reftable/error: discern locked/outdated errors

[alt-git.git] / reftable / stack.c

/*
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 "../write-or-die.h"
#include "system.h"
#include "merged.h"
#include "reader.h"
#include "refname.h"
#include "reftable-error.h"
#include "reftable-record.h"
#include "reftable-merged.h"
#include "writer.h"
#include "tempfile.h"

static int stack_try_add(struct reftable_stack *st,
                         int (*write_table)(struct reftable_writer *wr,
                                            void *arg),
                         void *arg);
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);
static int stack_check_addition(struct reftable_stack *st,
                                const char *new_tab_name);
static void reftable_addition_close(struct reftable_addition *add);
static int reftable_stack_reload_maybe_reuse(struct reftable_stack *st,
                                             int reuse_open);

static void stack_filename(struct strbuf *dest, struct reftable_stack *st,
                           const char *name)
{
        strbuf_reset(dest);
        strbuf_addstr(dest, st->reftable_dir);
        strbuf_addstr(dest, "/");
        strbuf_addstr(dest, name);
}

static ssize_t reftable_fd_write(void *arg, const void *data, size_t sz)
{
        int *fdp = (int *)arg;
        return write_in_full(*fdp, data, sz);
}

static int reftable_fd_flush(void *arg)
{
        int *fdp = (int *)arg;

        return fsync_component(FSYNC_COMPONENT_REFERENCE, *fdp);
}

int reftable_new_stack(struct reftable_stack **dest, const char *dir,
                       struct reftable_write_options config)
{
        struct reftable_stack *p = reftable_calloc(1, sizeof(*p));
        struct strbuf list_file_name = STRBUF_INIT;
        int err = 0;

        if (config.hash_id == 0) {
                config.hash_id = GIT_SHA1_FORMAT_ID;
        }

        *dest = NULL;

        strbuf_reset(&list_file_name);
        strbuf_addstr(&list_file_name, dir);
        strbuf_addstr(&list_file_name, "/tables.list");

        p->list_file = strbuf_detach(&list_file_name, NULL);
        p->list_fd = -1;
        p->reftable_dir = xstrdup(dir);
        p->config = config;

        err = reftable_stack_reload_maybe_reuse(p, 1);
        if (err < 0) {
                reftable_stack_destroy(p);
        } else {
                *dest = p;
        }
        return err;
}

static int fd_read_lines(int fd, char ***namesp)
{
        off_t size = lseek(fd, 0, SEEK_END);
        char *buf = NULL;
        int err = 0;
        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 (read_in_full(fd, buf, size) != size) {
                err = REFTABLE_IO_ERROR;
                goto done;
        }
        buf[size] = 0;

        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);
                        return 0;
                }

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

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

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

        if (st->readers) {
                int i = 0;
                struct strbuf filename = STRBUF_INIT;
                for (i = 0; i < st->readers_len; i++) {
                        const char *name = reader_name(st->readers[i]);
                        strbuf_reset(&filename);
                        if (names && !has_name(names, name)) {
                                stack_filename(&filename, st, name);
                        }
                        reftable_reader_free(st->readers[i]);

                        if (filename.len) {
                                /* On Windows, can only unlink after closing. */
                                unlink(filename.buf);
                        }
                }
                strbuf_release(&filename);
                st->readers_len = 0;
                FREE_AND_NULL(st->readers);
        }

