/*
 * Copyright 2020 Google LLC
 *
 * Use of this source code is governed by a BSD-style
 * license that can be found in the LICENSE file or at
 * https://developers.google.com/open-source/licenses/bsd
 */

#include "stack.h"

#include "system.h"
#include "constants.h"
#include "merged.h"
#include "reftable-error.h"
#include "reftable-record.h"
#include "reftable-merged.h"
#include "table.h"
#include "writer.h"

static int stack_filename(struct reftable_buf *dest, struct reftable_stack *st,
			  const char *name)
{
	int err;
	reftable_buf_reset(dest);
	if ((err = reftable_buf_addstr(dest, st->reftable_dir)) < 0 ||
	    (err = reftable_buf_addstr(dest, "/")) < 0 ||
	    (err = reftable_buf_addstr(dest, name)) < 0)
		return err;
	return 0;
}

static ssize_t reftable_write_data(int fd, const void *data, size_t size)
{
	size_t total_written = 0;
	const char *p = data;

	while (total_written < size) {
		ssize_t bytes_written = write(fd, p, size - total_written);
		if (bytes_written < 0 && (errno == EAGAIN || errno == EINTR))
			continue;
		if (bytes_written < 0)
			return REFTABLE_IO_ERROR;

		total_written += bytes_written;
		p += bytes_written;
	}

	return total_written;
}

struct fd_writer {
	const struct reftable_write_options *opts;
	int fd;
};

static ssize_t fd_writer_write(void *arg, const void *data, size_t sz)
{
	struct fd_writer *writer = arg;
	return reftable_write_data(writer->fd, data, sz);
}

static int fd_writer_flush(void *arg)
{
	struct fd_writer *writer = arg;
	return fsync(writer->fd);
}

static int fd_read_lines(int fd, char ***namesp)
{
	char *buf = NULL;
	int err = 0;
	off_t size;

	size = lseek(fd, 0, SEEK_END);
	if (size < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}

	err = lseek(fd, 0, SEEK_SET);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}

	REFTABLE_ALLOC_ARRAY(buf, size + 1);
	if (!buf) {
		err = REFTABLE_OUT_OF_MEMORY_ERROR;
		goto done;
	}

	for (off_t total_read = 0; total_read < size; ) {
		ssize_t bytes_read = read(fd, buf + total_read, size - total_read);
		if (bytes_read < 0 && (errno == EAGAIN || errno == EINTR))
			continue;
		if (bytes_read < 0 || !bytes_read) {
			err = REFTABLE_IO_ERROR;
			goto done;
		}

		total_read += bytes_read;
	}
	buf[size] = 0;

	err = parse_names(buf, size, namesp);
done:
	reftable_free(buf);
	return err;
}

int read_lines(const char *filename, char ***namesp)
{
	int fd = open(filename, O_RDONLY);
	int err = 0;
	if (fd < 0) {
		if (errno == ENOENT) {
			REFTABLE_CALLOC_ARRAY(*namesp, 1);
			if (!*namesp)
				return REFTABLE_OUT_OF_MEMORY_ERROR;
			return 0;
		}

		return REFTABLE_IO_ERROR;
	}
	err = fd_read_lines(fd, namesp);
	close(fd);
	return err;
}

int reftable_stack_init_ref_iterator(struct reftable_stack *st,
				      struct reftable_iterator *it)
{
	return merged_table_init_iter(reftable_stack_merged_table(st),
				      it, REFTABLE_BLOCK_TYPE_REF);
}

int reftable_stack_init_log_iterator(struct reftable_stack *st,
				     struct reftable_iterator *it)
{
	return merged_table_init_iter(reftable_stack_merged_table(st),
				      it, REFTABLE_BLOCK_TYPE_LOG);
}

struct reftable_merged_table *
reftable_stack_merged_table(struct reftable_stack *st)
{
	return st->merged;
}

static int has_name(char **names, const char *name)
{
	while (*names) {
		if (!strcmp(*names, name))
			return 1;
		names++;
	}
	return 0;
}

/* Close and free the stack */
void reftable_stack_destroy(struct reftable_stack *st)
{
	char **names = NULL;
	int err = 0;

	if (!st)
		return;

	if (st->merged) {
		reftable_merged_table_free(st->merged);
		st->merged = NULL;
	}

	if (st->list_file)
		err = read_lines(st->list_file, &names);
	if (err < 0) {
		REFTABLE_FREE_AND_NULL(names);
	}

	if (st->tables) {
		struct reftable_buf filename = REFTABLE_BUF_INIT;

		for (size_t i = 0; i < st->tables_len; i++) {
			const char *name = reftable_table_name(st->tables[i]);
			int try_unlinking = 1;

			reftable_buf_reset(&filename);
			if (names && !has_name(names, name)) {
				if (stack_filename(&filename, st, name) < 0)
					try_unlinking = 0;
			}
			reftable_table_decref(st->tables[i]);

			if (try_unlinking && filename.len) {
				/* On Windows, can only unlink after closing. */
				unlink(filename.buf);
			}
		}

		reftable_buf_release(&filename);
		st->tables_len = 0;
		REFTABLE_FREE_AND_NULL(st->tables);
	}

	if (st->list_fd >= 0) {
		close(st->list_fd);
		st->list_fd = -1;
	}

	REFTABLE_FREE_AND_NULL(st->list_file);
	REFTABLE_FREE_AND_NULL(st->reftable_dir);
	reftable_free(st);
	free_names(names);
}

static struct reftable_table **stack_copy_tables(struct reftable_stack *st,
						 size_t cur_len)
{
	struct reftable_table **cur = reftable_calloc(cur_len, sizeof(*cur));
	if (!cur)
		return NULL;
	for (size_t i = 0; i < cur_len; i++)
		cur[i] = st->tables[i];
	return cur;
}

static int reftable_stack_reload_once(struct reftable_stack *st,
				      const char **names,
				      int reuse_open)
{
	size_t cur_len = !st->merged ? 0 : st->merged->tables_len;
	struct reftable_table **cur = NULL;
	struct reftable_table **reused = NULL;
	struct reftable_table **new_tables = NULL;
	size_t reused_len = 0, reused_alloc = 0, names_len;
	size_t new_tables_len = 0;
	struct reftable_merged_table *new_merged = NULL;
	struct reftable_buf table_path = REFTABLE_BUF_INIT;
	int err = 0;
	size_t i;

	if (cur_len) {
		cur = stack_copy_tables(st, cur_len);
		if (!cur) {
			err = REFTABLE_OUT_OF_MEMORY_ERROR;
			goto done;
		}
	}

	names_len = names_length(names);

	if (names_len) {
		new_tables = reftable_calloc(names_len, sizeof(*new_tables));
		if (!new_tables) {
			err = REFTABLE_OUT_OF_MEMORY_ERROR;
			goto done;
		}
	}

	while (*names) {
		struct reftable_table *table = NULL;
		const char *name = *names++;

