Merge branch 'ps/pack-refs-auto' into jt/reftable-geometric-compaction

[thirdparty/git.git] / refs / reftable-backend.c

#include "../git-compat-util.h"
#include "../abspath.h"
#include "../chdir-notify.h"
#include "../environment.h"
#include "../gettext.h"
#include "../hash.h"
#include "../hex.h"
#include "../iterator.h"
#include "../ident.h"
#include "../lockfile.h"
#include "../object.h"
#include "../path.h"
#include "../refs.h"
#include "../reftable/reftable-stack.h"
#include "../reftable/reftable-record.h"
#include "../reftable/reftable-error.h"
#include "../reftable/reftable-iterator.h"
#include "../reftable/reftable-merged.h"
#include "../setup.h"
#include "../strmap.h"
#include "refs-internal.h"

/*
 * Used as a flag in ref_update::flags when the ref_update was via an
 * update to HEAD.
 */
#define REF_UPDATE_VIA_HEAD (1 << 8)

struct reftable_ref_store {
        struct ref_store base;

        /*
         * The main stack refers to the common dir and thus contains common
         * refs as well as refs of the main repository.
         */
        struct reftable_stack *main_stack;
        /*
         * The worktree stack refers to the gitdir in case the refdb is opened
         * via a worktree. It thus contains the per-worktree refs.
         */
        struct reftable_stack *worktree_stack;
        /*
         * Map of worktree stacks by their respective worktree names. The map
         * is populated lazily when we try to resolve `worktrees/$worktree` refs.
         */
        struct strmap worktree_stacks;
        struct reftable_write_options write_options;

        unsigned int store_flags;
        int err;
};

/*
 * Downcast ref_store to reftable_ref_store. Die if ref_store is not a
 * reftable_ref_store. required_flags is compared with ref_store's store_flags
 * to ensure the ref_store has all required capabilities. "caller" is used in
 * any necessary error messages.
 */
static struct reftable_ref_store *reftable_be_downcast(struct ref_store *ref_store,
                                                       unsigned int required_flags,
                                                       const char *caller)
{
        struct reftable_ref_store *refs;

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

        refs = (struct reftable_ref_store *)ref_store;

        if ((refs->store_flags & required_flags) != required_flags)
                BUG("operation %s requires abilities 0x%x, but only have 0x%x",
                    caller, required_flags, refs->store_flags);

        return refs;
}

/*
 * Some refs are global to the repository (refs/heads/{*}), while others are
 * local to the worktree (eg. HEAD, refs/bisect/{*}). We solve this by having
 * multiple separate databases (ie. multiple reftable/ directories), one for
 * the shared refs, one for the current worktree refs, and one for each
 * additional worktree. For reading, we merge the view of both the shared and
 * the current worktree's refs, when necessary.
 *
 * This function also optionally assigns the rewritten reference name that is
 * local to the stack. This translation is required when using worktree refs
 * like `worktrees/$worktree/refs/heads/foo` as worktree stacks will store
 * those references in their normalized form.
 */
static struct reftable_stack *stack_for(struct reftable_ref_store *store,
                                        const char *refname,
                                        const char **rewritten_ref)
{
        const char *wtname;
        int wtname_len;

        if (!refname)
                return store->main_stack;

        switch (parse_worktree_ref(refname, &wtname, &wtname_len, rewritten_ref)) {
        case REF_WORKTREE_OTHER: {
                static struct strbuf wtname_buf = STRBUF_INIT;
                struct strbuf wt_dir = STRBUF_INIT;
                struct reftable_stack *stack;

                /*
                 * We're using a static buffer here so that we don't need to
                 * allocate the worktree name whenever we look up a reference.
                 * This could be avoided if the strmap interface knew how to
                 * handle keys with a length.
                 */
                strbuf_reset(&wtname_buf);
                strbuf_add(&wtname_buf, wtname, wtname_len);

                /*
                 * There is an edge case here: when the worktree references the
                 * current worktree, then we set up the stack once via
                 * `worktree_stacks` and once via `worktree_stack`. This is
                 * wasteful, but in the reading case it shouldn't matter. And
                 * in the writing case we would notice that the stack is locked
                 * already and error out when trying to write a reference via
                 * both stacks.
                 */
                stack = strmap_get(&store->worktree_stacks, wtname_buf.buf);
                if (!stack) {
                        strbuf_addf(&wt_dir, "%s/worktrees/%s/reftable",
                                    store->base.repo->commondir, wtname_buf.buf);

                        store->err = reftable_new_stack(&stack, wt_dir.buf,
                                                        store->write_options);
                        assert(store->err != REFTABLE_API_ERROR);
                        strmap_put(&store->worktree_stacks, wtname_buf.buf, stack);
                }

                strbuf_release(&wt_dir);
                return stack;
        }
        case REF_WORKTREE_CURRENT:
                /*
                 * If there is no worktree stack then we're currently in the
                 * main worktree. We thus return the main stack in that case.
                 */
                if (!store->worktree_stack)
                        return store->main_stack;
                return store->worktree_stack;
        case REF_WORKTREE_MAIN:
        case REF_WORKTREE_SHARED:
                return store->main_stack;
        default:
                BUG("unhandled worktree reference type");
        }
}

static int should_write_log(struct ref_store *refs, const char *refname)
{
        if (log_all_ref_updates == LOG_REFS_UNSET)
                log_all_ref_updates = is_bare_repository() ? LOG_REFS_NONE : LOG_REFS_NORMAL;

        switch (log_all_ref_updates) {
        case LOG_REFS_NONE:
                return refs_reflog_exists(refs, refname);
        case LOG_REFS_ALWAYS:
                return 1;
        case LOG_REFS_NORMAL:
                if (should_autocreate_reflog(refname))
                        return 1;
                return refs_reflog_exists(refs, refname);
        default:
                BUG("unhandled core.logAllRefUpdates value %d", log_all_ref_updates);
        }
}

static void fill_reftable_log_record(struct reftable_log_record *log)
{
        const char *info = git_committer_info(0);
        struct ident_split split = {0};
        int sign = 1;

        if (split_ident_line(&split, info, strlen(info)))
                BUG("failed splitting committer info");

        reftable_log_record_release(log);
        log->value_type = REFTABLE_LOG_UPDATE;
        log->value.update.name =
                xstrndup(split.name_begin, split.name_end - split.name_begin);
        log->value.update.email =
                xstrndup(split.mail_begin, split.mail_end - split.mail_begin);
        log->value.update.time = atol(split.date_begin);
        if (*split.tz_begin == '-') {
                sign = -1;
                split.tz_begin++;
        }
        if (*split.tz_begin == '+') {
                sign = 1;
                split.tz_begin++;
        }

        log->value.update.tz_offset = sign * atoi(split.tz_begin);
}

static int read_ref_without_reload(struct reftable_stack *stack,
                                   const char *refname,
                                   struct object_id *oid,
                                   struct strbuf *referent,
                                   unsigned int *type)
{
        struct reftable_ref_record ref = {0};
        int ret;

        ret = reftable_stack_read_ref(stack, refname, &ref);
        if (ret)
                goto done;

        if (ref.value_type == REFTABLE_REF_SYMREF) {
                strbuf_reset(referent);
                strbuf_addstr(referent, ref.value.symref);
                *type |= REF_ISSYMREF;
        } else if (reftable_ref_record_val1(&ref)) {
                oidread(oid, reftable_ref_record_val1(&ref));
        } else {
                /* We got a tombstone, which should not happen. */
                BUG("unhandled reference value type %d", ref.value_type);
        }

done:
        assert(ret != REFTABLE_API_ERROR);
        reftable_ref_record_release(&ref);
        return ret;
}

static struct ref_store *reftable_be_init(struct repository *repo,
                                          const char *gitdir,
                                          unsigned int store_flags)
{
        struct reftable_ref_store *refs = xcalloc(1, sizeof(*refs));
        struct strbuf path = STRBUF_INIT;
        int is_worktree;
        mode_t mask;

        mask = umask(0);
        umask(mask);

        base_ref_store_init(&refs->base, repo, gitdir, &refs_be_reftable);
        strmap_init(&refs->worktree_stacks);
        refs->store_flags = store_flags;
        refs->write_options.block_size = 4096;
        refs->write_options.hash_id = repo->hash_algo->format_id;
        refs->write_options.default_permissions = calc_shared_perm(0666 & ~mask);

        /*
         * Set up the main reftable stack that is hosted in GIT_COMMON_DIR.
         * This stack contains both the shared and the main worktree refs.
         *
         * Note that we don't try to resolve the path in case we have a
         * worktree because `get_common_dir_noenv()` already does it for us.
         */
        is_worktree = get_common_dir_noenv(&path, gitdir);
        if (!is_worktree) {
                strbuf_reset(&path);
                strbuf_realpath(&path, gitdir, 0);
        }
        strbuf_addstr(&path, "/reftable");
        refs->err = reftable_new_stack(&refs->main_stack, path.buf,
                                       refs->write_options);
        if (refs->err)
                goto done;