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

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

static struct reftable_reader **stack_copy_readers(struct reftable_stack *st,
                                                   int cur_len)
{
        struct reftable_reader **cur = reftable_calloc(cur_len, sizeof(*cur));
        int i = 0;
        for (i = 0; i < cur_len; i++) {
                cur[i] = st->readers[i];
        }
        return cur;
}

static int reftable_stack_reload_once(struct reftable_stack *st, char **names,
                                      int reuse_open)
{
        size_t cur_len = !st->merged ? 0 : st->merged->stack_len;
        struct reftable_reader **cur = stack_copy_readers(st, cur_len);
        size_t names_len = names_length(names);
        struct reftable_reader **new_readers =
                reftable_calloc(names_len, sizeof(*new_readers));
        struct reftable_table *new_tables =
                reftable_calloc(names_len, sizeof(*new_tables));
        size_t new_readers_len = 0;
        struct reftable_merged_table *new_merged = NULL;
        struct strbuf table_path = STRBUF_INIT;
        int err = 0;
        size_t i;

        while (*names) {
                struct reftable_reader *rd = NULL;
                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)) {
                                rd = cur[i];
                                cur[i] = NULL;
                                break;
                        }
                }

                if (!rd) {
                        struct reftable_block_source src = { NULL };
                        stack_filename(&table_path, st, name);

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

                        err = reftable_new_reader(&rd, &src, name);
                        if (err < 0)
                                goto done;
                }

                new_readers[new_readers_len] = rd;
                reftable_table_from_reader(&new_tables[new_readers_len], rd);
                new_readers_len++;
        }

        /* success! */
        err = reftable_new_merged_table(&new_merged, new_tables,
                                        new_readers_len, st->config.hash_id);
        if (err < 0)
                goto done;

        new_tables = NULL;
        st->readers_len = new_readers_len;
        if (st->merged) {
                merged_table_release(st->merged);
                reftable_merged_table_free(st->merged);
        }
        if (st->readers) {
                reftable_free(st->readers);
        }
        st->readers = new_readers;
        new_readers = NULL;
        new_readers_len = 0;

        new_merged->suppress_deletions = 1;
        st->merged = new_merged;
        for (i = 0; i < cur_len; i++) {
                if (cur[i]) {
                        const char *name = reader_name(cur[i]);
                        stack_filename(&table_path, st, name);

                        reader_close(cur[i]);
                        reftable_reader_free(cur[i]);

                        /* On Windows, can only unlink after closing. */
                        unlink(table_path.buf);
                }
        }

done:
        for (i = 0; i < new_readers_len; i++) {
                reader_close(new_readers[i]);
                reftable_reader_free(new_readers[i]);
        }
        reftable_free(new_readers);
        reftable_free(new_tables);
        reftable_free(cur);
        strbuf_release(&table_path);
        return err;
}

/* return negative if a before b. */
static int tv_cmp(struct timeval *a, struct timeval *b)
{
        time_t diff = a->tv_sec - b->tv_sec;
        int udiff = a->tv_usec - b->tv_usec;

        if (diff != 0)
                return diff;

        return udiff;
}

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

        err = gettimeofday(&deadline, NULL);
        if (err < 0)
                goto out;
        deadline.tv_sec += 3;

        while (1) {
                struct timeval now;

                err = gettimeofday(&now, NULL);
                if (err < 0)
                        goto out;

                /*
                 * Only look at deadlines after the first few times. This
                 * simplifies debugging in GDB.
                 */
                tries++;
                if (tries > 3 && tv_cmp(&now, &deadline) >= 0)
                        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);
                } else {
                        err = fd_read_lines(fd, &names);
                        if (err < 0)
                                goto out;
                }

                err = reftable_stack_reload_once(st, 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(names_after, 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 * rand()) / RAND_MAX + 1;
                sleep_millisec(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);
        return err;
}

/* -1 = error
 0 = up to date
 1 = changed. */
static int stack_uptodate(struct reftable_stack *st)
{
        char **names = NULL;
        int err;
        int i = 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 readers.
                         */
                        if (errno == ENOENT)
                                return !!st->readers_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 (i = 0; i < st->readers_len; i++) {
                if (!names[i]) {
                        err = 1;
                        goto done;
                }