		/* this is linear; we assume compaction keeps the number of
		   tables under control so this is not quadratic. */
		for (i = 0; reuse_open && i < cur_len; i++) {
			if (cur[i] && 0 == strcmp(cur[i]->name, name)) {
				table = cur[i];
				cur[i] = NULL;

				/*
				 * When reloading the stack fails, we end up
				 * releasing all new tables. This also
				 * includes the reused tables, even though
				 * they are still in used by the old stack. We
				 * thus need to keep them alive here, which we
				 * do by bumping their refcount.
				 */
				REFTABLE_ALLOC_GROW_OR_NULL(reused,
							    reused_len + 1,
							    reused_alloc);
				if (!reused) {
					err = REFTABLE_OUT_OF_MEMORY_ERROR;
					goto done;
				}
				reused[reused_len++] = table;
				reftable_table_incref(table);
				break;
			}
		}

		if (!table) {
			struct reftable_block_source src = { NULL };

			err = stack_filename(&table_path, st, name);
			if (err < 0)
				goto done;

			err = reftable_block_source_from_file(&src,
							      table_path.buf);
			if (err < 0)
				goto done;

			err = reftable_table_new(&table, &src, name);
			if (err < 0)
				goto done;
		}

		new_tables[new_tables_len] = table;
		new_tables_len++;
	}

	/* success! */
	err = reftable_merged_table_new(&new_merged, new_tables,
					new_tables_len, st->opts.hash_id);
	if (err < 0)
		goto done;

	/*
	 * Close the old, non-reused tables and proactively try to unlink
	 * them. This is done for systems like Windows, where the underlying
	 * file of such an open table wouldn't have been possible to be
	 * unlinked by the compacting process.
	 */
	for (i = 0; i < cur_len; i++) {
		if (cur[i]) {
			const char *name = reftable_table_name(cur[i]);

			err = stack_filename(&table_path, st, name);
			if (err < 0)
				goto done;

			reftable_table_decref(cur[i]);
			unlink(table_path.buf);
		}
	}

	/* Update the stack to point to the new tables. */
	if (st->merged)
		reftable_merged_table_free(st->merged);
	new_merged->suppress_deletions = st->opts.suppress_deletions;
	st->merged = new_merged;

	if (st->tables)
		reftable_free(st->tables);
	st->tables = new_tables;
	st->tables_len = new_tables_len;
	new_tables = NULL;
	new_tables_len = 0;

	/*
	 * Decrement the refcount of reused tables again. This only needs to
	 * happen on the successful case, because on the unsuccessful one we
	 * decrement their refcount via `new_tables`.
	 */
	for (i = 0; i < reused_len; i++)
		reftable_table_decref(reused[i]);

done:
	for (i = 0; i < new_tables_len; i++)
		reftable_table_decref(new_tables[i]);
	reftable_free(new_tables);
	reftable_free(reused);
	reftable_free(cur);
	reftable_buf_release(&table_path);
	return err;
}

static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
					     int reuse_open)
{
	char **names = NULL, **names_after = NULL;
	uint64_t deadline;
	int64_t delay = 0;
	int tries = 0, err;
	int fd = -1;

	deadline = reftable_time_ms() + 3000;

	while (1) {
		uint64_t now = reftable_time_ms();

		/*
		 * Only look at deadlines after the first few times. This
		 * simplifies debugging in GDB.
		 */
		tries++;
		if (tries > 3 && now >= deadline)
			goto out;

		fd = open(st->list_file, O_RDONLY);
		if (fd < 0) {
			if (errno != ENOENT) {
				err = REFTABLE_IO_ERROR;
				goto out;
			}

			REFTABLE_CALLOC_ARRAY(names, 1);
			if (!names) {
				err = REFTABLE_OUT_OF_MEMORY_ERROR;
				goto out;
			}
		} else {
			err = fd_read_lines(fd, &names);
			if (err < 0)
				goto out;
		}

		err = reftable_stack_reload_once(st, (const char **) names, reuse_open);
		if (!err)
			break;
		if (err != REFTABLE_NOT_EXIST_ERROR)
			goto out;

		/*
		 * REFTABLE_NOT_EXIST_ERROR can be caused by a concurrent
		 * writer. Check if there was one by checking if the name list
		 * changed.
		 */
		err = read_lines(st->list_file, &names_after);
		if (err < 0)
			goto out;
		if (names_equal((const char **) names_after,
				(const char **) names)) {
			err = REFTABLE_NOT_EXIST_ERROR;
			goto out;
		}

		free_names(names);
		names = NULL;
		free_names(names_after);
		names_after = NULL;
		close(fd);
		fd = -1;

		delay = delay + (delay * reftable_rand()) / UINT32_MAX + 1;
		poll(NULL, 0, delay);
	}

out:
	/*
	 * Invalidate the stat cache. It is sufficient to only close the file
	 * descriptor and keep the cached stat info because we never use the
	 * latter when the former is negative.
	 */
	if (st->list_fd >= 0) {
		close(st->list_fd);
		st->list_fd = -1;
	}

	/*
	 * Cache stat information in case it provides a useful signal to us.
	 * According to POSIX, "The st_ino and st_dev fields taken together
	 * uniquely identify the file within the system." That being said,
	 * Windows is not POSIX compliant and we do not have these fields
	 * available. So the information we have there is insufficient to
	 * determine whether two file descriptors point to the same file.
	 *
	 * While we could fall back to using other signals like the file's
	 * mtime, those are not sufficient to avoid races. We thus refrain from
	 * using the stat cache on such systems and fall back to the secondary
	 * caching mechanism, which is to check whether contents of the file
	 * have changed.
	 *
	 * On other systems which are POSIX compliant we must keep the file
	 * descriptor open. This is to avoid a race condition where two
	 * processes access the reftable stack at the same point in time:
	 *
	 *   1. A reads the reftable stack and caches its stat info.
	 *
	 *   2. B updates the stack, appending a new table to "tables.list".
	 *      This will both use a new inode and result in a different file
	 *      size, thus invalidating A's cache in theory.
	 *
	 *   3. B decides to auto-compact the stack and merges two tables. The
	 *      file size now matches what A has cached again. Furthermore, the
	 *      filesystem may decide to recycle the inode number of the file
	 *      we have replaced in (2) because it is not in use anymore.
	 *
	 *   4. A reloads the reftable stack. Neither the inode number nor the
	 *      file size changed. If the timestamps did not change either then
	 *      we think the cached copy of our stack is up-to-date.
	 *
	 * By keeping the file descriptor open the inode number cannot be
	 * recycled, mitigating the race.
	 */
	if (!err && fd >= 0 && !fstat(fd, &st->list_st) &&
	    st->list_st.st_dev && st->list_st.st_ino) {
		st->list_fd = fd;
		fd = -1;
	}