        /*
         * If we're in a worktree we also need to set up the worktree reftable
         * stack that is contained in the per-worktree GIT_DIR.
         *
         * Ideally, we would also add the stack to our worktree stack map. But
         * we have no way to figure out the worktree name here and thus can't
         * do it efficiently.
         */
        if (is_worktree) {
                strbuf_reset(&path);
                strbuf_addf(&path, "%s/reftable", gitdir);

                refs->err = reftable_new_stack(&refs->worktree_stack, path.buf,
                                               refs->write_options);
                if (refs->err)
                        goto done;
        }

        chdir_notify_reparent("reftables-backend $GIT_DIR", &refs->base.gitdir);

done:
        assert(refs->err != REFTABLE_API_ERROR);
        strbuf_release(&path);
        return &refs->base;
}

static int reftable_be_init_db(struct ref_store *ref_store,
                               int flags UNUSED,
                               struct strbuf *err UNUSED)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "init_db");
        struct strbuf sb = STRBUF_INIT;

        strbuf_addf(&sb, "%s/reftable", refs->base.gitdir);
        safe_create_dir(sb.buf, 1);
        strbuf_reset(&sb);

        strbuf_addf(&sb, "%s/HEAD", refs->base.gitdir);
        write_file(sb.buf, "ref: refs/heads/.invalid");
        adjust_shared_perm(sb.buf);
        strbuf_reset(&sb);

        strbuf_addf(&sb, "%s/refs", refs->base.gitdir);
        safe_create_dir(sb.buf, 1);
        strbuf_reset(&sb);

        strbuf_addf(&sb, "%s/refs/heads", refs->base.gitdir);
        write_file(sb.buf, "this repository uses the reftable format");
        adjust_shared_perm(sb.buf);

        strbuf_release(&sb);
        return 0;
}

struct reftable_ref_iterator {
        struct ref_iterator base;
        struct reftable_ref_store *refs;
        struct reftable_iterator iter;
        struct reftable_ref_record ref;
        struct object_id oid;

        const char *prefix;
        size_t prefix_len;
        unsigned int flags;
        int err;
};

static int reftable_ref_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct reftable_ref_iterator *iter =
                (struct reftable_ref_iterator *)ref_iterator;
        struct reftable_ref_store *refs = iter->refs;

        while (!iter->err) {
                int flags = 0;

                iter->err = reftable_iterator_next_ref(&iter->iter, &iter->ref);
                if (iter->err)
                        break;

                /*
                 * The files backend only lists references contained in "refs/" unless
                 * the root refs are to be included. We emulate the same behaviour here.
                 */
                if (!starts_with(iter->ref.refname, "refs/") &&
                    !(iter->flags & DO_FOR_EACH_INCLUDE_ROOT_REFS &&
                     (is_pseudoref(&iter->refs->base, iter->ref.refname) ||
                      is_headref(&iter->refs->base, iter->ref.refname)))) {
                        continue;
                }

                if (iter->prefix_len &&
                    strncmp(iter->prefix, iter->ref.refname, iter->prefix_len)) {
                        iter->err = 1;
                        break;
                }

                if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
                    parse_worktree_ref(iter->ref.refname, NULL, NULL, NULL) !=
                            REF_WORKTREE_CURRENT)
                        continue;

                switch (iter->ref.value_type) {
                case REFTABLE_REF_VAL1:
                        oidread(&iter->oid, iter->ref.value.val1);
                        break;
                case REFTABLE_REF_VAL2:
                        oidread(&iter->oid, iter->ref.value.val2.value);
                        break;
                case REFTABLE_REF_SYMREF:
                        if (!refs_resolve_ref_unsafe(&iter->refs->base, iter->ref.refname,
                                                     RESOLVE_REF_READING, &iter->oid, &flags))
                                oidclr(&iter->oid);
                        break;
                default:
                        BUG("unhandled reference value type %d", iter->ref.value_type);
                }

                if (is_null_oid(&iter->oid))
                        flags |= REF_ISBROKEN;

                if (check_refname_format(iter->ref.refname, REFNAME_ALLOW_ONELEVEL)) {
                        if (!refname_is_safe(iter->ref.refname))
                                die(_("refname is dangerous: %s"), iter->ref.refname);
                        oidclr(&iter->oid);
                        flags |= REF_BAD_NAME | REF_ISBROKEN;
                }

                if (iter->flags & DO_FOR_EACH_OMIT_DANGLING_SYMREFS &&
                    flags & REF_ISSYMREF &&
                    flags & REF_ISBROKEN)
                        continue;

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

                iter->base.refname = iter->ref.refname;
                iter->base.oid = &iter->oid;
                iter->base.flags = flags;

                break;
        }

        if (iter->err > 0) {
                if (ref_iterator_abort(ref_iterator) != ITER_DONE)
                        return ITER_ERROR;
                return ITER_DONE;
        }

        if (iter->err < 0) {
                ref_iterator_abort(ref_iterator);
                return ITER_ERROR;
        }

        return ITER_OK;
}

static int reftable_ref_iterator_peel(struct ref_iterator *ref_iterator,
                                      struct object_id *peeled)
{
        struct reftable_ref_iterator *iter =
                (struct reftable_ref_iterator *)ref_iterator;

        if (iter->ref.value_type == REFTABLE_REF_VAL2) {
                oidread(peeled, iter->ref.value.val2.target_value);
                return 0;
        }

        return -1;
}

static int reftable_ref_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct reftable_ref_iterator *iter =
                (struct reftable_ref_iterator *)ref_iterator;
        reftable_ref_record_release(&iter->ref);
        reftable_iterator_destroy(&iter->iter);
        free(iter);
        return ITER_DONE;
}

static struct ref_iterator_vtable reftable_ref_iterator_vtable = {
        .advance = reftable_ref_iterator_advance,
        .peel = reftable_ref_iterator_peel,
        .abort = reftable_ref_iterator_abort
};

static struct reftable_ref_iterator *ref_iterator_for_stack(struct reftable_ref_store *refs,
                                                            struct reftable_stack *stack,
                                                            const char *prefix,
                                                            int flags)
{
        struct reftable_merged_table *merged_table;
        struct reftable_ref_iterator *iter;
        int ret;

        iter = xcalloc(1, sizeof(*iter));
        base_ref_iterator_init(&iter->base, &reftable_ref_iterator_vtable);
        iter->prefix = prefix;
        iter->prefix_len = prefix ? strlen(prefix) : 0;
        iter->base.oid = &iter->oid;
        iter->flags = flags;
        iter->refs = refs;

        ret = refs->err;
        if (ret)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret)
                goto done;

        merged_table = reftable_stack_merged_table(stack);

        ret = reftable_merged_table_seek_ref(merged_table, &iter->iter, prefix);
        if (ret)
                goto done;

done:
        iter->err = ret;
        return iter;
}

static struct ref_iterator *reftable_be_iterator_begin(struct ref_store *ref_store,
                                                       const char *prefix,
                                                       const char **exclude_patterns,
                                                       unsigned int flags)
{
        struct reftable_ref_iterator *main_iter, *worktree_iter;
        struct reftable_ref_store *refs;
        unsigned int required_flags = REF_STORE_READ;

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

        main_iter = ref_iterator_for_stack(refs, refs->main_stack, prefix, flags);

        /*
         * The worktree stack is only set when we're in an actual worktree
         * right now. If we aren't, then we return the common reftable
         * iterator, only.
         */
         if (!refs->worktree_stack)
                return &main_iter->base;