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

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

done:
        free_names(names);
        return err;
}

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

int reftable_stack_add(struct reftable_stack *st,
                       int (*write)(struct reftable_writer *wr, void *arg),
                       void *arg)
{
        int err = stack_try_add(st, write, arg);
        if (err < 0) {
                if (err == REFTABLE_OUTDATED_ERROR) {
                        /* Ignore error return, we want to propagate
                           REFTABLE_OUTDATED_ERROR.
                        */
                        reftable_stack_reload(st);
                }
                return err;
        }

        return 0;
}

static void format_name(struct strbuf *dest, uint64_t min, uint64_t max)
{
        char buf[100];
        uint32_t rnd = (uint32_t)git_rand();
        snprintf(buf, sizeof(buf), "0x%012" PRIx64 "-0x%012" PRIx64 "-%08x",
                 min, max, rnd);
        strbuf_reset(dest);
        strbuf_addstr(dest, buf);
}

struct reftable_addition {
        struct tempfile *lock_file;
        struct reftable_stack *stack;

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

#define REFTABLE_ADDITION_INIT {0}

static int reftable_stack_init_addition(struct reftable_addition *add,
                                        struct reftable_stack *st)
{
        struct strbuf lock_file_name = STRBUF_INIT;
        int err = 0;
        add->stack = st;

        strbuf_addf(&lock_file_name, "%s.lock", st->list_file);

        add->lock_file = create_tempfile(lock_file_name.buf);
        if (!add->lock_file) {
                if (errno == EEXIST) {
                        err = REFTABLE_LOCK_ERROR;
                } else {
                        err = REFTABLE_IO_ERROR;
                }
                goto done;
        }
        if (st->config.default_permissions) {
                if (chmod(add->lock_file->filename.buf, st->config.default_permissions) < 0) {
                        err = REFTABLE_IO_ERROR;
                        goto done;
                }
        }

        err = stack_uptodate(st);
        if (err < 0)
                goto done;
        if (err > 0) {
                err = REFTABLE_OUTDATED_ERROR;
                goto done;
        }

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

static void reftable_addition_close(struct reftable_addition *add)
{
        struct strbuf nm = STRBUF_INIT;
        size_t i;

        for (i = 0; i < add->new_tables_len; i++) {
                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;

        delete_tempfile(&add->lock_file);
        strbuf_release(&nm);
}

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 strbuf table_list = STRBUF_INIT;
        int lock_file_fd = get_tempfile_fd(add->lock_file);
        int err = 0;
        size_t i;

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

        for (i = 0; i < add->stack->merged->stack_len; i++) {
                strbuf_addstr(&table_list, add->stack->readers[i]->name);
                strbuf_addstr(&table_list, "\n");
        }
        for (i = 0; i < add->new_tables_len; i++) {
                strbuf_addstr(&table_list, add->new_tables[i]);
                strbuf_addstr(&table_list, "\n");
        }

        err = write_in_full(lock_file_fd, table_list.buf, table_list.len);
        strbuf_release(&table_list);
        if (err < 0) {
                err = REFTABLE_IO_ERROR;
                goto done;
        }

        fsync_component_or_die(FSYNC_COMPONENT_REFERENCE, lock_file_fd,
                               get_tempfile_path(add->lock_file));

        err = rename_tempfile(&add->lock_file, add->stack->list_file);
        if (err < 0) {
                err = REFTABLE_IO_ERROR;
                goto done;
        }

        /* 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->stack->disable_auto_compact)
                err = reftable_stack_auto_compact(add->stack);

done:
        reftable_addition_close(add);
        return err;
}

int reftable_stack_new_addition(struct reftable_addition **dest,
                                struct reftable_stack *st)
{
        int err = 0;
        struct reftable_addition empty = REFTABLE_ADDITION_INIT;
        REFTABLE_CALLOC_ARRAY(*dest, 1);
        **dest = empty;
        err = reftable_stack_init_addition(*dest, st);
        if (err) {
                reftable_free(*dest);
                *dest = NULL;
        }
        return err;
}