	if (fd >= 0)
		close(fd);
	free_names(names);
	free_names(names_after);

	if (st->opts.on_reload)
		st->opts.on_reload(st->opts.on_reload_payload);

	return err;
}

int reftable_new_stack(struct reftable_stack **dest, const char *dir,
		       const struct reftable_stack_options *_opts)
{
	struct reftable_buf list_file_name = REFTABLE_BUF_INIT;
	struct reftable_stack_options opts = { 0 };
	struct reftable_stack *p;
	int err;

	p = reftable_calloc(1, sizeof(*p));
	if (!p) {
		err = REFTABLE_OUT_OF_MEMORY_ERROR;
		goto out;
	}

	if (_opts)
		opts = *_opts;
	if (opts.hash_id == 0)
		opts.hash_id = REFTABLE_HASH_SHA1;

	*dest = NULL;

	reftable_buf_reset(&list_file_name);
	if ((err = reftable_buf_addstr(&list_file_name, dir)) < 0 ||
	    (err = reftable_buf_addstr(&list_file_name, "/tables.list")) < 0)
		goto out;

	p->list_file = reftable_buf_detach(&list_file_name);
	p->list_fd = -1;
	p->opts = opts;
	p->reftable_dir = reftable_strdup(dir);
	if (!p->reftable_dir) {
		err = REFTABLE_OUT_OF_MEMORY_ERROR;
		goto out;
	}

	p->list_lock = REFTABLE_FLOCK_INIT;

	err = reftable_stack_reload_maybe_reuse(p, 1);
	if (err < 0)
		goto out;

	*dest = p;
	err = 0;

out:
	if (err < 0)
		reftable_stack_destroy(p);
	return err;
}

/*
 * Check whether the given stack is up-to-date with what we have in memory.
 * If skip_if_locked is set skip stack reloading if the stack is currently
 * locked. Stack reloading must _not_ be skipped right after obtaining the
 * lock, to check for concurrent updates which may have happened.
 *
 * Returns 0 if so, 1 if the stack is out-of-date or a negative error code
 * otherwise.
 */
static int stack_uptodate(struct reftable_stack *st, int skip_if_locked)
{
	char **names = NULL;
	int err;

	if (skip_if_locked && st->list_lock.fd != -1)
		return 0;

	/*
	 * When we have cached stat information available then we use it to
	 * verify whether the file has been rewritten.
	 *
	 * Note that we explicitly do not want to use `stat_validity_check()`
	 * and friends here because they may end up not comparing the `st_dev`
	 * and `st_ino` fields. These functions thus cannot guarantee that we
	 * indeed still have the same file.
	 */
	if (st->list_fd >= 0) {
		struct stat list_st;

		if (stat(st->list_file, &list_st) < 0) {
			/*
			 * It's fine for "tables.list" to not exist. In that
			 * case, we have to refresh when the loaded stack has
			 * any tables.
			 */
			if (errno == ENOENT)
				return !!st->tables_len;
			return REFTABLE_IO_ERROR;
		}

		/*
		 * When "tables.list" refers to the same file we can assume
		 * that it didn't change. This is because we always use
		 * rename(3P) to update the file and never write to it
		 * directly.
		 */
		if (st->list_st.st_dev == list_st.st_dev &&
		    st->list_st.st_ino == list_st.st_ino)
			return 0;
	}

	err = read_lines(st->list_file, &names);
	if (err < 0)
		return err;

	for (size_t i = 0; i < st->tables_len; i++) {
		if (!names[i]) {
			err = 1;
			goto done;
		}

		if (strcmp(st->tables[i]->name, names[i])) {
			err = 1;
			goto done;
		}
	}

	if (names[st->merged->tables_len]) {
		err = 1;
		goto done;
	}

done:
	free_names(names);
	return err;
}

int reftable_stack_reload(struct reftable_stack *st)
{
	int err = stack_uptodate(st, 1);
	if (err > 0)
		return reftable_stack_reload_maybe_reuse(st, 1);
	return err;
}

struct reftable_addition {
	struct reftable_stack *stack;
	struct reftable_write_options opts;

	/*
	 * While the list lock is acquired on the stack, we need to distinguish
	 * which 'reftable_addition' is responsible for the lock. This avoids
	 * clearing the lock of another 'reftable_addition'.
	 */
	unsigned int locked : 1;

	char **new_tables;
	size_t new_tables_len, new_tables_cap;
	uint64_t next_update_index;
};

static void reftable_addition_close(struct reftable_addition *add)
{
	struct reftable_buf nm = REFTABLE_BUF_INIT;
	size_t i;

	for (i = 0; i < add->new_tables_len; i++) {
		if (!stack_filename(&nm, add->stack, add->new_tables[i]))
			unlink(nm.buf);
		reftable_free(add->new_tables[i]);
		add->new_tables[i] = NULL;
	}
	reftable_free(add->new_tables);
	add->new_tables = NULL;
	add->new_tables_len = 0;
	add->new_tables_cap = 0;

	if (add->locked)
		flock_release(&add->stack->list_lock);
	add->locked = 0;
	reftable_buf_release(&nm);
}

static int reftable_stack_init_addition(struct reftable_addition *add,
					struct reftable_stack *st,
					const struct reftable_write_options *opts)
{
	struct reftable_buf lock_file_name = REFTABLE_BUF_INIT;
	int err;

	memset(add, 0, sizeof(*add));
	add->stack = st;
	if (opts)
		add->opts = *opts;

	err = flock_acquire(&add->stack->list_lock, st->list_file,
			    add->opts.lock_timeout_ms);
	if (err < 0)
		goto done;
	add->locked = 1;

	if (add->opts.default_permissions) {
		if (chmod(add->stack->list_lock.path,
			  add->opts.default_permissions) < 0) {
			err = REFTABLE_IO_ERROR;
			goto done;
		}
	}

	err = stack_uptodate(st, 0);
	if (err < 0)
		goto done;
	if (err > 0) {
		err = reftable_stack_reload_maybe_reuse(add->stack, 1);
		if (err)
			goto done;
	}

	add->next_update_index = reftable_stack_next_update_index(st);
done:
	if (err)
		reftable_addition_close(add);
	reftable_buf_release(&lock_file_name);
	return err;
}