        /*
         * Otherwise we merge both the common and the per-worktree refs into a
         * single iterator.
         */
        worktree_iter = ref_iterator_for_stack(refs, refs->worktree_stack, prefix, flags);
        return merge_ref_iterator_begin(&worktree_iter->base, &main_iter->base,
                                        ref_iterator_select, NULL);
}

static int reftable_be_read_raw_ref(struct ref_store *ref_store,
                                    const char *refname,
                                    struct object_id *oid,
                                    struct strbuf *referent,
                                    unsigned int *type,
                                    int *failure_errno)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        int ret;

        if (refs->err < 0)
                return refs->err;

        ret = reftable_stack_reload(stack);
        if (ret)
                return ret;

        ret = read_ref_without_reload(stack, refname, oid, referent, type);
        if (ret < 0)
                return ret;
        if (ret > 0) {
                *failure_errno = ENOENT;
                return -1;
        }

        return 0;
}

static int reftable_be_read_symbolic_ref(struct ref_store *ref_store,
                                         const char *refname,
                                         struct strbuf *referent)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_READ, "read_symbolic_ref");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct reftable_ref_record ref = {0};
        int ret;

        ret = reftable_stack_reload(stack);
        if (ret)
                return ret;

        ret = reftable_stack_read_ref(stack, refname, &ref);
        if (ret == 0 && ref.value_type == REFTABLE_REF_SYMREF)
                strbuf_addstr(referent, ref.value.symref);
        else
                ret = -1;

        reftable_ref_record_release(&ref);
        return ret;
}

/*
 * Return the refname under which update was originally requested.
 */
static const char *original_update_refname(struct ref_update *update)
{
        while (update->parent_update)
                update = update->parent_update;
        return update->refname;
}

struct reftable_transaction_update {
        struct ref_update *update;
        struct object_id current_oid;
};

struct write_transaction_table_arg {
        struct reftable_ref_store *refs;
        struct reftable_stack *stack;
        struct reftable_addition *addition;
        struct reftable_transaction_update *updates;
        size_t updates_nr;
        size_t updates_alloc;
        size_t updates_expected;
};

struct reftable_transaction_data {
        struct write_transaction_table_arg *args;
        size_t args_nr, args_alloc;
};

static void free_transaction_data(struct reftable_transaction_data *tx_data)
{
        if (!tx_data)
                return;
        for (size_t i = 0; i < tx_data->args_nr; i++) {
                reftable_addition_destroy(tx_data->args[i].addition);
                free(tx_data->args[i].updates);
        }
        free(tx_data->args);
        free(tx_data);
}

/*
 * Prepare transaction update for the given reference update. This will cause
 * us to lock the corresponding reftable stack for concurrent modification.
 */
static int prepare_transaction_update(struct write_transaction_table_arg **out,
                                      struct reftable_ref_store *refs,
                                      struct reftable_transaction_data *tx_data,
                                      struct ref_update *update,
                                      struct strbuf *err)
{
        struct reftable_stack *stack = stack_for(refs, update->refname, NULL);
        struct write_transaction_table_arg *arg = NULL;
        size_t i;
        int ret;

        /*
         * Search for a preexisting stack update. If there is one then we add
         * the update to it, otherwise we set up a new stack update.
         */
        for (i = 0; !arg && i < tx_data->args_nr; i++)
                if (tx_data->args[i].stack == stack)
                        arg = &tx_data->args[i];

        if (!arg) {
                struct reftable_addition *addition;

                ret = reftable_stack_reload(stack);
                if (ret)
                        return ret;

                ret = reftable_stack_new_addition(&addition, stack);
                if (ret) {
                        if (ret == REFTABLE_LOCK_ERROR)
                                strbuf_addstr(err, "cannot lock references");
                        return ret;
                }

                ALLOC_GROW(tx_data->args, tx_data->args_nr + 1,
                           tx_data->args_alloc);
                arg = &tx_data->args[tx_data->args_nr++];
                arg->refs = refs;
                arg->stack = stack;
                arg->addition = addition;
                arg->updates = NULL;
                arg->updates_nr = 0;
                arg->updates_alloc = 0;
                arg->updates_expected = 0;
        }

        arg->updates_expected++;

        if (out)
                *out = arg;

        return 0;
}

/*
 * Queue a reference update for the correct stack. We potentially need to
 * handle multiple stack updates in a single transaction when it spans across
 * multiple worktrees.
 */
static int queue_transaction_update(struct reftable_ref_store *refs,
                                    struct reftable_transaction_data *tx_data,
                                    struct ref_update *update,
                                    struct object_id *current_oid,
                                    struct strbuf *err)
{
        struct write_transaction_table_arg *arg = NULL;
        int ret;

        if (update->backend_data)
                BUG("reference update queued more than once");

        ret = prepare_transaction_update(&arg, refs, tx_data, update, err);
        if (ret < 0)
                return ret;

        ALLOC_GROW(arg->updates, arg->updates_nr + 1,
                   arg->updates_alloc);
        arg->updates[arg->updates_nr].update = update;
        oidcpy(&arg->updates[arg->updates_nr].current_oid, current_oid);
        update->backend_data = &arg->updates[arg->updates_nr++];

        return 0;
}

static int reftable_be_transaction_prepare(struct ref_store *ref_store,
                                           struct ref_transaction *transaction,
                                           struct strbuf *err)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE|REF_STORE_MAIN, "ref_transaction_prepare");
        struct strbuf referent = STRBUF_INIT, head_referent = STRBUF_INIT;
        struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
        struct reftable_transaction_data *tx_data = NULL;
        struct object_id head_oid;
        unsigned int head_type = 0;
        size_t i;
        int ret;

        ret = refs->err;
        if (ret < 0)
                goto done;

        tx_data = xcalloc(1, sizeof(*tx_data));

        /*
         * Preprocess all updates. For one we check that there are no duplicate
         * reference updates in this transaction. Second, we lock all stacks
         * that will be modified during the transaction.
         */
        for (i = 0; i < transaction->nr; i++) {
                ret = prepare_transaction_update(NULL, refs, tx_data,
                                                 transaction->updates[i], err);
                if (ret)
                        goto done;

                string_list_append(&affected_refnames,
                                   transaction->updates[i]->refname);
        }

        /*
         * Now that we have counted updates per stack we can preallocate their
         * arrays. This avoids having to reallocate many times.
         */
        for (i = 0; i < tx_data->args_nr; i++) {
                CALLOC_ARRAY(tx_data->args[i].updates, tx_data->args[i].updates_expected);
                tx_data->args[i].updates_alloc = tx_data->args[i].updates_expected;
        }

        /*
         * Fail if a refname appears more than once in the transaction.
         * This code is taken from the files backend and is a good candidate to
         * be moved into the generic layer.
         */
        string_list_sort(&affected_refnames);
        if (ref_update_reject_duplicates(&affected_refnames, err)) {
                ret = TRANSACTION_GENERIC_ERROR;
                goto done;
        }

        ret = read_ref_without_reload(stack_for(refs, "HEAD", NULL), "HEAD", &head_oid,
                                      &head_referent, &head_type);
        if (ret < 0)
                goto done;
        ret = 0;

        for (i = 0; i < transaction->nr; i++) {
                struct ref_update *u = transaction->updates[i];
                struct object_id current_oid = {0};
                struct reftable_stack *stack;
                const char *rewritten_ref;

                stack = stack_for(refs, u->refname, &rewritten_ref);

                /* Verify that the new object ID is valid. */
                if ((u->flags & REF_HAVE_NEW) && !is_null_oid(&u->new_oid) &&
                    !(u->flags & REF_SKIP_OID_VERIFICATION) &&
                    !(u->flags & REF_LOG_ONLY)) {
                        struct object *o = parse_object(refs->base.repo, &u->new_oid);
                        if (!o) {
                                strbuf_addf(err,
                                            _("trying to write ref '%s' with nonexistent object %s"),
                                            u->refname, oid_to_hex(&u->new_oid));
                                ret = -1;
                                goto done;
                        }

                        if (o->type != OBJ_COMMIT && is_branch(u->refname)) {
                                strbuf_addf(err, _("trying to write non-commit object %s to branch '%s'"),
                                            oid_to_hex(&u->new_oid), u->refname);
                                ret = -1;
                                goto done;
                        }
                }

                /*
                 * When we update the reference that HEAD points to we enqueue
                 * a second log-only update for HEAD so that its reflog is
                 * updated accordingly.
                 */
                if (head_type == REF_ISSYMREF &&
                    !(u->flags & REF_LOG_ONLY) &&
                    !(u->flags & REF_UPDATE_VIA_HEAD) &&
                    !strcmp(rewritten_ref, head_referent.buf)) {
                        struct ref_update *new_update;

                        /*
                         * First make sure that HEAD is not already in the
                         * transaction. This check is O(lg N) in the transaction
                         * size, but it happens at most once per transaction.
                         */
                        if (string_list_has_string(&affected_refnames, "HEAD")) {
                                /* An entry already existed */
                                strbuf_addf(err,
                                            _("multiple updates for 'HEAD' (including one "
                                            "via its referent '%s') are not allowed"),
                                            u->refname);
                                ret = TRANSACTION_NAME_CONFLICT;
                                goto done;
                        }

