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

struct packed_ref_store;

struct packed_ref_cache {
        /*
         * A back-pointer to the packed_ref_store with which this
         * cache is associated:
         */
        struct packed_ref_store *refs;

        struct ref_cache *cache;

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

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

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

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

/*
 * A container for `packed-refs`-related data. It is not (yet) a
 * `ref_store`.
 */
struct packed_ref_store {
        struct ref_store base;

        unsigned int store_flags;

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

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

        /*
         * 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;
};

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);
        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)
                die("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)
                die("BUG: unallowed operation (%s), requires %x, has %x\n",
                    caller, required_flags, refs->store_flags);

        return refs;
}

static void clear_packed_ref_cache(struct packed_ref_store *refs)
{
        if (refs->cache) {
                struct packed_ref_cache *cache = refs->cache;

                refs->cache = NULL;
                release_packed_ref_cache(cache);
        }
}

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

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

        if (parse_oid_hex(line->buf, oid, &ref) < 0)
                return NULL;
        if (!isspace(*ref++))
                return NULL;

        if (isspace(*ref))
                return NULL;

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

        return ref;
}

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);

}

/*
 * Read from the `packed-refs` file into a newly-allocated
 * `packed_ref_cache` 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:
 *
 *   No traits:
 *
 *      Probably no references are peeled. But if the file contains a
 *      peeled value for a reference, we will use it.
 *
 *   peeled:
 *
 *      References under "refs/tags/", if they *can* be peeled, *are*
 *      peeled in this file. References outside of "refs/tags/" are
 *      probably not peeled even if they could have been, but if we find
 *      a peeled value for such a reference we will use it.
 *
 *   fully-peeled:
 *
 *      All references in the file that can be peeled are peeled.
 *      Inversely (and this is more important), any references in the
 *      file for which no peeled value is recorded is not peelable. This
 *      trait should typically be written alongside "peeled" for
 *      compatibility with older clients, but we do not require it
 *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
 */
static struct packed_ref_cache *read_packed_refs(struct packed_ref_store *refs)
{
        FILE *f;
        struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
        struct ref_entry *last = NULL;
        struct strbuf line = STRBUF_INIT;
        enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
        struct ref_dir *dir;

        packed_refs->refs = refs;
        acquire_packed_ref_cache(packed_refs);
        packed_refs->cache = create_ref_cache(NULL, NULL);
        packed_refs->cache->root->flag &= ~REF_INCOMPLETE;

        f = fopen(refs->path, "r");
        if (!f) {
                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.
                         */
                        return packed_refs;
                } else {
                        die_errno("couldn't read %s", refs->path);
                }
        }

        stat_validity_update(&packed_refs->validity, fileno(f));

        dir = get_ref_dir(packed_refs->cache->root);
        while (strbuf_getwholeline(&line, f, '\n') != EOF) {
                struct object_id oid;
                const char *refname;
                const char *traits;

                if (!line.len || line.buf[line.len - 1] != '\n')
                        die_unterminated_line(refs->path, line.buf, line.len);

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

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

                        if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
                                if (!refname_is_safe(refname))
                                        die("packed refname is dangerous: %s", refname);
                                oidclr(&oid);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }
                        last = create_ref_entry(refname, &oid, flag);
                        if (peeled == PEELED_FULLY ||
                            (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
                                last->flag |= REF_KNOWS_PEELED;
                        add_ref_entry(dir, last);
                } else if (last &&
                    line.buf[0] == '^' &&
                    line.len == PEELED_LINE_LENGTH &&
                    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
                    !get_oid_hex(line.buf + 1, &oid)) {
                        oidcpy(&last->u.value.peeled, &oid);
                        /*
                         * Regardless of what the file header said,
                         * we definitely know the value of *this*
                         * reference:
                         */
                        last->flag |= REF_KNOWS_PEELED;
                } else {
                        die_invalid_line(refs->path, line.buf, line.len);
                }
        }

        fclose(f);
        strbuf_release(&line);

        return packed_refs;
}

/*
 * Check that the packed refs cache (if any) still reflects the
 * contents of the file. If not, clear the cache.
 */
static void validate_packed_ref_cache(struct packed_ref_store *refs)
{
        if (refs->cache &&
            !stat_validity_check(&refs->cache->validity, refs->path))
                clear_packed_ref_cache(refs);
}