static int stack_try_add(struct reftable_stack *st,
			 int (*write_table)(struct reftable_writer *wr,
					    void *arg),
			 void *arg,
			 const struct reftable_write_options *opts)
{
	struct reftable_addition add;
	int err;

	err = reftable_stack_init_addition(&add, st, opts);
	if (err < 0)
		goto done;

	err = reftable_addition_add(&add, write_table, arg);
	if (err < 0)
		goto done;

	err = reftable_addition_commit(&add);
done:
	reftable_addition_close(&add);
	return err;
}

int reftable_stack_add(struct reftable_stack *st,
		       int (*write)(struct reftable_writer *wr, void *arg),
		       void *arg,
		       const struct reftable_write_options *opts)
{
	int err = stack_try_add(st, write, arg, opts);
	if (err < 0) {
		return err;
	}

	return 0;
}

static int format_name(struct reftable_buf *dest, uint64_t min, uint64_t max)
{
	char buf[100];
	uint32_t rnd = reftable_rand();
	snprintf(buf, sizeof(buf), "0x%012" PRIx64 "-0x%012" PRIx64 "-%08x",
		 min, max, rnd);
	reftable_buf_reset(dest);
	return reftable_buf_addstr(dest, buf);
}

void reftable_addition_destroy(struct reftable_addition *add)
{
	if (!add) {
		return;
	}
	reftable_addition_close(add);
	reftable_free(add);
}

int reftable_addition_commit(struct reftable_addition *add)
{
	struct reftable_buf table_list = REFTABLE_BUF_INIT;
	int err = 0;
	size_t i;

	if (add->new_tables_len == 0)
		goto done;

	for (i = 0; i < add->stack->merged->tables_len; i++) {
		if ((err = reftable_buf_addstr(&table_list, add->stack->tables[i]->name)) < 0 ||
		    (err = reftable_buf_addstr(&table_list, "\n")) < 0)
			goto done;
	}
	for (i = 0; i < add->new_tables_len; i++) {
		if ((err = reftable_buf_addstr(&table_list, add->new_tables[i])) < 0 ||
		    (err = reftable_buf_addstr(&table_list, "\n")) < 0)
			goto done;
	}

	err = reftable_write_data(add->stack->list_lock.fd,
				  table_list.buf, table_list.len);
	reftable_buf_release(&table_list);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}

	err = fsync(add->stack->list_lock.fd);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}

	err = flock_commit(&add->stack->list_lock);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}
	add->locked = 0;

	/* success, no more state to clean up. */
	for (i = 0; i < add->new_tables_len; i++)
		reftable_free(add->new_tables[i]);
	reftable_free(add->new_tables);
	add->new_tables = NULL;
	add->new_tables_len = 0;
	add->new_tables_cap = 0;

	err = reftable_stack_reload_maybe_reuse(add->stack, 1);
	if (err)
		goto done;

	if (!add->opts.disable_auto_compact) {
		/*
		 * Auto-compact the stack to keep the number of tables in
		 * control. It is possible that a concurrent writer is already
		 * trying to compact parts of the stack, which would lead to a
		 * `REFTABLE_LOCK_ERROR` because parts of the stack are locked
		 * already. Similarly, the stack may have been rewritten by a
		 * concurrent writer, which causes `REFTABLE_OUTDATED_ERROR`.
		 * Both of these errors are benign, so we simply ignore them.
		 */
		err = reftable_stack_auto_compact(add->stack, &add->opts);
		if (err < 0 && err != REFTABLE_LOCK_ERROR &&
		    err != REFTABLE_OUTDATED_ERROR)
			goto done;
		err = 0;
	}

done:
	reftable_addition_close(add);
	return err;
}

int reftable_stack_addition_new(struct reftable_addition **dest,
				struct reftable_stack *st,
				const struct reftable_write_options *opts)
{
	int err;

	REFTABLE_CALLOC_ARRAY(*dest, 1);
	if (!*dest)
		return REFTABLE_OUT_OF_MEMORY_ERROR;

	err = reftable_stack_init_addition(*dest, st, opts);
	if (err) {
		reftable_free(*dest);
		*dest = NULL;
	}

	return err;
}

int reftable_addition_add(struct reftable_addition *add,
			  int (*write_table)(struct reftable_writer *wr,
					     void *arg),
			  void *arg)
{
	struct reftable_buf temp_tab_file_name = REFTABLE_BUF_INIT;
	struct reftable_buf tab_file_name = REFTABLE_BUF_INIT;
	struct reftable_buf next_name = REFTABLE_BUF_INIT;
	struct reftable_writer *wr = NULL;
	struct reftable_tmpfile tab_file = REFTABLE_TMPFILE_INIT;
	struct fd_writer writer = {
		.opts = &add->opts,
	};
	int err = 0;

	reftable_buf_reset(&next_name);

	err = format_name(&next_name, add->next_update_index, add->next_update_index);
	if (err < 0)
		goto done;

	err = stack_filename(&temp_tab_file_name, add->stack, next_name.buf);
	if (err < 0)
		goto done;

	err = reftable_buf_addstr(&temp_tab_file_name, ".temp.XXXXXX");
	if (err < 0)
		goto done;

	err = tmpfile_from_pattern(&tab_file, temp_tab_file_name.buf);
	if (err < 0)
		goto done;
	if (add->opts.default_permissions) {
		if (chmod(tab_file.path,
			  add->opts.default_permissions)) {
			err = REFTABLE_IO_ERROR;
			goto done;
		}
	}

	writer.fd = tab_file.fd;
	err = reftable_writer_new(&wr, fd_writer_write, fd_writer_flush,
				  &writer, add->stack->opts.hash_id, &add->opts);
	if (err < 0)
		goto done;

	err = write_table(wr, arg);
	if (err < 0)
		goto done;

	err = reftable_writer_close(wr);
	if (err == REFTABLE_EMPTY_TABLE_ERROR) {
		err = 0;
		goto done;
	}
	if (err < 0)
		goto done;

	err = tmpfile_close(&tab_file);
	if (err < 0)
		goto done;

	if (wr->min_update_index < add->next_update_index) {
		err = REFTABLE_API_ERROR;
		goto done;
	}

	err = format_name(&next_name, wr->min_update_index, wr->max_update_index);
	if (err < 0)
		goto done;

	err = reftable_buf_addstr(&next_name, ".ref");
	if (err < 0)
		goto done;

	err = stack_filename(&tab_file_name, add->stack, next_name.buf);
	if (err < 0)
		goto done;