                        new_update = ref_transaction_add_update(
                                        transaction, "HEAD",
                                        u->flags | REF_LOG_ONLY | REF_NO_DEREF,
                                        &u->new_oid, &u->old_oid, u->msg);
                        string_list_insert(&affected_refnames, new_update->refname);
                }

                ret = read_ref_without_reload(stack, rewritten_ref,
                                              &current_oid, &referent, &u->type);
                if (ret < 0)
                        goto done;
                if (ret > 0 && (!(u->flags & REF_HAVE_OLD) || is_null_oid(&u->old_oid))) {
                        /*
                         * The reference does not exist, and we either have no
                         * old object ID or expect the reference to not exist.
                         * We can thus skip below safety checks as well as the
                         * symref splitting. But we do want to verify that
                         * there is no conflicting reference here so that we
                         * can output a proper error message instead of failing
                         * at a later point.
                         */
                        ret = refs_verify_refname_available(ref_store, u->refname,
                                                            &affected_refnames, NULL, err);
                        if (ret < 0)
                                goto done;

                        /*
                         * There is no need to write the reference deletion
                         * when the reference in question doesn't exist.
                         */
                         if (u->flags & REF_HAVE_NEW && !is_null_oid(&u->new_oid)) {
                                 ret = queue_transaction_update(refs, tx_data, u,
                                                                &current_oid, err);
                                 if (ret)
                                         goto done;
                         }

                        continue;
                }
                if (ret > 0) {
                        /* The reference does not exist, but we expected it to. */
                        strbuf_addf(err, _("cannot lock ref '%s': "
                                    "unable to resolve reference '%s'"),
                                    original_update_refname(u), u->refname);
                        ret = -1;
                        goto done;
                }

                if (u->type & REF_ISSYMREF) {
                        /*
                         * The reftable stack is locked at this point already,
                         * so it is safe to call `refs_resolve_ref_unsafe()`
                         * here without causing races.
                         */
                        const char *resolved = refs_resolve_ref_unsafe(&refs->base, u->refname, 0,
                                                                       &current_oid, NULL);

                        if (u->flags & REF_NO_DEREF) {
                                if (u->flags & REF_HAVE_OLD && !resolved) {
                                        strbuf_addf(err, _("cannot lock ref '%s': "
                                                    "error reading reference"), u->refname);
                                        ret = -1;
                                        goto done;
                                }
                        } else {
                                struct ref_update *new_update;
                                int new_flags;

                                new_flags = u->flags;
                                if (!strcmp(rewritten_ref, "HEAD"))
                                        new_flags |= REF_UPDATE_VIA_HEAD;

                                /*
                                 * If we are updating a symref (eg. HEAD), we should also
                                 * update the branch that the symref points to.
                                 *
                                 * This is generic functionality, and would be better
                                 * done in refs.c, but the current implementation is
                                 * intertwined with the locking in files-backend.c.
                                 */
                                new_update = ref_transaction_add_update(
                                                transaction, referent.buf, new_flags,
                                                &u->new_oid, &u->old_oid, u->msg);
                                new_update->parent_update = u;

                                /*
                                 * Change the symbolic ref update to log only. Also, it
                                 * doesn't need to check its old OID value, as that will be
                                 * done when new_update is processed.
                                 */
                                u->flags |= REF_LOG_ONLY | REF_NO_DEREF;
                                u->flags &= ~REF_HAVE_OLD;

                                if (string_list_has_string(&affected_refnames, new_update->refname)) {
                                        strbuf_addf(err,
                                                    _("multiple updates for '%s' (including one "
                                                    "via symref '%s') are not allowed"),
                                                    referent.buf, u->refname);
                                        ret = TRANSACTION_NAME_CONFLICT;
                                        goto done;
                                }
                                string_list_insert(&affected_refnames, new_update->refname);
                        }
                }

                /*
                 * Verify that the old object matches our expectations. Note
                 * that the error messages here do not make a lot of sense in
                 * the context of the reftable backend as we never lock
                 * individual refs. But the error messages match what the files
                 * backend returns, which keeps our tests happy.
                 */
                if (u->flags & REF_HAVE_OLD && !oideq(&current_oid, &u->old_oid)) {
                        if (is_null_oid(&u->old_oid))
                                strbuf_addf(err, _("cannot lock ref '%s': "
                                            "reference already exists"),
                                            original_update_refname(u));
                        else if (is_null_oid(&current_oid))
                                strbuf_addf(err, _("cannot lock ref '%s': "
                                            "reference is missing but expected %s"),
                                            original_update_refname(u),
                                            oid_to_hex(&u->old_oid));
                        else
                                strbuf_addf(err, _("cannot lock ref '%s': "
                                            "is at %s but expected %s"),
                                            original_update_refname(u),
                                            oid_to_hex(&current_oid),
                                            oid_to_hex(&u->old_oid));
                        ret = -1;
                        goto done;
                }

                /*
                 * If all of the following conditions are true:
                 *
                 *   - We're not about to write a symref.
                 *   - We're not about to write a log-only entry.
                 *   - Old and new object ID are different.
                 *
                 * Then we're essentially doing a no-op update that can be
                 * skipped. This is not only for the sake of efficiency, but
                 * also skips writing unneeded reflog entries.
                 */
                if ((u->type & REF_ISSYMREF) ||
                    (u->flags & REF_LOG_ONLY) ||
                    (u->flags & REF_HAVE_NEW && !oideq(&current_oid, &u->new_oid))) {
                        ret = queue_transaction_update(refs, tx_data, u,
                                                       &current_oid, err);
                        if (ret)
                                goto done;
                }
        }

        transaction->backend_data = tx_data;
        transaction->state = REF_TRANSACTION_PREPARED;

done:
        assert(ret != REFTABLE_API_ERROR);
        if (ret < 0) {
                free_transaction_data(tx_data);
                transaction->state = REF_TRANSACTION_CLOSED;
                if (!err->len)
                        strbuf_addf(err, _("reftable: transaction prepare: %s"),
                                    reftable_error_str(ret));
        }
        string_list_clear(&affected_refnames, 0);
        strbuf_release(&referent);
        strbuf_release(&head_referent);

        return ret;
}

static int reftable_be_transaction_abort(struct ref_store *ref_store,
                                         struct ref_transaction *transaction,
                                         struct strbuf *err)
{
        struct reftable_transaction_data *tx_data = transaction->backend_data;
        free_transaction_data(tx_data);
        transaction->state = REF_TRANSACTION_CLOSED;
        return 0;
}

static int transaction_update_cmp(const void *a, const void *b)
{
        return strcmp(((struct reftable_transaction_update *)a)->update->refname,
                      ((struct reftable_transaction_update *)b)->update->refname);
}

static int write_transaction_table(struct reftable_writer *writer, void *cb_data)
{
        struct write_transaction_table_arg *arg = cb_data;
        struct reftable_merged_table *mt =
                reftable_stack_merged_table(arg->stack);
        uint64_t ts = reftable_stack_next_update_index(arg->stack);
        struct reftable_log_record *logs = NULL;
        size_t logs_nr = 0, logs_alloc = 0, i;
        int ret = 0;

        QSORT(arg->updates, arg->updates_nr, transaction_update_cmp);

        reftable_writer_set_limits(writer, ts, ts);

        for (i = 0; i < arg->updates_nr; i++) {
                struct reftable_transaction_update *tx_update = &arg->updates[i];
                struct ref_update *u = tx_update->update;

                /*
                 * Write a reflog entry when updating a ref to point to
                 * something new in either of the following cases:
                 *
                 * - The reference is about to be deleted. We always want to
                 *   delete the reflog in that case.
                 * - REF_FORCE_CREATE_REFLOG is set, asking us to always create
                 *   the reflog entry.
                 * - `core.logAllRefUpdates` tells us to create the reflog for
                 *   the given ref.
                 */
                if (u->flags & REF_HAVE_NEW && !(u->type & REF_ISSYMREF) && is_null_oid(&u->new_oid)) {
                        struct reftable_log_record log = {0};
                        struct reftable_iterator it = {0};

                        /*
                         * When deleting refs we also delete all reflog entries
                         * with them. While it is not strictly required to
                         * delete reflogs together with their refs, this
                         * matches the behaviour of the files backend.
                         *
                         * Unfortunately, we have no better way than to delete
                         * all reflog entries one by one.
                         */
                        ret = reftable_merged_table_seek_log(mt, &it, u->refname);
                        while (ret == 0) {
                                struct reftable_log_record *tombstone;

                                ret = reftable_iterator_next_log(&it, &log);
                                if (ret < 0)
                                        break;
                                if (ret > 0 || strcmp(log.refname, u->refname)) {
                                        ret = 0;
                                        break;
                                }

                                ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                                tombstone = &logs[logs_nr++];
                                tombstone->refname = xstrdup(u->refname);
                                tombstone->value_type = REFTABLE_LOG_DELETION;
                                tombstone->update_index = log.update_index;
                        }

                        reftable_log_record_release(&log);
                        reftable_iterator_destroy(&it);

                        if (ret)
                                goto done;
                } else if (u->flags & REF_HAVE_NEW &&
                           (u->flags & REF_FORCE_CREATE_REFLOG ||
                            should_write_log(&arg->refs->base, u->refname))) {
                        struct reftable_log_record *log;

