
        if (cur_msg[cur_len - 1] == '\n')
                cur_len--;

        /* concatenate cur_msg and new_msg into buf */
        buf_len = cur_len + 2 + new_len;
        buf = (char *) xmalloc(buf_len);
        memcpy(buf, cur_msg, cur_len);
        buf[cur_len] = '\n';
        buf[cur_len + 1] = '\n';
        memcpy(buf + cur_len + 2, new_msg, new_len);
        free(cur_msg);
        free(new_msg);

        /* create a new blob object from buf */
        ret = write_sha1_file(buf, buf_len, blob_type, cur_sha1);
        free(buf);
        return ret;
}

int combine_notes_overwrite(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        hashcpy(cur_sha1, new_sha1);
        return 0;
}

int combine_notes_ignore(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        return 0;
}

/*
 * Add the lines from the named object to list, with trailing
 * newlines removed.
 */
static int string_list_add_note_lines(struct string_list *list,
                                      const unsigned char *sha1)
{
        char *data;
        unsigned long len;
        enum object_type t;

        if (is_null_sha1(sha1))
                return 0;

        /* read_sha1_file NUL-terminates */
        data = read_sha1_file(sha1, &t, &len);
        if (t != OBJ_BLOB || !data || !len) {
                free(data);
                return t != OBJ_BLOB || !data;
        }

        /*
         * If the last line of the file is EOL-terminated, this will
         * add an empty string to the list.  But it will be removed
         * later, along with any empty strings that came from empty
         * lines within the file.
         */
        string_list_split(list, data, '\n', -1);
        free(data);
        return 0;
}

static int string_list_join_lines_helper(struct string_list_item *item,
                                         void *cb_data)
{
        struct strbuf *buf = cb_data;
        strbuf_addstr(buf, item->string);
        strbuf_addch(buf, '\n');
        return 0;
}

int combine_notes_cat_sort_uniq(unsigned char *cur_sha1,
                const unsigned char *new_sha1)
{
        struct string_list sort_uniq_list = STRING_LIST_INIT_DUP;
        struct strbuf buf = STRBUF_INIT;
        int ret = 1;

        /* read both note blob objects into unique_lines */
        if (string_list_add_note_lines(&sort_uniq_list, cur_sha1))
                goto out;
        if (string_list_add_note_lines(&sort_uniq_list, new_sha1))
                goto out;
        string_list_remove_empty_items(&sort_uniq_list, 0);
        string_list_sort(&sort_uniq_list);
        string_list_remove_duplicates(&sort_uniq_list, 0);

        /* create a new blob object from sort_uniq_list */
        if (for_each_string_list(&sort_uniq_list,
                                 string_list_join_lines_helper, &buf))
                goto out;

        ret = write_sha1_file(buf.buf, buf.len, blob_type, cur_sha1);

out:
        strbuf_release(&buf);
        string_list_clear(&sort_uniq_list, 0);
        return ret;
}

static int string_list_add_one_ref(const char *refname, const struct object_id *oid,
                                   int flag, void *cb)
{
        struct string_list *refs = cb;
        if (!unsorted_string_list_has_string(refs, refname))
                string_list_append(refs, refname);
        return 0;
}

/*
 * The list argument must have strdup_strings set on it.
 */
void string_list_add_refs_by_glob(struct string_list *list, const char *glob)
{
        assert(list->strdup_strings);
        if (has_glob_specials(glob)) {
                for_each_glob_ref(string_list_add_one_ref, glob, list);
        } else {
                struct object_id oid;
                if (get_oid(glob, &oid))
                        warning("notes ref %s is invalid", glob);
                if (!unsorted_string_list_has_string(list, glob))
                        string_list_append(list, glob);
        }
}

void string_list_add_refs_from_colon_sep(struct string_list *list,
                                         const char *globs)
{
        struct string_list split = STRING_LIST_INIT_NODUP;
        char *globs_copy = xstrdup(globs);
        int i;

        string_list_split_in_place(&split, globs_copy, ':', -1);
        string_list_remove_empty_items(&split, 0);

        for (i = 0; i < split.nr; i++)
                string_list_add_refs_by_glob(list, split.items[i].string);

        string_list_clear(&split, 0);
        free(globs_copy);
}

static int notes_display_config(const char *k, const char *v, void *cb)
{
        int *load_refs = cb;

        if (*load_refs && !strcmp(k, "notes.displayref")) {
                if (!v)
                        config_error_nonbool(k);
                string_list_add_refs_by_glob(&display_notes_refs, v);
        }

        return 0;
}

const char *default_notes_ref(void)
{
        const char *notes_ref = NULL;
        if (!notes_ref)
                notes_ref = getenv(GIT_NOTES_REF_ENVIRONMENT);
        if (!notes_ref)
                notes_ref = notes_ref_name; /* value of core.notesRef config */
        if (!notes_ref)
                notes_ref = GIT_NOTES_DEFAULT_REF;
        return notes_ref;
}

void init_notes(struct notes_tree *t, const char *notes_ref,
                combine_notes_fn combine_notes, int flags)
{
        struct object_id oid, object_oid;
        unsigned mode;
        struct leaf_node root_tree;