	/*
	  On windows, this relies on rand() picking a unique destination name.
	  Maybe we should do retry loop as well?
	 */
	err = tmpfile_rename(&tab_file, tab_file_name.buf);
	if (err < 0)
		goto done;

	REFTABLE_ALLOC_GROW_OR_NULL(add->new_tables, add->new_tables_len + 1,
				    add->new_tables_cap);
	if (!add->new_tables) {
		err = REFTABLE_OUT_OF_MEMORY_ERROR;
		goto done;
	}
	add->new_tables[add->new_tables_len++] = reftable_buf_detach(&next_name);

done:
	tmpfile_delete(&tab_file);
	reftable_buf_release(&temp_tab_file_name);
	reftable_buf_release(&tab_file_name);
	reftable_buf_release(&next_name);
	reftable_writer_free(wr);
	return err;
}

uint64_t reftable_stack_next_update_index(struct reftable_stack *st)
{
	int sz = st->merged->tables_len;
	if (sz > 0)
		return reftable_table_max_update_index(st->tables[sz - 1]) +
		       1;
	return 1;
}

static int stack_write_compact(struct reftable_stack *st,
			       struct reftable_writer *wr,
			       size_t first, size_t last,
			       struct reftable_log_expiry_config *config)
{
	struct reftable_merged_table *mt = NULL;
	struct reftable_iterator it = { NULL };
	struct reftable_ref_record ref = { NULL };
	struct reftable_log_record log = { NULL };
	size_t subtabs_len = last - first + 1;
	uint64_t entries = 0;
	int err = 0;

	for (size_t i = first; i <= last; i++)
		st->stats.bytes += st->tables[i]->size;
	err = reftable_writer_set_limits(wr, st->tables[first]->min_update_index,
					 st->tables[last]->max_update_index);
	if (err < 0)
		goto done;

	err = reftable_merged_table_new(&mt, st->tables + first, subtabs_len,
					st->opts.hash_id);
	if (err < 0)
		goto done;

	err = merged_table_init_iter(mt, &it, REFTABLE_BLOCK_TYPE_REF);
	if (err < 0)
		goto done;

	err = reftable_iterator_seek_ref(&it, "");
	if (err < 0)
		goto done;

	while (1) {
		err = reftable_iterator_next_ref(&it, &ref);
		if (err > 0) {
			err = 0;
			break;
		}
		if (err < 0)
			goto done;

		if (first == 0 && reftable_ref_record_is_deletion(&ref)) {
			continue;
		}

		err = reftable_writer_add_ref(wr, &ref);
		if (err < 0)
			goto done;
		entries++;
	}
	reftable_iterator_destroy(&it);

	err = merged_table_init_iter(mt, &it, REFTABLE_BLOCK_TYPE_LOG);
	if (err < 0)
		goto done;

	err = reftable_iterator_seek_log(&it, "");
	if (err < 0)
		goto done;

	while (1) {
		err = reftable_iterator_next_log(&it, &log);
		if (err > 0) {
			err = 0;
			break;
		}
		if (err < 0)
			goto done;
		if (first == 0 && reftable_log_record_is_deletion(&log)) {
			continue;
		}

		if (config && config->min_update_index > 0 &&
		    log.update_index < config->min_update_index) {
			continue;
		}

		if (config && config->time > 0 &&
		    log.value.update.time < config->time) {
			continue;
		}

		err = reftable_writer_add_log(wr, &log);
		if (err < 0)
			goto done;
		entries++;
	}

done:
	reftable_iterator_destroy(&it);
	if (mt)
		reftable_merged_table_free(mt);
	reftable_ref_record_release(&ref);
	reftable_log_record_release(&log);
	st->stats.entries_written += entries;
	return err;
}

static int stack_compact_locked(struct reftable_stack *st,
				size_t first, size_t last,
				struct reftable_log_expiry_config *config,
				const struct reftable_write_options *opts,
				struct reftable_tmpfile *tab_file_out)
{
	struct reftable_buf next_name = REFTABLE_BUF_INIT;
	struct reftable_buf tab_file_path = REFTABLE_BUF_INIT;
	struct reftable_writer *wr = NULL;
	struct fd_writer writer=  {
		.opts = opts,
	};
	struct reftable_tmpfile tab_file = REFTABLE_TMPFILE_INIT;
	int err = 0;

	err = format_name(&next_name, reftable_table_min_update_index(st->tables[first]),
			  reftable_table_max_update_index(st->tables[last]));
	if (err < 0)
		goto done;

	err = stack_filename(&tab_file_path, st, next_name.buf);
	if (err < 0)
		goto done;

	err = reftable_buf_addstr(&tab_file_path, ".temp.XXXXXX");
	if (err < 0)
		goto done;

	err = tmpfile_from_pattern(&tab_file, tab_file_path.buf);
	if (err < 0)
		goto done;

	if (opts->default_permissions &&
	    chmod(tab_file.path, opts->default_permissions) < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}

	writer.fd = tab_file.fd;
	err = reftable_writer_new(&wr, fd_writer_write, fd_writer_flush,
				  &writer, st->opts.hash_id, opts);
	if (err < 0)
		goto done;

	err = stack_write_compact(st, wr, first, last, config);
	if (err < 0)
		goto done;

	err = reftable_writer_close(wr);
	if (err < 0)
		goto done;

	err = tmpfile_close(&tab_file);
	if (err < 0)
		goto done;

	*tab_file_out = tab_file;
	tab_file = REFTABLE_TMPFILE_INIT;

done:
	tmpfile_delete(&tab_file);
	reftable_writer_free(wr);
	reftable_buf_release(&next_name);
	reftable_buf_release(&tab_file_path);
	return err;
}

enum stack_compact_range_flags {
	/*
	 * Perform a best-effort compaction. That is, even if we cannot lock
	 * all tables in the specified range, we will try to compact the
	 * remaining slice.
	 */
	STACK_COMPACT_RANGE_BEST_EFFORT = (1 << 0),
};

/*
 * Compact all tables in the range `[first, last)` into a single new table.
 *
 * This function returns `0` on success or a code `< 0` on failure. When the
 * stack or any of the tables in the specified range are already locked then
 * this function returns `REFTABLE_LOCK_ERROR`. This is a benign error that
 * callers can either ignore, or they may choose to retry compaction after some
 * amount of time.
 */
static int stack_compact_range(struct reftable_stack *st,
			       size_t first, size_t last,
			       struct reftable_log_expiry_config *expiry,
			       const struct reftable_write_options *opts,
			       unsigned int flags)
{
	struct reftable_buf tables_list_buf = REFTABLE_BUF_INIT;
	struct reftable_buf new_table_name = REFTABLE_BUF_INIT;
	struct reftable_buf new_table_path = REFTABLE_BUF_INIT;
	struct reftable_buf table_name = REFTABLE_BUF_INIT;
	struct reftable_flock tables_list_lock = REFTABLE_FLOCK_INIT;
	struct reftable_flock *table_locks = NULL;
	struct reftable_tmpfile new_table = REFTABLE_TMPFILE_INIT;
	int is_empty_table = 0, err = 0;
	size_t first_to_replace, last_to_replace;
	size_t i, nlocks = 0;
	char **names = NULL;

	if (first > last || (!expiry && first == last)) {
		err = 0;
		goto done;
	}

	st->stats.attempts++;