static int stack_try_add(struct reftable_stack *st,
                         int (*write_table)(struct reftable_writer *wr,
                                            void *arg),
                         void *arg)
{
        struct reftable_addition add = REFTABLE_ADDITION_INIT;
        int err = reftable_stack_init_addition(&add, st);
        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_addition_add(struct reftable_addition *add,
                          int (*write_table)(struct reftable_writer *wr,
                                             void *arg),
                          void *arg)
{
        struct strbuf temp_tab_file_name = STRBUF_INIT;
        struct strbuf tab_file_name = STRBUF_INIT;
        struct strbuf next_name = STRBUF_INIT;
        struct reftable_writer *wr = NULL;
        struct tempfile *tab_file = NULL;
        int err = 0;
        int tab_fd;

        strbuf_reset(&next_name);
        format_name(&next_name, add->next_update_index, add->next_update_index);

        stack_filename(&temp_tab_file_name, add->stack, next_name.buf);
        strbuf_addstr(&temp_tab_file_name, ".temp.XXXXXX");

        tab_file = mks_tempfile(temp_tab_file_name.buf);
        if (!tab_file) {
                err = REFTABLE_IO_ERROR;
                goto done;
        }
        if (add->stack->config.default_permissions) {
                if (chmod(get_tempfile_path(tab_file),
                          add->stack->config.default_permissions)) {
                        err = REFTABLE_IO_ERROR;
                        goto done;
                }
        }
        tab_fd = get_tempfile_fd(tab_file);

        wr = reftable_new_writer(reftable_fd_write, reftable_fd_flush, &tab_fd,
                                 &add->stack->config);
        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 = close_tempfile_gently(tab_file);
        if (err < 0) {
                err = REFTABLE_IO_ERROR;
                goto done;
        }

        err = stack_check_addition(add->stack, get_tempfile_path(tab_file));
        if (err < 0)
                goto done;

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

        format_name(&next_name, wr->min_update_index, wr->max_update_index);
        strbuf_addstr(&next_name, ".ref");
        stack_filename(&tab_file_name, add->stack, next_name.buf);

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

        REFTABLE_ALLOC_GROW(add->new_tables, add->new_tables_len + 1,
                            add->new_tables_cap);
        add->new_tables[add->new_tables_len++] = strbuf_detach(&next_name, NULL);
done:
        delete_tempfile(&tab_file);
        strbuf_release(&temp_tab_file_name);
        strbuf_release(&tab_file_name);
        strbuf_release(&next_name);
        reftable_writer_free(wr);
        return err;
}

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

static int stack_compact_locked(struct reftable_stack *st,
                                size_t first, size_t last,
                                struct reftable_log_expiry_config *config,
                                struct tempfile **tab_file_out)
{
        struct strbuf next_name = STRBUF_INIT;
        struct strbuf tab_file_path = STRBUF_INIT;
        struct reftable_writer *wr = NULL;
        struct tempfile *tab_file;
        int tab_fd, err = 0;

        format_name(&next_name,
                    reftable_reader_min_update_index(st->readers[first]),
                    reftable_reader_max_update_index(st->readers[last]));
        stack_filename(&tab_file_path, st, next_name.buf);
        strbuf_addstr(&tab_file_path, ".temp.XXXXXX");

        tab_file = mks_tempfile(tab_file_path.buf);
        if (!tab_file) {
                err = REFTABLE_IO_ERROR;
                goto done;
        }
        tab_fd = get_tempfile_fd(tab_file);

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

        wr = reftable_new_writer(reftable_fd_write, reftable_fd_flush,
                                 &tab_fd, &st->config);
        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 = close_tempfile_gently(tab_file);
        if (err < 0)
                goto done;

        *tab_file_out = tab_file;
        tab_file = NULL;

done:
        delete_tempfile(&tab_file);
        reftable_writer_free(wr);
        strbuf_release(&next_name);
        strbuf_release(&tab_file_path);
        return err;
}