                        ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                        log = &logs[logs_nr++];
                        memset(log, 0, sizeof(*log));

                        fill_reftable_log_record(log);
                        log->update_index = ts;
                        log->refname = xstrdup(u->refname);
                        memcpy(log->value.update.new_hash, u->new_oid.hash, GIT_MAX_RAWSZ);
                        memcpy(log->value.update.old_hash, tx_update->current_oid.hash, GIT_MAX_RAWSZ);
                        log->value.update.message =
                                xstrndup(u->msg, arg->refs->write_options.block_size / 2);
                }

                if (u->flags & REF_LOG_ONLY)
                        continue;

                if (u->flags & REF_HAVE_NEW && is_null_oid(&u->new_oid)) {
                        struct reftable_ref_record ref = {
                                .refname = (char *)u->refname,
                                .update_index = ts,
                                .value_type = REFTABLE_REF_DELETION,
                        };

                        ret = reftable_writer_add_ref(writer, &ref);
                        if (ret < 0)
                                goto done;
                } else if (u->flags & REF_HAVE_NEW) {
                        struct reftable_ref_record ref = {0};
                        struct object_id peeled;
                        int peel_error;

                        ref.refname = (char *)u->refname;
                        ref.update_index = ts;

                        peel_error = peel_object(&u->new_oid, &peeled);
                        if (!peel_error) {
                                ref.value_type = REFTABLE_REF_VAL2;
                                memcpy(ref.value.val2.target_value, peeled.hash, GIT_MAX_RAWSZ);
                                memcpy(ref.value.val2.value, u->new_oid.hash, GIT_MAX_RAWSZ);
                        } else if (!is_null_oid(&u->new_oid)) {
                                ref.value_type = REFTABLE_REF_VAL1;
                                memcpy(ref.value.val1, u->new_oid.hash, GIT_MAX_RAWSZ);
                        }

                        ret = reftable_writer_add_ref(writer, &ref);
                        if (ret < 0)
                                goto done;
                }
        }

        /*
         * Logs are written at the end so that we do not have intermixed ref
         * and log blocks.
         */
        if (logs) {
                ret = reftable_writer_add_logs(writer, logs, logs_nr);
                if (ret < 0)
                        goto done;
        }

done:
        assert(ret != REFTABLE_API_ERROR);
        for (i = 0; i < logs_nr; i++)
                reftable_log_record_release(&logs[i]);
        free(logs);
        return ret;
}

static int reftable_be_transaction_finish(struct ref_store *ref_store,
                                          struct ref_transaction *transaction,
                                          struct strbuf *err)
{
        struct reftable_transaction_data *tx_data = transaction->backend_data;
        int ret = 0;

        for (size_t i = 0; i < tx_data->args_nr; i++) {
                ret = reftable_addition_add(tx_data->args[i].addition,
                                            write_transaction_table, &tx_data->args[i]);
                if (ret < 0)
                        goto done;

                ret = reftable_addition_commit(tx_data->args[i].addition);
                if (ret < 0)
                        goto done;
        }

done:
        assert(ret != REFTABLE_API_ERROR);
        free_transaction_data(tx_data);
        transaction->state = REF_TRANSACTION_CLOSED;

        if (ret) {
                strbuf_addf(err, _("reftable: transaction failure: %s"),
                            reftable_error_str(ret));
                return -1;
        }
        return ret;
}

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

static int reftable_be_pack_refs(struct ref_store *ref_store,
                                 struct pack_refs_opts *opts)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE | REF_STORE_ODB, "pack_refs");
        struct reftable_stack *stack;
        int ret;

        if (refs->err)
                return refs->err;

        stack = refs->worktree_stack;
        if (!stack)
                stack = refs->main_stack;

        if (opts->flags & PACK_REFS_AUTO)
                ret = reftable_stack_auto_compact(stack);
        else
                ret = reftable_stack_compact_all(stack, NULL);
        if (ret < 0) {
                ret = error(_("unable to compact stack: %s"),
                            reftable_error_str(ret));
                goto out;
        }

        ret = reftable_stack_clean(stack);
        if (ret)
                goto out;

out:
        return ret;
}

struct write_create_symref_arg {
        struct reftable_ref_store *refs;
        struct reftable_stack *stack;
        const char *refname;
        const char *target;
        const char *logmsg;
};

static int write_create_symref_table(struct reftable_writer *writer, void *cb_data)
{
        struct write_create_symref_arg *create = cb_data;
        uint64_t ts = reftable_stack_next_update_index(create->stack);
        struct reftable_ref_record ref = {
                .refname = (char *)create->refname,
                .value_type = REFTABLE_REF_SYMREF,
                .value.symref = (char *)create->target,
                .update_index = ts,
        };
        struct reftable_log_record log = {0};
        struct object_id new_oid;
        struct object_id old_oid;
        int ret;

        reftable_writer_set_limits(writer, ts, ts);

        ret = reftable_writer_add_ref(writer, &ref);
        if (ret)
                return ret;

        /*
         * Note that it is important to try and resolve the reference before we
         * write the log entry. This is because `should_write_log()` will munge
         * `core.logAllRefUpdates`, which is undesirable when we create a new
         * repository because it would be written into the config. As HEAD will
         * not resolve for new repositories this ordering will ensure that this
         * never happens.
         */
        if (!create->logmsg ||
            !refs_resolve_ref_unsafe(&create->refs->base, create->target,
                                     RESOLVE_REF_READING, &new_oid, NULL) ||
            !should_write_log(&create->refs->base, create->refname))
                return 0;

        fill_reftable_log_record(&log);
        log.refname = xstrdup(create->refname);
        log.update_index = ts;
        log.value.update.message = xstrndup(create->logmsg,
                                            create->refs->write_options.block_size / 2);
        memcpy(log.value.update.new_hash, new_oid.hash, GIT_MAX_RAWSZ);
        if (refs_resolve_ref_unsafe(&create->refs->base, create->refname,
                                    RESOLVE_REF_READING, &old_oid, NULL))
                memcpy(log.value.update.old_hash, old_oid.hash, GIT_MAX_RAWSZ);

        ret = reftable_writer_add_log(writer, &log);
        reftable_log_record_release(&log);
        return ret;
}

static int reftable_be_create_symref(struct ref_store *ref_store,
                                     const char *refname,
                                     const char *target,
                                     const char *logmsg)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "create_symref");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct write_create_symref_arg arg = {
                .refs = refs,
                .stack = stack,
                .refname = refname,
                .target = target,
                .logmsg = logmsg,
        };
        int ret;

        ret = refs->err;
        if (ret < 0)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret)
                goto done;

        ret = reftable_stack_add(stack, &write_create_symref_table, &arg);

done:
        assert(ret != REFTABLE_API_ERROR);
        if (ret)
                error("unable to write symref for %s: %s", refname,
                      reftable_error_str(ret));
        return ret;
}

struct write_copy_arg {
        struct reftable_ref_store *refs;
        struct reftable_stack *stack;
        const char *oldname;
        const char *newname;
        const char *logmsg;
        int delete_old;
};

static int write_copy_table(struct reftable_writer *writer, void *cb_data)
{
        struct write_copy_arg *arg = cb_data;
        uint64_t deletion_ts, creation_ts;
        struct reftable_merged_table *mt = reftable_stack_merged_table(arg->stack);
        struct reftable_ref_record old_ref = {0}, refs[2] = {0};
        struct reftable_log_record old_log = {0}, *logs = NULL;
        struct reftable_iterator it = {0};
        struct string_list skip = STRING_LIST_INIT_NODUP;
        struct strbuf errbuf = STRBUF_INIT;
        size_t logs_nr = 0, logs_alloc = 0, i;
        int ret;

        if (reftable_stack_read_ref(arg->stack, arg->oldname, &old_ref)) {
                ret = error(_("refname %s not found"), arg->oldname);
                goto done;
        }
        if (old_ref.value_type == REFTABLE_REF_SYMREF) {
                ret = error(_("refname %s is a symbolic ref, copying it is not supported"),
                            arg->oldname);
                goto done;
        }

        /*
         * There's nothing to do in case the old and new name are the same, so
         * we exit early in that case.
         */
        if (!strcmp(arg->oldname, arg->newname)) {
                ret = 0;
                goto done;
        }

        /*
         * Verify that the new refname is available.
         */
        string_list_insert(&skip, arg->oldname);
        ret = refs_verify_refname_available(&arg->refs->base, arg->newname,
                                            NULL, &skip, &errbuf);
        if (ret < 0) {
                error("%s", errbuf.buf);
                goto done;
        }