	/*
	 * Hold the lock so that we can read "tables.list" and lock all tables
	 * which are part of the user-specified range.
	 */
	err = flock_acquire(&tables_list_lock, st->list_file, opts->lock_timeout_ms);
	if (err < 0)
		goto done;

	/*
	 * Check whether the stack is up-to-date. We unfortunately cannot
	 * handle the situation gracefully in case it's _not_ up-to-date
	 * because the range of tables that the user has requested us to
	 * compact may have been changed. So instead we abort.
	 *
	 * We could in theory improve the situation by having the caller not
	 * pass in a range, but instead the list of tables to compact. If so,
	 * we could check that relevant tables still exist. But for now it's
	 * good enough to just abort.
	 */
	err = stack_uptodate(st, 0);
	if (err < 0)
		goto done;
	if (err > 0) {
		err = REFTABLE_OUTDATED_ERROR;
		goto done;
	}

	/*
	 * Lock all tables in the user-provided range. This is the slice of our
	 * stack which we'll compact.
	 *
	 * Note that we lock tables in reverse order from last to first. The
	 * intent behind this is to allow a newer process to perform best
	 * effort compaction of tables that it has added in the case where an
	 * older process is still busy compacting tables which are preexisting
	 * from the point of view of the newer process.
	 */
	REFTABLE_ALLOC_ARRAY(table_locks, last - first + 1);
	if (!table_locks) {
		err = REFTABLE_OUT_OF_MEMORY_ERROR;
		goto done;
	}
	for (i = 0; i < last - first + 1; i++)
		table_locks[i] = REFTABLE_FLOCK_INIT;

	for (i = last + 1; i > first; i--) {
		err = stack_filename(&table_name, st, reftable_table_name(st->tables[i - 1]));
		if (err < 0)
			goto done;

		err = flock_acquire(&table_locks[nlocks], table_name.buf, 0);
		if (err < 0) {
			/*
			 * When the table is locked already we may do a
			 * best-effort compaction and compact only the tables
			 * that we have managed to lock so far. This of course
			 * requires that we have been able to lock at least two
			 * tables, otherwise there would be nothing to compact.
			 * In that case, we return a lock error to our caller.
			 */
			if (err == REFTABLE_LOCK_ERROR && last - (i - 1) >= 2 &&
			    flags & STACK_COMPACT_RANGE_BEST_EFFORT) {
				err = 0;
				/*
				 * The subtraction is to offset the index, the
				 * addition is to only compact up to the table
				 * of the preceding iteration. They obviously
				 * cancel each other out, but that may be
				 * non-obvious when it was omitted.
				 */
				first = (i - 1) + 1;
				break;
			}

			goto done;
		}

		/*
		 * We need to close the lockfiles as we might otherwise easily
		 * run into file descriptor exhaustion when we compress a lot
		 * of tables.
		 */
		err = flock_close(&table_locks[nlocks++]);
		if (err < 0)
			goto done;
	}

	/*
	 * We have locked all tables in our range and can thus release the
	 * "tables.list" lock while compacting the locked tables. This allows
	 * concurrent updates to the stack to proceed.
	 */
	err = flock_release(&tables_list_lock);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		goto done;
	}

	/*
	 * Compact the now-locked tables into a new table. Note that compacting
	 * these tables may end up with an empty new table in case tombstones
	 * end up cancelling out all refs in that range.
	 */
	err = stack_compact_locked(st, first, last, expiry, opts, &new_table);
	if (err < 0) {
		if (err != REFTABLE_EMPTY_TABLE_ERROR)
			goto done;
		is_empty_table = 1;
	}

	/*
	 * Now that we have written the new, compacted table we need to re-lock
	 * "tables.list". We'll then replace the compacted range of tables with
	 * the new table.
	 */
	err = flock_acquire(&tables_list_lock, st->list_file, opts->lock_timeout_ms);
	if (err < 0)
		goto done;

	if (opts->default_permissions) {
		if (chmod(tables_list_lock.path,
			  opts->default_permissions) < 0) {
			err = REFTABLE_IO_ERROR;
			goto done;
		}
	}

	/*
	 * As we have unlocked the stack while compacting our slice of tables
	 * it may have happened that a concurrently running process has updated
	 * the stack while we were compacting. In that case, we need to check
	 * whether the tables that we have just compacted still exist in the
	 * stack in the exact same order as we have compacted them.
	 *
	 * If they do exist, then it is fine to continue and replace those
	 * tables with our compacted version. If they don't, then we need to
	 * abort.
	 */
	err = stack_uptodate(st, 0);
	if (err < 0)
		goto done;
	if (err > 0) {
		ssize_t new_offset = -1;
		int fd;

		fd = open(st->list_file, O_RDONLY);
		if (fd < 0) {
			err = REFTABLE_IO_ERROR;
			goto done;
		}

		err = fd_read_lines(fd, &names);
		close(fd);
		if (err < 0)
			goto done;

		/*
		 * Search for the offset of the first table that we have
		 * compacted in the updated "tables.list" file.
		 */
		for (size_t i = 0; names[i]; i++) {
			if (strcmp(names[i], st->tables[first]->name))
				continue;

			/*
			 * We have found the first entry. Verify that all the
			 * subsequent tables we have compacted still exist in
			 * the modified stack in the exact same order as we
			 * have compacted them.
			 */
			for (size_t j = 1; j < last - first + 1; j++) {
				const char *old = first + j < st->merged->tables_len ?
					st->tables[first + j]->name : NULL;
				const char *new = names[i + j];

				/*
				 * If some entries are missing or in case the tables
				 * have changed then we need to bail out. Again, this
				 * shouldn't ever happen because we have locked the
				 * tables we are compacting.
				 */
				if (!old || !new || strcmp(old, new)) {
					err = REFTABLE_OUTDATED_ERROR;
					goto done;
				}
			}

			new_offset = i;
			break;
		}

		/*
		 * In case we didn't find our compacted tables in the stack we
		 * need to bail out. In theory, this should have never happened
		 * because we locked the tables we are compacting.
		 */
		if (new_offset < 0) {
			err = REFTABLE_OUTDATED_ERROR;
			goto done;
		}