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)
{
        size_t subtabs_len = last - first + 1;
        struct reftable_table *subtabs = reftable_calloc(
                last - first + 1, sizeof(*subtabs));
        struct reftable_merged_table *mt = NULL;
        struct reftable_iterator it = { NULL };
        struct reftable_ref_record ref = { NULL };
        struct reftable_log_record log = { NULL };
        uint64_t entries = 0;
        int err = 0;

        for (size_t i = first, j = 0; i <= last; i++) {
                struct reftable_reader *t = st->readers[i];
                reftable_table_from_reader(&subtabs[j++], t);
                st->stats.bytes += t->size;
        }
        reftable_writer_set_limits(wr, st->readers[first]->min_update_index,
                                   st->readers[last]->max_update_index);

        err = reftable_new_merged_table(&mt, subtabs, subtabs_len,
                                        st->config.hash_id);
        if (err < 0) {
                reftable_free(subtabs);
                goto done;
        }

        err = reftable_merged_table_seek_ref(mt, &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 = reftable_merged_table_seek_log(mt, &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) {
                merged_table_release(mt);
                reftable_merged_table_free(mt);
        }
        reftable_ref_record_release(&ref);
        reftable_log_record_release(&log);
        st->stats.entries_written += entries;
        return err;
}

/* <  0: error. 0 == OK, > 0 attempt failed; could retry. */
static int stack_compact_range(struct reftable_stack *st,
                               size_t first, size_t last,
                               struct reftable_log_expiry_config *expiry)
{
        struct strbuf tables_list_buf = STRBUF_INIT;
        struct strbuf new_table_name = STRBUF_INIT;
        struct strbuf new_table_path = STRBUF_INIT;
        struct strbuf table_name = STRBUF_INIT;
        struct lock_file tables_list_lock = LOCK_INIT;
        struct lock_file *table_locks = NULL;
        struct tempfile *new_table = NULL;
        int is_empty_table = 0, err = 0;
        size_t i;

        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 = hold_lock_file_for_update(&tables_list_lock, st->list_file,
                                        LOCK_NO_DEREF);
        if (err < 0) {
                if (errno == EEXIST)
                        err = 1;
                else
                        err = REFTABLE_IO_ERROR;
                goto done;
        }

        err = stack_uptodate(st);
        if (err)
                goto done;

        /*
         * Lock all tables in the user-provided range. This is the slice of our
         * stack which we'll compact.
         */
        REFTABLE_CALLOC_ARRAY(table_locks, last - first + 1);
        for (i = first; i <= last; i++) {
                stack_filename(&table_name, st, reader_name(st->readers[i]));

                err = hold_lock_file_for_update(&table_locks[i - first],
                                                table_name.buf, LOCK_NO_DEREF);
                if (err < 0) {
                        if (errno == EEXIST)
                                err = 1;
                        else
                                err = REFTABLE_IO_ERROR;
                        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 = close_lock_file_gently(&table_locks[i - first]);
                if (err < 0) {
                        err = REFTABLE_IO_ERROR;
                        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 = rollback_lock_file(&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, &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 = hold_lock_file_for_update(&tables_list_lock, st->list_file,
                                        LOCK_NO_DEREF);
        if (err < 0) {
                if (errno == EEXIST)
                        err = 1;
                else
                        err = REFTABLE_IO_ERROR;
                goto done;
        }

        if (st->config.default_permissions) {
                if (chmod(get_lock_file_path(&tables_list_lock),
                          st->config.default_permissions) < 0) {
                        err = REFTABLE_IO_ERROR;
                        goto done;
                }
        }