        if (!t)
                t = &default_notes_tree;
        assert(!t->initialized);

        if (!notes_ref)
                notes_ref = default_notes_ref();

        if (!combine_notes)
                combine_notes = combine_notes_concatenate;

        t->root = (struct int_node *) xcalloc(1, sizeof(struct int_node));
        t->first_non_note = NULL;
        t->prev_non_note = NULL;
        t->ref = xstrdup_or_null(notes_ref);
        t->update_ref = (flags & NOTES_INIT_WRITABLE) ? t->ref : NULL;
        t->combine_notes = combine_notes;
        t->initialized = 1;
        t->dirty = 0;

        if (flags & NOTES_INIT_EMPTY || !notes_ref ||
            get_sha1_treeish(notes_ref, object_oid.hash))
                return;
        if (flags & NOTES_INIT_WRITABLE && read_ref(notes_ref, object_oid.hash))
                die("Cannot use notes ref %s", notes_ref);
        if (get_tree_entry(object_oid.hash, "", oid.hash, &mode))
                die("Failed to read notes tree referenced by %s (%s)",
                    notes_ref, oid_to_hex(&object_oid));

        oidclr(&root_tree.key_oid);
        oidcpy(&root_tree.val_oid, &oid);
        load_subtree(t, &root_tree, t->root, 0);
}

struct notes_tree **load_notes_trees(struct string_list *refs, int flags)
{
        struct string_list_item *item;
        int counter = 0;
        struct notes_tree **trees;
        ALLOC_ARRAY(trees, refs->nr + 1);
        for_each_string_list_item(item, refs) {
                struct notes_tree *t = xcalloc(1, sizeof(struct notes_tree));
                init_notes(t, item->string, combine_notes_ignore, flags);
                trees[counter++] = t;
        }
        trees[counter] = NULL;
        return trees;
}

void init_display_notes(struct display_notes_opt *opt)
{
        char *display_ref_env;
        int load_config_refs = 0;
        display_notes_refs.strdup_strings = 1;

        assert(!display_notes_trees);

        if (!opt || opt->use_default_notes > 0 ||
            (opt->use_default_notes == -1 && !opt->extra_notes_refs.nr)) {
                string_list_append(&display_notes_refs, default_notes_ref());
                display_ref_env = getenv(GIT_NOTES_DISPLAY_REF_ENVIRONMENT);
                if (display_ref_env) {
                        string_list_add_refs_from_colon_sep(&display_notes_refs,
                                                            display_ref_env);
                        load_config_refs = 0;
                } else
                        load_config_refs = 1;
        }

        git_config(notes_display_config, &load_config_refs);

        if (opt) {
                struct string_list_item *item;
                for_each_string_list_item(item, &opt->extra_notes_refs)
                        string_list_add_refs_by_glob(&display_notes_refs,
                                                     item->string);
        }

        display_notes_trees = load_notes_trees(&display_notes_refs, 0);
        string_list_clear(&display_notes_refs, 0);
}

int add_note(struct notes_tree *t, const struct object_id *object_oid,
                const struct object_id *note_oid, combine_notes_fn combine_notes)
{
        struct leaf_node *l;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        t->dirty = 1;
        if (!combine_notes)
                combine_notes = t->combine_notes;
        l = (struct leaf_node *) xmalloc(sizeof(struct leaf_node));
        oidcpy(&l->key_oid, object_oid);
        oidcpy(&l->val_oid, note_oid);
        return note_tree_insert(t, t->root, 0, l, PTR_TYPE_NOTE, combine_notes);
}

int remove_note(struct notes_tree *t, const unsigned char *object_sha1)
{
        struct leaf_node l;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        hashcpy(l.key_oid.hash, object_sha1);
        oidclr(&l.val_oid);
        note_tree_remove(t, t->root, 0, &l);
        if (is_null_oid(&l.val_oid)) /* no note was removed */
                return 1;
        t->dirty = 1;
        return 0;
}

const struct object_id *get_note(struct notes_tree *t,
                const struct object_id *oid)
{
        struct leaf_node *found;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        found = note_tree_find(t, t->root, 0, oid->hash);
        return found ? &found->val_oid : NULL;
}

int for_each_note(struct notes_tree *t, int flags, each_note_fn fn,
                void *cb_data)
{
        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);
        return for_each_note_helper(t, t->root, 0, 0, flags, fn, cb_data);
}

int write_notes_tree(struct notes_tree *t, unsigned char *result)
{
        struct tree_write_stack root;
        struct write_each_note_data cb_data;
        int ret;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);

        /* Prepare for traversal of current notes tree */
        root.next = NULL; /* last forward entry in list is grounded */
        strbuf_init(&root.buf, 256 * (32 + GIT_SHA1_HEXSZ)); /* assume 256 entries */
        root.path[0] = root.path[1] = '\0';
        cb_data.root = &root;
        cb_data.next_non_note = t->first_non_note;