		/*
		 * We have found the new range that we want to replace, so
		 * let's update the range of tables that we want to replace.
		 */
		first_to_replace = new_offset;
		last_to_replace = last + (new_offset - first);
	} else {
		/*
		 * `fd_read_lines()` uses a `NULL` sentinel to indicate that
		 * the array is at its end. As we use `free_names()` to free
		 * the array, we need to include this sentinel value here and
		 * thus have to allocate `tables_len + 1` many entries.
		 */
		REFTABLE_CALLOC_ARRAY(names, st->merged->tables_len + 1);
		if (!names) {
			err = REFTABLE_OUT_OF_MEMORY_ERROR;
			goto done;
		}

		for (size_t i = 0; i < st->merged->tables_len; i++) {
			names[i] = reftable_strdup(st->tables[i]->name);
			if (!names[i]) {
				err = REFTABLE_OUT_OF_MEMORY_ERROR;
				goto done;
			}
		}
		first_to_replace = first;
		last_to_replace = last;
	}

	/*
	 * If the resulting compacted table is not empty, then we need to move
	 * it into place now.
	 */
	if (!is_empty_table) {
		err = format_name(&new_table_name, st->tables[first]->min_update_index,
				  st->tables[last]->max_update_index);
		if (err < 0)
			goto done;

		err = reftable_buf_addstr(&new_table_name, ".ref");
		if (err < 0)
			goto done;

		err = stack_filename(&new_table_path, st, new_table_name.buf);
		if (err < 0)
			goto done;

		err = tmpfile_rename(&new_table, new_table_path.buf);
		if (err < 0)
			goto done;
	}

	/*
	 * Write the new "tables.list" contents with the compacted table we
	 * have just written. In case the compacted table became empty we
	 * simply skip writing it.
	 */
	for (i = 0; i < first_to_replace; i++) {
		if ((err = reftable_buf_addstr(&tables_list_buf, names[i])) < 0 ||
		    (err = reftable_buf_addstr(&tables_list_buf, "\n")) < 0)
		      goto done;
	}
	if (!is_empty_table) {
		if ((err = reftable_buf_addstr(&tables_list_buf, new_table_name.buf)) < 0 ||
		    (err = reftable_buf_addstr(&tables_list_buf, "\n")) < 0)
			goto done;
	}
	for (i = last_to_replace + 1; names[i]; i++) {
		if ((err = reftable_buf_addstr(&tables_list_buf, names[i])) < 0 ||
		    (err = reftable_buf_addstr(&tables_list_buf, "\n")) < 0)
			goto done;
	}

	err = reftable_write_data(tables_list_lock.fd,
				  tables_list_buf.buf, tables_list_buf.len);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		unlink(new_table_path.buf);
		goto done;
	}

	err = fsync(tables_list_lock.fd);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		unlink(new_table_path.buf);
		goto done;
	}

	err = flock_commit(&tables_list_lock);
	if (err < 0) {
		err = REFTABLE_IO_ERROR;
		unlink(new_table_path.buf);
		goto done;
	}

	/*
	 * Reload the stack before deleting the compacted tables. We can only
	 * delete the files after we closed them on Windows, so this needs to
	 * happen first.
	 */
	err = reftable_stack_reload_maybe_reuse(st, first < last);
	if (err < 0)
		goto done;

	/*
	 * Delete the old tables. They may still be in use by concurrent
	 * readers, so it is expected that unlinking tables may fail.
	 */
	for (i = 0; i < nlocks; i++) {
		struct reftable_flock *table_lock = &table_locks[i];

		reftable_buf_reset(&table_name);
		err = reftable_buf_add(&table_name, table_lock->path,
				       strlen(table_lock->path) - strlen(".lock"));
		if (err)
			continue;

		unlink(table_name.buf);
	}

done:
	flock_release(&tables_list_lock);
	for (i = 0; table_locks && i < nlocks; i++)
		flock_release(&table_locks[i]);
	reftable_free(table_locks);

	tmpfile_delete(&new_table);
	reftable_buf_release(&new_table_name);
	reftable_buf_release(&new_table_path);
	reftable_buf_release(&tables_list_buf);
	reftable_buf_release(&table_name);
	free_names(names);

	if (err == REFTABLE_LOCK_ERROR)
		st->stats.failures++;

	return err;
}

int reftable_stack_compact_all(struct reftable_stack *st,
			       const struct reftable_write_options *opts,
			       struct reftable_log_expiry_config *config)
{
	struct reftable_write_options opts_default = { 0 };
	size_t last = st->merged->tables_len ? st->merged->tables_len - 1 : 0;

	if (!opts)
		opts = &opts_default;

	return stack_compact_range(st, 0, last, config, opts, 0);
}

static int segment_size(struct segment *s)
{
	return s->end - s->start;
}

struct segment suggest_compaction_segment(uint64_t *sizes, size_t n,
					  uint8_t factor)
{
	struct segment seg = { 0 };
	uint64_t bytes;
	size_t i;

	if (!factor)
		factor = DEFAULT_GEOMETRIC_FACTOR;

	/*
	 * If there are no tables or only a single one then we don't have to
	 * compact anything. The sequence is geometric by definition already.
	 */
	if (n <= 1)
		return seg;

	/*
	 * Find the ending table of the compaction segment needed to restore the
	 * geometric sequence. Note that the segment end is exclusive.
	 *
	 * To do so, we iterate backwards starting from the most recent table
	 * until a valid segment end is found. If the preceding table is smaller
	 * than the current table multiplied by the geometric factor (2), the
	 * compaction segment end has been identified.
	 *
	 * Tables after the ending point are not added to the byte count because
	 * they are already valid members of the geometric sequence. Due to the
	 * properties of a geometric sequence, it is not possible for the sum of
	 * these tables to exceed the value of the ending point table.
	 *
	 * Example table size sequence requiring no compaction:
	 * 	64, 32, 16, 8, 4, 2, 1
	 *
	 * Example table size sequence where compaction segment end is set to
	 * the last table. Since the segment end is exclusive, the last table is
	 * excluded during subsequent compaction and the table with size 3 is
	 * the final table included:
	 * 	64, 32, 16, 8, 4, 3, 1
	 */
	for (i = n - 1; i > 0; i--) {
		if (sizes[i - 1] < sizes[i] * factor) {
			seg.end = i + 1;
			bytes = sizes[i];
			break;
		}
	}