        /*
         * When deleting the old reference we have to use two update indices:
         * once to delete the old ref and its reflog, and once to create the
         * new ref and its reflog. They need to be staged with two separate
         * indices because the new reflog needs to encode both the deletion of
         * the old branch and the creation of the new branch, and we cannot do
         * two changes to a reflog in a single update.
         */
        deletion_ts = creation_ts = reftable_stack_next_update_index(arg->stack);
        if (arg->delete_old)
                creation_ts++;
        reftable_writer_set_limits(writer, deletion_ts, creation_ts);

        /*
         * Add the new reference. If this is a rename then we also delete the
         * old reference.
         */
        refs[0] = old_ref;
        refs[0].refname = (char *)arg->newname;
        refs[0].update_index = creation_ts;
        if (arg->delete_old) {
                refs[1].refname = (char *)arg->oldname;
                refs[1].value_type = REFTABLE_REF_DELETION;
                refs[1].update_index = deletion_ts;
        }
        ret = reftable_writer_add_refs(writer, refs, arg->delete_old ? 2 : 1);
        if (ret < 0)
                goto done;

        /*
         * When deleting the old branch we need to create a reflog entry on the
         * new branch name that indicates that the old branch has been deleted
         * and then recreated. This is a tad weird, but matches what the files
         * backend does.
         */
        if (arg->delete_old) {
                struct strbuf head_referent = STRBUF_INIT;
                struct object_id head_oid;
                int append_head_reflog;
                unsigned head_type = 0;

                ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                memset(&logs[logs_nr], 0, sizeof(logs[logs_nr]));
                fill_reftable_log_record(&logs[logs_nr]);
                logs[logs_nr].refname = (char *)arg->newname;
                logs[logs_nr].update_index = deletion_ts;
                logs[logs_nr].value.update.message =
                        xstrndup(arg->logmsg, arg->refs->write_options.block_size / 2);
                memcpy(logs[logs_nr].value.update.old_hash, old_ref.value.val1, GIT_MAX_RAWSZ);
                logs_nr++;

                ret = read_ref_without_reload(arg->stack, "HEAD", &head_oid, &head_referent, &head_type);
                if (ret < 0)
                        goto done;
                append_head_reflog = (head_type & REF_ISSYMREF) && !strcmp(head_referent.buf, arg->oldname);
                strbuf_release(&head_referent);

                /*
                 * The files backend uses `refs_delete_ref()` to delete the old
                 * branch name, which will append a reflog entry for HEAD in
                 * case it points to the old branch.
                 */
                if (append_head_reflog) {
                        ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                        logs[logs_nr] = logs[logs_nr - 1];
                        logs[logs_nr].refname = "HEAD";
                        logs_nr++;
                }
        }

        /*
         * Create the reflog entry for the newly created branch.
         */
        ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
        memset(&logs[logs_nr], 0, sizeof(logs[logs_nr]));
        fill_reftable_log_record(&logs[logs_nr]);
        logs[logs_nr].refname = (char *)arg->newname;
        logs[logs_nr].update_index = creation_ts;
        logs[logs_nr].value.update.message =
                xstrndup(arg->logmsg, arg->refs->write_options.block_size / 2);
        memcpy(logs[logs_nr].value.update.new_hash, old_ref.value.val1, GIT_MAX_RAWSZ);
        logs_nr++;

        /*
         * In addition to writing the reflog entry for the new branch, we also
         * copy over all log entries from the old reflog. Last but not least,
         * when renaming we also have to delete all the old reflog entries.
         */
        ret = reftable_merged_table_seek_log(mt, &it, arg->oldname);
        if (ret < 0)
                goto done;

        while (1) {
                ret = reftable_iterator_next_log(&it, &old_log);
                if (ret < 0)
                        goto done;
                if (ret > 0 || strcmp(old_log.refname, arg->oldname)) {
                        ret = 0;
                        break;
                }

                free(old_log.refname);

                /*
                 * Copy over the old reflog entry with the new refname.
                 */
                ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                logs[logs_nr] = old_log;
                logs[logs_nr].refname = (char *)arg->newname;
                logs_nr++;

                /*
                 * Delete the old reflog entry in case we are renaming.
                 */
                if (arg->delete_old) {
                        ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                        memset(&logs[logs_nr], 0, sizeof(logs[logs_nr]));
                        logs[logs_nr].refname = (char *)arg->oldname;
                        logs[logs_nr].value_type = REFTABLE_LOG_DELETION;
                        logs[logs_nr].update_index = old_log.update_index;
                        logs_nr++;
                }

                /*
                 * Transfer ownership of the log record we're iterating over to
                 * the array of log records. Otherwise, the pointers would get
                 * free'd or reallocated by the iterator.
                 */
                memset(&old_log, 0, sizeof(old_log));
        }

        ret = reftable_writer_add_logs(writer, logs, logs_nr);
        if (ret < 0)
                goto done;

done:
        assert(ret != REFTABLE_API_ERROR);
        reftable_iterator_destroy(&it);
        string_list_clear(&skip, 0);
        strbuf_release(&errbuf);
        for (i = 0; i < logs_nr; i++) {
                if (!strcmp(logs[i].refname, "HEAD"))
                        continue;
                logs[i].refname = NULL;
                reftable_log_record_release(&logs[i]);
        }
        free(logs);
        reftable_ref_record_release(&old_ref);
        reftable_log_record_release(&old_log);
        return ret;
}

static int reftable_be_rename_ref(struct ref_store *ref_store,
                                  const char *oldrefname,
                                  const char *newrefname,
                                  const char *logmsg)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "rename_ref");
        struct reftable_stack *stack = stack_for(refs, newrefname, &newrefname);
        struct write_copy_arg arg = {
                .refs = refs,
                .stack = stack,
                .oldname = oldrefname,
                .newname = newrefname,
                .logmsg = logmsg,
                .delete_old = 1,
        };
        int ret;

        ret = refs->err;
        if (ret < 0)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret)
                goto done;
        ret = reftable_stack_add(stack, &write_copy_table, &arg);

done:
        assert(ret != REFTABLE_API_ERROR);
        return ret;
}

static int reftable_be_copy_ref(struct ref_store *ref_store,
                                const char *oldrefname,
                                const char *newrefname,
                                const char *logmsg)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "copy_ref");
        struct reftable_stack *stack = stack_for(refs, newrefname, &newrefname);
        struct write_copy_arg arg = {
                .refs = refs,
                .stack = stack,
                .oldname = oldrefname,
                .newname = newrefname,
                .logmsg = logmsg,
        };
        int ret;

        ret = refs->err;
        if (ret < 0)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret)
                goto done;
        ret = reftable_stack_add(stack, &write_copy_table, &arg);

done:
        assert(ret != REFTABLE_API_ERROR);
        return ret;
}

struct reftable_reflog_iterator {
        struct ref_iterator base;
        struct reftable_ref_store *refs;
        struct reftable_iterator iter;
        struct reftable_log_record log;
        struct strbuf last_name;
        int err;
};

static int reftable_reflog_iterator_advance(struct ref_iterator *ref_iterator)
{
        struct reftable_reflog_iterator *iter =
                (struct reftable_reflog_iterator *)ref_iterator;

        while (!iter->err) {
                iter->err = reftable_iterator_next_log(&iter->iter, &iter->log);
                if (iter->err)
                        break;

                /*
                 * We want the refnames that we have reflogs for, so we skip if
                 * we've already produced this name. This could be faster by
                 * seeking directly to reflog@update_index==0.
                 */
                if (!strcmp(iter->log.refname, iter->last_name.buf))
                        continue;

                if (check_refname_format(iter->log.refname,
                                         REFNAME_ALLOW_ONELEVEL))
                        continue;

                strbuf_reset(&iter->last_name);
                strbuf_addstr(&iter->last_name, iter->log.refname);
                iter->base.refname = iter->log.refname;

                break;
        }

        if (iter->err > 0) {
                if (ref_iterator_abort(ref_iterator) != ITER_DONE)
                        return ITER_ERROR;
                return ITER_DONE;
        }

        if (iter->err < 0) {
                ref_iterator_abort(ref_iterator);
                return ITER_ERROR;
        }

        return ITER_OK;
}

static int reftable_reflog_iterator_peel(struct ref_iterator *ref_iterator,
                                                 struct object_id *peeled)
{
        BUG("reftable reflog iterator cannot be peeled");
        return -1;
}

static int reftable_reflog_iterator_abort(struct ref_iterator *ref_iterator)
{
        struct reftable_reflog_iterator *iter =
                (struct reftable_reflog_iterator *)ref_iterator;
        reftable_log_record_release(&iter->log);
        reftable_iterator_destroy(&iter->iter);
        strbuf_release(&iter->last_name);
        free(iter);
        return ITER_DONE;
}