/*
 * Get the packed_ref_cache 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()`.
 */
static struct packed_ref_cache *get_packed_ref_cache(struct packed_ref_store *refs)
{
        if (!is_lock_file_locked(&refs->lock))
                validate_packed_ref_cache(refs);

        if (!refs->cache)
                refs->cache = read_packed_refs(refs);

        return refs->cache;
}

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

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

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

static int packed_read_raw_ref(struct ref_store *ref_store,
                               const char *refname, unsigned char *sha1,
                               struct strbuf *referent, unsigned int *type)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");

        struct ref_entry *entry;

        *type = 0;

        entry = get_packed_ref(refs, refname);
        if (!entry) {
                errno = ENOENT;
                return -1;
        }

        hashcpy(sha1, entry->u.value.oid.hash);
        *type = REF_ISPACKED;
        return 0;
}

static int packed_peel_ref(struct ref_store *ref_store,
                           const char *refname, unsigned char *sha1)
{
        struct packed_ref_store *refs =
                packed_downcast(ref_store, REF_STORE_READ | REF_STORE_ODB,
                                "peel_ref");
        struct ref_entry *r = get_packed_ref(refs, refname);

        if (!r || peel_entry(r, 0))
                return -1;

        hashcpy(sha1, r->u.value.peeled.hash);
        return 0;
}

struct packed_ref_iterator {
        struct ref_iterator base;

        struct packed_ref_cache *cache;
        struct ref_iterator *iter0;
        unsigned int flags;
};

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 = ref_iterator_advance(iter->iter0)) == ITER_OK) {
                if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
                    ref_type(iter->iter0->refname) != REF_TYPE_PER_WORKTREE)
                        continue;

                if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
                    !ref_resolves_to_object(iter->iter0->refname,
                                            iter->iter0->oid,
                                            iter->iter0->flags))
                        continue;

                iter->base.refname = iter->iter0->refname;
                iter->base.oid = iter->iter0->oid;
                iter->base.flags = iter->iter0->flags;
                return ITER_OK;
        }

        iter->iter0 = NULL;
        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;

        return ref_iterator_peel(iter->iter0, 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;

        if (iter->iter0)
                ok = ref_iterator_abort(iter->iter0);

        release_packed_ref_cache(iter->cache);
        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 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");

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

        /*
         * Note that get_packed_ref_cache() internally checks whether
         * the packed-ref cache is up to date with what is on disk,
         * and re-reads it if not.
         */

        iter->cache = get_packed_ref_cache(refs);
        acquire_packed_ref_cache(iter->cache);
        iter->iter0 = cache_ref_iterator_begin(iter->cache->cache, prefix, 0);

        iter->flags = flags;

        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 unsigned char *sha1,
                              const unsigned char *peeled)
{
        if (fprintf(fh, "%s %s\n", sha1_to_hex(sha1), refname) < 0 ||
            (peeled && fprintf(fh, "^%s\n", sha1_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(&refs->lock)) {
                strbuf_addf(err, "unable to close %s: %s", refs->path, strerror(errno));
                return -1;
        }

        /*
         * Now that we hold the `packed-refs` lock, make sure that our
         * cache matches the current version of the file. Normally
         * `get_packed_ref_cache()` does that for us, but that
         * function assumes that when the file is locked, any existing
         * cache is still valid. We've just locked the file, but it
         * might have changed the moment *before* we locked it.
         */
        validate_packed_ref_cache(refs);

        /*
         * Now make sure that the packed-refs file as it exists in the
         * locked state is loaded into the cache:
         */
        get_packed_ref_cache(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))
                die("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.  The trailing space is required.
 */
static const char PACKED_REFS_HEADER[] =
        "# pack-refs with: peeled fully-peeled \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 cache 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))
                die("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);
        if (create_tempfile(&refs->tempfile, sb.buf) < 0) {
                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 (oidcmp(&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->hash,
                                               peel_error ? NULL : peeled.hash))
                                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.hash,
                                                     peeled.hash);

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

                        i++;
                }
        }

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

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

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.
         * 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;

        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;
        }

        clear_packed_ref_cache(refs);
        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)
{
        die("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)
{
        die("BUG: packed reference store does not support renaming 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)
{
        die("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 unsigned char *sha1,
                                unsigned int flags,
                                reflog_expiry_prepare_fn prepare_fn,
                                reflog_expiry_should_prune_fn should_prune_fn,
                                reflog_expiry_cleanup_fn cleanup_fn,
                                void *policy_cb_data)
{
        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_peel_ref,
        packed_create_symref,
        packed_delete_refs,
        packed_rename_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
};