	/*
	 * Find the starting table of the compaction segment by iterating
	 * through the remaining tables and keeping track of the accumulated
	 * size of all tables seen from the segment end table. The previous
	 * table is compared to the accumulated size because the tables from the
	 * segment end are merged backwards recursively.
	 *
	 * Note that we keep iterating even after we have found the first
	 * starting point. This is because there may be tables in the stack
	 * preceding that first starting point which violate the geometric
	 * sequence.
	 *
	 * Example compaction segment start set to table with size 32:
	 * 	128, 32, 16, 8, 4, 3, 1
	 */
	for (; i > 0; i--) {
		uint64_t curr = bytes;
		bytes += sizes[i - 1];

		if (sizes[i - 1] < curr * factor) {
			seg.start = i - 1;
			seg.bytes = bytes;
		}
	}

	return seg;
}

static int stack_segments_for_compaction(struct reftable_stack *st,
					 const struct reftable_write_options *opts,
					 struct segment *seg)
{
	int version = (st->opts.hash_id == REFTABLE_HASH_SHA1) ? 1 : 2;
	int overhead = header_size(version) - 1;
	uint64_t *sizes;

	REFTABLE_CALLOC_ARRAY(sizes, st->merged->tables_len);
	if (!sizes)
		return REFTABLE_OUT_OF_MEMORY_ERROR;

	for (size_t i = 0; i < st->merged->tables_len; i++)
		sizes[i] = st->tables[i]->size - overhead;

	*seg = suggest_compaction_segment(sizes, st->merged->tables_len,
					  opts->auto_compaction_factor);
	reftable_free(sizes);

	return 0;
}

static int update_segment_if_compaction_required(struct reftable_stack *st,
						 const struct reftable_write_options *opts,
						 struct segment *seg,
						 bool use_geometric,
						 bool *required)
{
	int err;

	if (st->merged->tables_len < 2) {
		*required = false;
		return 0;
	}

	if (!use_geometric) {
		*required = true;
		return 0;
	}

	err = stack_segments_for_compaction(st, opts, seg);
	if (err)
		return err;

	*required = segment_size(seg) > 0;
	return 0;
}

int reftable_stack_compaction_required(struct reftable_stack *st,
				       const struct reftable_write_options *opts,
				       bool use_heuristics,
				       bool *required)
{
	struct reftable_write_options opts_default = { 0 };
	struct segment seg;

	if (!opts)
		opts = &opts_default;

	return update_segment_if_compaction_required(st, opts, &seg,
						     use_heuristics, required);
}

int reftable_stack_auto_compact(struct reftable_stack *st,
				const struct reftable_write_options *opts)
{
	struct reftable_write_options opts_default = { 0 };
	struct segment seg;
	bool required;
	int err;

	if (!opts)
		opts = &opts_default;

	err = update_segment_if_compaction_required(st, opts, &seg, true,
						    &required);
	if (err)
		return err;

	if (required)
		return stack_compact_range(st, seg.start, seg.end - 1,
					   NULL, opts,
					   STACK_COMPACT_RANGE_BEST_EFFORT);

	return 0;
}

struct reftable_compaction_stats *
reftable_stack_compaction_stats(struct reftable_stack *st)
{
	return &st->stats;
}

int reftable_stack_read_ref(struct reftable_stack *st, const char *refname,
			    struct reftable_ref_record *ref)
{
	struct reftable_iterator it = { 0 };
	int ret;

	ret = reftable_merged_table_init_ref_iterator(st->merged, &it);
	if (ret)
		goto out;

	ret = reftable_iterator_seek_ref(&it, refname);
	if (ret)
		goto out;

	ret = reftable_iterator_next_ref(&it, ref);
	if (ret)
		goto out;

	if (strcmp(ref->refname, refname) ||
	    reftable_ref_record_is_deletion(ref)) {
		reftable_ref_record_release(ref);
		ret = 1;
		goto out;
	}

out:
	reftable_iterator_destroy(&it);
	return ret;
}

int reftable_stack_read_log(struct reftable_stack *st, const char *refname,
			    struct reftable_log_record *log)
{
	struct reftable_iterator it = {0};
	int err;

	err = reftable_stack_init_log_iterator(st, &it);
	if (err)
		goto done;

	err = reftable_iterator_seek_log(&it, refname);
	if (err)
		goto done;

	err = reftable_iterator_next_log(&it, log);
	if (err)
		goto done;

	if (strcmp(log->refname, refname) ||
	    reftable_log_record_is_deletion(log)) {
		err = 1;
		goto done;
	}

done:
	if (err) {
		reftable_log_record_release(log);
	}
	reftable_iterator_destroy(&it);
	return err;
}

static int is_table_name(const char *s)
{
	const char *dot = strrchr(s, '.');
	return dot && !strcmp(dot, ".ref");
}

static void remove_maybe_stale_table(struct reftable_stack *st, uint64_t max,
				     const char *name)
{
	int err = 0;
	uint64_t update_idx = 0;
	struct reftable_block_source src = { NULL };
	struct reftable_table *table = NULL;
	struct reftable_buf table_path = REFTABLE_BUF_INIT;

	err = stack_filename(&table_path, st, name);
	if (err < 0)
		goto done;

	err = reftable_block_source_from_file(&src, table_path.buf);
	if (err < 0)
		goto done;

	err = reftable_table_new(&table, &src, name);
	if (err < 0)
		goto done;

	update_idx = reftable_table_max_update_index(table);
	reftable_table_decref(table);

	if (update_idx <= max) {
		unlink(table_path.buf);
	}
done:
	reftable_buf_release(&table_path);
}

static int reftable_stack_clean_locked(struct reftable_stack *st)
{
	uint64_t max = reftable_merged_table_max_update_index(
		reftable_stack_merged_table(st));
	DIR *dir = opendir(st->reftable_dir);
	struct dirent *d = NULL;
	if (!dir) {
		return REFTABLE_IO_ERROR;
	}

	while ((d = readdir(dir))) {
		int found = 0;
		if (!is_table_name(d->d_name))
			continue;

		for (size_t i = 0; !found && i < st->tables_len; i++)
			found = !strcmp(reftable_table_name(st->tables[i]), d->d_name);
		if (found)
			continue;

		remove_maybe_stale_table(st, max, d->d_name);
	}

	closedir(dir);
	return 0;
}

int reftable_stack_clean(struct reftable_stack *st)
{
	struct reftable_addition *add = NULL;
	int err = reftable_stack_addition_new(&add, st, NULL);
	if (err < 0) {
		goto done;
	}

	err = reftable_stack_clean_locked(st);

done:
	reftable_addition_destroy(add);
	return err;
}

enum reftable_hash reftable_stack_hash_id(struct reftable_stack *st)
{
	return reftable_merged_table_hash_id(st->merged);
}