        /*
         * If the resulting compacted table is not empty, then we need to move
         * it into place now.
         */
        if (!is_empty_table) {
                format_name(&new_table_name, st->readers[first]->min_update_index,
                            st->readers[last]->max_update_index);
                strbuf_addstr(&new_table_name, ".ref");
                stack_filename(&new_table_path, st, new_table_name.buf);

                err = rename_tempfile(&new_table, new_table_path.buf);
                if (err < 0) {
                        err = REFTABLE_IO_ERROR;
                        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; i++)
                strbuf_addf(&tables_list_buf, "%s\n", st->readers[i]->name);
        if (!is_empty_table)
                strbuf_addf(&tables_list_buf, "%s\n", new_table_name.buf);
        for (i = last + 1; i < st->merged->stack_len; i++)
                strbuf_addf(&tables_list_buf, "%s\n", st->readers[i]->name);

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

        err = fsync_component(FSYNC_COMPONENT_REFERENCE, get_lock_file_fd(&tables_list_lock));
        if (err < 0) {
                err = REFTABLE_IO_ERROR;
                unlink(new_table_path.buf);
                goto done;
        }

        err = commit_lock_file(&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 = first; i <= last; i++) {
                struct lock_file *table_lock = &table_locks[i - first];
                char *table_path = get_locked_file_path(table_lock);
                unlink(table_path);
                free(table_path);
        }

done:
        rollback_lock_file(&tables_list_lock);
        for (i = first; table_locks && i <= last; i++)
                rollback_lock_file(&table_locks[i - first]);
        reftable_free(table_locks);

        delete_tempfile(&new_table);
        strbuf_release(&new_table_name);
        strbuf_release(&new_table_path);

        strbuf_release(&tables_list_buf);
        strbuf_release(&table_name);
        return err;
}

int reftable_stack_compact_all(struct reftable_stack *st,
                               struct reftable_log_expiry_config *config)
{
        return stack_compact_range(st, 0, st->merged->stack_len ?
                        st->merged->stack_len - 1 : 0, config);
}

static int stack_compact_range_stats(struct reftable_stack *st,
                                     size_t first, size_t last,
                                     struct reftable_log_expiry_config *config)
{
        int err = stack_compact_range(st, first, last, config);
        if (err > 0)
                st->stats.failures++;
        return err;
}

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

int fastlog2(uint64_t sz)
{
        int l = 0;
        if (sz == 0)
                return 0;
        for (; sz; sz /= 2) {
                l++;
        }
        return l - 1;
}

struct segment *sizes_to_segments(size_t *seglen, uint64_t *sizes, size_t n)
{
        struct segment *segs = reftable_calloc(n, sizeof(*segs));
        struct segment cur = { 0 };
        size_t next = 0, i;

        if (n == 0) {
                *seglen = 0;
                return segs;
        }
        for (i = 0; i < n; i++) {
                int log = fastlog2(sizes[i]);
                if (cur.log != log && cur.bytes > 0) {
                        struct segment fresh = {
                                .start = i,
                        };

                        segs[next++] = cur;
                        cur = fresh;
                }

                cur.log = log;
                cur.end = i + 1;
                cur.bytes += sizes[i];
        }
        segs[next++] = cur;
        *seglen = next;
        return segs;
}

struct segment suggest_compaction_segment(uint64_t *sizes, size_t n)
{
        struct segment min_seg = {
                .log = 64,
        };
        struct segment *segs;
        size_t seglen = 0, i;

        segs = sizes_to_segments(&seglen, sizes, n);
        for (i = 0; i < seglen; i++) {
                if (segment_size(&segs[i]) == 1)
                        continue;

                if (segs[i].log < min_seg.log)
                        min_seg = segs[i];
        }

        while (min_seg.start > 0) {
                size_t prev = min_seg.start - 1;
                if (fastlog2(min_seg.bytes) < fastlog2(sizes[prev]))
                        break;

                min_seg.start = prev;
                min_seg.bytes += sizes[prev];
        }

        reftable_free(segs);
        return min_seg;
}

static uint64_t *stack_table_sizes_for_compaction(struct reftable_stack *st)
{
        uint64_t *sizes =
                reftable_calloc(st->merged->stack_len, sizeof(*sizes));
        int version = (st->config.hash_id == GIT_SHA1_FORMAT_ID) ? 1 : 2;
        int overhead = header_size(version) - 1;
        int i = 0;
        for (i = 0; i < st->merged->stack_len; i++) {
                sizes[i] = st->readers[i]->size - overhead;
        }
        return sizes;
}