static struct ref_iterator_vtable reftable_reflog_iterator_vtable = {
        .advance = reftable_reflog_iterator_advance,
        .peel = reftable_reflog_iterator_peel,
        .abort = reftable_reflog_iterator_abort
};

static struct reftable_reflog_iterator *reflog_iterator_for_stack(struct reftable_ref_store *refs,
                                                                  struct reftable_stack *stack)
{
        struct reftable_merged_table *merged_table;
        struct reftable_reflog_iterator *iter;
        int ret;

        iter = xcalloc(1, sizeof(*iter));
        base_ref_iterator_init(&iter->base, &reftable_reflog_iterator_vtable);
        strbuf_init(&iter->last_name, 0);
        iter->refs = refs;

        ret = refs->err;
        if (ret)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret < 0)
                goto done;

        merged_table = reftable_stack_merged_table(stack);

        ret = reftable_merged_table_seek_log(merged_table, &iter->iter, "");
        if (ret < 0)
                goto done;

done:
        iter->err = ret;
        return iter;
}

static struct ref_iterator *reftable_be_reflog_iterator_begin(struct ref_store *ref_store)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_READ, "reflog_iterator_begin");
        struct reftable_reflog_iterator *main_iter, *worktree_iter;

        main_iter = reflog_iterator_for_stack(refs, refs->main_stack);
        if (!refs->worktree_stack)
                return &main_iter->base;

        worktree_iter = reflog_iterator_for_stack(refs, refs->worktree_stack);

        return merge_ref_iterator_begin(&worktree_iter->base, &main_iter->base,
                                        ref_iterator_select, NULL);
}

static int yield_log_record(struct reftable_log_record *log,
                            each_reflog_ent_fn fn,
                            void *cb_data)
{
        struct object_id old_oid, new_oid;
        const char *full_committer;

        oidread(&old_oid, log->value.update.old_hash);
        oidread(&new_oid, log->value.update.new_hash);

        /*
         * When both the old object ID and the new object ID are null
         * then this is the reflog existence marker. The caller must
         * not be aware of it.
         */
        if (is_null_oid(&old_oid) && is_null_oid(&new_oid))
                return 0;

        full_committer = fmt_ident(log->value.update.name, log->value.update.email,
                                   WANT_COMMITTER_IDENT, NULL, IDENT_NO_DATE);
        return fn(&old_oid, &new_oid, full_committer,
                  log->value.update.time, log->value.update.tz_offset,
                  log->value.update.message, cb_data);
}

static int reftable_be_for_each_reflog_ent_reverse(struct ref_store *ref_store,
                                                   const char *refname,
                                                   each_reflog_ent_fn fn,
                                                   void *cb_data)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_READ, "for_each_reflog_ent_reverse");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct reftable_merged_table *mt = NULL;
        struct reftable_log_record log = {0};
        struct reftable_iterator it = {0};
        int ret;

        if (refs->err < 0)
                return refs->err;

        mt = reftable_stack_merged_table(stack);
        ret = reftable_merged_table_seek_log(mt, &it, refname);
        while (!ret) {
                ret = reftable_iterator_next_log(&it, &log);
                if (ret < 0)
                        break;
                if (ret > 0 || strcmp(log.refname, refname)) {
                        ret = 0;
                        break;
                }

                ret = yield_log_record(&log, fn, cb_data);
                if (ret)
                        break;
        }

        reftable_log_record_release(&log);
        reftable_iterator_destroy(&it);
        return ret;
}

static int reftable_be_for_each_reflog_ent(struct ref_store *ref_store,
                                           const char *refname,
                                           each_reflog_ent_fn fn,
                                           void *cb_data)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_READ, "for_each_reflog_ent");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct reftable_merged_table *mt = NULL;
        struct reftable_log_record *logs = NULL;
        struct reftable_iterator it = {0};
        size_t logs_alloc = 0, logs_nr = 0, i;
        int ret;

        if (refs->err < 0)
                return refs->err;

        mt = reftable_stack_merged_table(stack);
        ret = reftable_merged_table_seek_log(mt, &it, refname);
        while (!ret) {
                struct reftable_log_record log = {0};

                ret = reftable_iterator_next_log(&it, &log);
                if (ret < 0)
                        goto done;
                if (ret > 0 || strcmp(log.refname, refname)) {
                        reftable_log_record_release(&log);
                        ret = 0;
                        break;
                }

                ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                logs[logs_nr++] = log;
        }

        for (i = logs_nr; i--;) {
                ret = yield_log_record(&logs[i], fn, cb_data);
                if (ret)
                        goto done;
        }

done:
        reftable_iterator_destroy(&it);
        for (i = 0; i < logs_nr; i++)
                reftable_log_record_release(&logs[i]);
        free(logs);
        return ret;
}

static int reftable_be_reflog_exists(struct ref_store *ref_store,
                                     const char *refname)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_READ, "reflog_exists");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct reftable_merged_table *mt = reftable_stack_merged_table(stack);
        struct reftable_log_record log = {0};
        struct reftable_iterator it = {0};
        int ret;

        ret = refs->err;
        if (ret < 0)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret < 0)
                goto done;

        ret = reftable_merged_table_seek_log(mt, &it, refname);
        if (ret < 0)
                goto done;

        /*
         * Check whether we get at least one log record for the given ref name.
         * If so, the reflog exists, otherwise it doesn't.
         */
        ret = reftable_iterator_next_log(&it, &log);
        if (ret < 0)
                goto done;
        if (ret > 0) {
                ret = 0;
                goto done;
        }

        ret = strcmp(log.refname, refname) == 0;

done:
        reftable_iterator_destroy(&it);
        reftable_log_record_release(&log);
        if (ret < 0)
                ret = 0;
        return ret;
}

struct write_reflog_existence_arg {
        struct reftable_ref_store *refs;
        const char *refname;
        struct reftable_stack *stack;
};

static int write_reflog_existence_table(struct reftable_writer *writer,
                                        void *cb_data)
{
        struct write_reflog_existence_arg *arg = cb_data;
        uint64_t ts = reftable_stack_next_update_index(arg->stack);
        struct reftable_log_record log = {0};
        int ret;

        ret = reftable_stack_read_log(arg->stack, arg->refname, &log);
        if (ret <= 0)
                goto done;

        reftable_writer_set_limits(writer, ts, ts);

        /*
         * The existence entry has both old and new object ID set to the the
         * null object ID. Our iterators are aware of this and will not present
         * them to their callers.
         */
        log.refname = xstrdup(arg->refname);
        log.update_index = ts;
        log.value_type = REFTABLE_LOG_UPDATE;
        ret = reftable_writer_add_log(writer, &log);

done:
        assert(ret != REFTABLE_API_ERROR);
        reftable_log_record_release(&log);
        return ret;
}

static int reftable_be_create_reflog(struct ref_store *ref_store,
                                     const char *refname,
                                     struct strbuf *errmsg)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "create_reflog");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct write_reflog_existence_arg arg = {
                .refs = refs,
                .stack = stack,
                .refname = refname,
        };
        int ret;

        ret = refs->err;
        if (ret < 0)
                goto done;

        ret = reftable_stack_reload(stack);
        if (ret)
                goto done;

        ret = reftable_stack_add(stack, &write_reflog_existence_table, &arg);

done:
        return ret;
}

struct write_reflog_delete_arg {
        struct reftable_stack *stack;
        const char *refname;
};

static int write_reflog_delete_table(struct reftable_writer *writer, void *cb_data)
{
        struct write_reflog_delete_arg *arg = cb_data;
        struct reftable_merged_table *mt =
                reftable_stack_merged_table(arg->stack);
        struct reftable_log_record log = {0}, tombstone = {0};
        struct reftable_iterator it = {0};
        uint64_t ts = reftable_stack_next_update_index(arg->stack);
        int ret;

        reftable_writer_set_limits(writer, ts, ts);