        /* Write tree objects representing current notes tree */
        ret = for_each_note(t, FOR_EACH_NOTE_DONT_UNPACK_SUBTREES |
                                FOR_EACH_NOTE_YIELD_SUBTREES,
                        write_each_note, &cb_data) ||
                write_each_non_note_until(NULL, &cb_data) ||
                tree_write_stack_finish_subtree(&root) ||
                write_sha1_file(root.buf.buf, root.buf.len, tree_type, result);
        strbuf_release(&root.buf);
        return ret;
}

void prune_notes(struct notes_tree *t, int flags)
{
        struct note_delete_list *l = NULL;

        if (!t)
                t = &default_notes_tree;
        assert(t->initialized);

        for_each_note(t, 0, prune_notes_helper, &l);

        while (l) {
                if (flags & NOTES_PRUNE_VERBOSE)
                        printf("%s\n", sha1_to_hex(l->sha1));
                if (!(flags & NOTES_PRUNE_DRYRUN))
                        remove_note(t, l->sha1);
                l = l->next;
        }
}

void free_notes(struct notes_tree *t)
{
        if (!t)
                t = &default_notes_tree;
        if (t->root)
                note_tree_free(t->root);
        free(t->root);
        while (t->first_non_note) {
                t->prev_non_note = t->first_non_note->next;
                free(t->first_non_note->path);
                free(t->first_non_note);
                t->first_non_note = t->prev_non_note;
        }
        free(t->ref);
        memset(t, 0, sizeof(struct notes_tree));
}

/*
 * Fill the given strbuf with the notes associated with the given object.
 *
 * If the given notes_tree structure is not initialized, it will be auto-
 * initialized to the default value (see documentation for init_notes() above).
 * If the given notes_tree is NULL, the internal/default notes_tree will be
 * used instead.
 *
 * (raw != 0) gives the %N userformat; otherwise, the note message is given
 * for human consumption.
 */
static void format_note(struct notes_tree *t, const struct object_id *object_oid,
                        struct strbuf *sb, const char *output_encoding, int raw)
{
        static const char utf8[] = "utf-8";
        const struct object_id *oid;
        char *msg, *msg_p;
        unsigned long linelen, msglen;
        enum object_type type;

        if (!t)
                t = &default_notes_tree;
        if (!t->initialized)
                init_notes(t, NULL, NULL, 0);

        oid = get_note(t, object_oid);
        if (!oid)
                return;

        if (!(msg = read_sha1_file(oid->hash, &type, &msglen)) || type != OBJ_BLOB) {
                free(msg);
                return;
        }

        if (output_encoding && *output_encoding &&
            !is_encoding_utf8(output_encoding)) {
                char *reencoded = reencode_string(msg, output_encoding, utf8);
                if (reencoded) {
                        free(msg);
                        msg = reencoded;
                        msglen = strlen(msg);
                }
        }

        /* we will end the annotation by a newline anyway */
        if (msglen && msg[msglen - 1] == '\n')
                msglen--;

        if (!raw) {
                const char *ref = t->ref;
                if (!ref || !strcmp(ref, GIT_NOTES_DEFAULT_REF)) {
                        strbuf_addstr(sb, "\nNotes:\n");
                } else {
                        if (starts_with(ref, "refs/"))
                                ref += 5;
                        if (starts_with(ref, "notes/"))
                                ref += 6;
                        strbuf_addf(sb, "\nNotes (%s):\n", ref);
                }
        }

        for (msg_p = msg; msg_p < msg + msglen; msg_p += linelen + 1) {
                linelen = strchrnul(msg_p, '\n') - msg_p;

                if (!raw)
                        strbuf_addstr(sb, "    ");
                strbuf_add(sb, msg_p, linelen);
                strbuf_addch(sb, '\n');
        }

        free(msg);
}

void format_display_notes(const struct object_id *object_oid,
                          struct strbuf *sb, const char *output_encoding, int raw)
{
        int i;
        assert(display_notes_trees);
        for (i = 0; display_notes_trees[i]; i++)
                format_note(display_notes_trees[i], object_oid, sb,
                            output_encoding, raw);
}

int copy_note(struct notes_tree *t,
              const struct object_id *from_obj, const struct object_id *to_obj,
              int force, combine_notes_fn combine_notes)
{
        const struct object_id *note = get_note(t, from_obj);
        const struct object_id *existing_note = get_note(t, to_obj);

        if (!force && existing_note)
                return 1;

        if (note)
                return add_note(t, to_obj, note, combine_notes);
        else if (existing_note)
                return add_note(t, to_obj, &null_oid, combine_notes);

        return 0;
}

void expand_notes_ref(struct strbuf *sb)
{
        if (starts_with(sb->buf, "refs/notes/"))
                return; /* we're happy */
        else if (starts_with(sb->buf, "notes/"))
                strbuf_insert(sb, 0, "refs/", 5);
        else
                strbuf_insert(sb, 0, "refs/notes/", 11);
}

void expand_loose_notes_ref(struct strbuf *sb)
{
        struct object_id object;

        if (get_oid(sb->buf, &object)) {
                /* fallback to expand_notes_ref */
                expand_notes_ref(sb);
        }
}