int reftable_stack_auto_compact(struct reftable_stack *st)
{
        uint64_t *sizes = stack_table_sizes_for_compaction(st);
        struct segment seg =
                suggest_compaction_segment(sizes, st->merged->stack_len);
        reftable_free(sizes);
        if (segment_size(&seg) > 0)
                return stack_compact_range_stats(st, seg.start, seg.end - 1,
                                                 NULL);

        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_table tab = { NULL };
        reftable_table_from_merged_table(&tab, reftable_stack_merged_table(st));
        return reftable_table_read_ref(&tab, refname, ref);
}

int reftable_stack_read_log(struct reftable_stack *st, const char *refname,
                            struct reftable_log_record *log)
{
        struct reftable_iterator it = { NULL };
        struct reftable_merged_table *mt = reftable_stack_merged_table(st);
        int err = reftable_merged_table_seek_log(mt, &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 stack_check_addition(struct reftable_stack *st,
                                const char *new_tab_name)
{
        int err = 0;
        struct reftable_block_source src = { NULL };
        struct reftable_reader *rd = NULL;
        struct reftable_table tab = { NULL };
        struct reftable_ref_record *refs = NULL;
        struct reftable_iterator it = { NULL };
        int cap = 0;
        int len = 0;
        int i = 0;

        if (st->config.skip_name_check)
                return 0;

        err = reftable_block_source_from_file(&src, new_tab_name);
        if (err < 0)
                goto done;

        err = reftable_new_reader(&rd, &src, new_tab_name);
        if (err < 0)
                goto done;

        err = reftable_reader_seek_ref(rd, &it, "");
        if (err > 0) {
                err = 0;
                goto done;
        }
        if (err < 0)
                goto done;

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

                REFTABLE_ALLOC_GROW(refs, len + 1, cap);
                refs[len++] = ref;
        }

        reftable_table_from_merged_table(&tab, reftable_stack_merged_table(st));

        err = validate_ref_record_addition(tab, refs, len);

done:
        for (i = 0; i < len; i++) {
                reftable_ref_record_release(&refs[i]);
        }

        free(refs);
        reftable_iterator_destroy(&it);
        reftable_reader_free(rd);
        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_reader *rd = NULL;
        struct strbuf table_path = STRBUF_INIT;
        stack_filename(&table_path, st, name);

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

        err = reftable_new_reader(&rd, &src, name);
        if (err < 0)
                goto done;

        update_idx = reftable_reader_max_update_index(rd);
        reftable_reader_free(rd);

        if (update_idx <= max) {
                unlink(table_path.buf);
        }
done:
        strbuf_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 i = 0;
                int found = 0;
                if (!is_table_name(d->d_name))
                        continue;

                for (i = 0; !found && i < st->readers_len; i++) {
                        found = !strcmp(reader_name(st->readers[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_new_addition(&add, st);
        if (err < 0) {
                goto done;
        }

        err = reftable_stack_reload(st);
        if (err < 0) {
                goto done;
        }

        err = reftable_stack_clean_locked(st);

done:
        reftable_addition_destroy(add);
        return err;
}

int reftable_stack_print_directory(const char *stackdir, uint32_t hash_id)
{
        struct reftable_stack *stack = NULL;
        struct reftable_write_options cfg = { .hash_id = hash_id };
        struct reftable_merged_table *merged = NULL;
        struct reftable_table table = { NULL };

        int err = reftable_new_stack(&stack, stackdir, cfg);
        if (err < 0)
                goto done;

        merged = reftable_stack_merged_table(stack);
        reftable_table_from_merged_table(&table, merged);
        err = reftable_table_print(&table);
done:
        if (stack)
                reftable_stack_destroy(stack);
        return err;
}