        /*
         * In order to delete a table we need to delete all reflog entries one
         * by one. This is inefficient, but the reftable format does not have a
         * better marker right now.
         */
        ret = reftable_merged_table_seek_log(mt, &it, arg->refname);
        while (ret == 0) {
                ret = reftable_iterator_next_log(&it, &log);
                if (ret < 0)
                        break;
                if (ret > 0 || strcmp(log.refname, arg->refname)) {
                        ret = 0;
                        break;
                }

                tombstone.refname = (char *)arg->refname;
                tombstone.value_type = REFTABLE_LOG_DELETION;
                tombstone.update_index = log.update_index;

                ret = reftable_writer_add_log(writer, &tombstone);
        }

        reftable_log_record_release(&log);
        reftable_iterator_destroy(&it);
        return ret;
}

static int reftable_be_delete_reflog(struct ref_store *ref_store,
                                     const char *refname)
{
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "delete_reflog");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct write_reflog_delete_arg arg = {
                .stack = stack,
                .refname = refname,
        };
        int ret;

        ret = reftable_stack_reload(stack);
        if (ret)
                return ret;
        ret = reftable_stack_add(stack, &write_reflog_delete_table, &arg);

        assert(ret != REFTABLE_API_ERROR);
        return ret;
}

struct reflog_expiry_arg {
        struct reftable_stack *stack;
        struct reftable_log_record *records;
        struct object_id update_oid;
        const char *refname;
        size_t len;
};

static int write_reflog_expiry_table(struct reftable_writer *writer, void *cb_data)
{
        struct reflog_expiry_arg *arg = cb_data;
        uint64_t ts = reftable_stack_next_update_index(arg->stack);
        uint64_t live_records = 0;
        size_t i;
        int ret;

        for (i = 0; i < arg->len; i++)
                if (arg->records[i].value_type == REFTABLE_LOG_UPDATE)
                        live_records++;

        reftable_writer_set_limits(writer, ts, ts);

        if (!is_null_oid(&arg->update_oid)) {
                struct reftable_ref_record ref = {0};
                struct object_id peeled;

                ref.refname = (char *)arg->refname;
                ref.update_index = ts;

                if (!peel_object(&arg->update_oid, &peeled)) {
                        ref.value_type = REFTABLE_REF_VAL2;
                        memcpy(ref.value.val2.target_value, peeled.hash, GIT_MAX_RAWSZ);
                        memcpy(ref.value.val2.value, arg->update_oid.hash, GIT_MAX_RAWSZ);
                } else {
                        ref.value_type = REFTABLE_REF_VAL1;
                        memcpy(ref.value.val1, arg->update_oid.hash, GIT_MAX_RAWSZ);
                }

                ret = reftable_writer_add_ref(writer, &ref);
                if (ret < 0)
                        return ret;
        }

        /*
         * When there are no more entries left in the reflog we empty it
         * completely, but write a placeholder reflog entry that indicates that
         * the reflog still exists.
         */
        if (!live_records) {
                struct reftable_log_record log = {
                        .refname = (char *)arg->refname,
                        .value_type = REFTABLE_LOG_UPDATE,
                        .update_index = ts,
                };

                ret = reftable_writer_add_log(writer, &log);
                if (ret)
                        return ret;
        }

        for (i = 0; i < arg->len; i++) {
                ret = reftable_writer_add_log(writer, &arg->records[i]);
                if (ret)
                        return ret;
        }

        return 0;
}

static int reftable_be_reflog_expire(struct ref_store *ref_store,
                                     const char *refname,
                                     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)
{
        /*
         * For log expiry, we write tombstones for every single reflog entry
         * that is to be expired. This means that the entries are still
         * retrievable by delving into the stack, and expiring entries
         * paradoxically takes extra memory. This memory is only reclaimed when
         * compacting the reftable stack.
         *
         * It would be better if the refs backend supported an API that sets a
         * criterion for all refs, passing the criterion to pack_refs().
         *
         * On the plus side, because we do the expiration per ref, we can easily
         * insert the reflog existence dummies.
         */
        struct reftable_ref_store *refs =
                reftable_be_downcast(ref_store, REF_STORE_WRITE, "reflog_expire");
        struct reftable_stack *stack = stack_for(refs, refname, &refname);
        struct reftable_merged_table *mt = reftable_stack_merged_table(stack);
        struct reftable_log_record *logs = NULL;
        struct reftable_log_record *rewritten = NULL;
        struct reftable_ref_record ref_record = {0};
        struct reftable_iterator it = {0};
        struct reftable_addition *add = NULL;
        struct reflog_expiry_arg arg = {0};
        struct object_id oid = {0};
        uint8_t *last_hash = NULL;
        size_t logs_nr = 0, logs_alloc = 0, i;
        int ret;

        if (refs->err < 0)
                return refs->err;

        ret = reftable_stack_reload(stack);
        if (ret < 0)
                goto done;

        ret = reftable_merged_table_seek_log(mt, &it, refname);
        if (ret < 0)
                goto done;

        ret = reftable_stack_new_addition(&add, stack);
        if (ret < 0)
                goto done;

        ret = reftable_stack_read_ref(stack, refname, &ref_record);
        if (ret < 0)
                goto done;
        if (reftable_ref_record_val1(&ref_record))
                oidread(&oid, reftable_ref_record_val1(&ref_record));
        prepare_fn(refname, &oid, policy_cb_data);

        while (1) {
                struct reftable_log_record log = {0};
                struct object_id old_oid, new_oid;

                ret = reftable_iterator_next_log(&it, &log);
                if (ret < 0)
                        goto done;
                if (ret > 0 || strcmp(log.refname, refname)) {
                        reftable_log_record_release(&log);
                        break;
                }

                oidread(&old_oid, log.value.update.old_hash);
                oidread(&new_oid, log.value.update.new_hash);

                /*
                 * Skip over the reflog existence marker. We will add it back
                 * in when there are no live reflog records.
                 */
                if (is_null_oid(&old_oid) && is_null_oid(&new_oid)) {
                        reftable_log_record_release(&log);
                        continue;
                }

                ALLOC_GROW(logs, logs_nr + 1, logs_alloc);
                logs[logs_nr++] = log;
        }

        /*
         * We need to rewrite all reflog entries according to the pruning
         * callback function:
         *
         *   - If a reflog entry shall be pruned we mark the record for
         *     deletion.
         *
         *   - Otherwise we may have to rewrite the chain of reflog entries so
         *     that gaps created by just-deleted records get backfilled.
         */
        CALLOC_ARRAY(rewritten, logs_nr);
        for (i = logs_nr; i--;) {
                struct reftable_log_record *dest = &rewritten[i];
                struct object_id old_oid, new_oid;

                *dest = logs[i];
                oidread(&old_oid, logs[i].value.update.old_hash);
                oidread(&new_oid, logs[i].value.update.new_hash);

                if (should_prune_fn(&old_oid, &new_oid, logs[i].value.update.email,
                                    (timestamp_t)logs[i].value.update.time,
                                    logs[i].value.update.tz_offset,
                                    logs[i].value.update.message,
                                    policy_cb_data)) {
                        dest->value_type = REFTABLE_LOG_DELETION;
                } else {
                        if ((flags & EXPIRE_REFLOGS_REWRITE) && last_hash)
                                memcpy(dest->value.update.old_hash, last_hash, GIT_MAX_RAWSZ);
                        last_hash = logs[i].value.update.new_hash;
                }
        }

        if (flags & EXPIRE_REFLOGS_UPDATE_REF && last_hash &&
            reftable_ref_record_val1(&ref_record))
                oidread(&arg.update_oid, last_hash);

        arg.records = rewritten;
        arg.len = logs_nr;
        arg.stack = stack,
        arg.refname = refname,

        ret = reftable_addition_add(add, &write_reflog_expiry_table, &arg);
        if (ret < 0)
                goto done;

        /*
         * Future improvement: we could skip writing records that were
         * not changed.
         */
        if (!(flags & EXPIRE_REFLOGS_DRY_RUN))
                ret = reftable_addition_commit(add);

done:
        if (add)
                cleanup_fn(policy_cb_data);
        assert(ret != REFTABLE_API_ERROR);

        reftable_ref_record_release(&ref_record);
        reftable_iterator_destroy(&it);
        reftable_addition_destroy(add);
        for (i = 0; i < logs_nr; i++)
                reftable_log_record_release(&logs[i]);
        free(logs);
        free(rewritten);
        return ret;
}

struct ref_storage_be refs_be_reftable = {
        .name = "reftable",
        .init = reftable_be_init,
        .init_db = reftable_be_init_db,
        .transaction_prepare = reftable_be_transaction_prepare,
        .transaction_finish = reftable_be_transaction_finish,
        .transaction_abort = reftable_be_transaction_abort,
        .initial_transaction_commit = reftable_be_initial_transaction_commit,

        .pack_refs = reftable_be_pack_refs,
        .create_symref = reftable_be_create_symref,
        .rename_ref = reftable_be_rename_ref,
        .copy_ref = reftable_be_copy_ref,

        .iterator_begin = reftable_be_iterator_begin,
        .read_raw_ref = reftable_be_read_raw_ref,
        .read_symbolic_ref = reftable_be_read_symbolic_ref,

        .reflog_iterator_begin = reftable_be_reflog_iterator_begin,
        .for_each_reflog_ent = reftable_be_for_each_reflog_ent,
        .for_each_reflog_ent_reverse = reftable_be_for_each_reflog_ent_reverse,
        .reflog_exists = reftable_be_reflog_exists,
        .create_reflog = reftable_be_create_reflog,
        .delete_reflog = reftable_be_delete_reflog,
        .reflog_expire = reftable_be_reflog_expire,
};