doc: add FSRVP commands to rpcclient man page

[Samba/gbeck.git] / lib / tdb2 / free.c

 /*
   Trivial Database 2: free list/block handling
   Copyright (C) Rusty Russell 2010

   This library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 3 of the License, or (at your option) any later version.

   This library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with this library; if not, see <http://www.gnu.org/licenses/>.
*/
#include "private.h"
#include <ccan/likely/likely.h>
#include <ccan/ilog/ilog.h>
#include <time.h>
#include <assert.h>
#include <limits.h>

static unsigned fls64(uint64_t val)
{
        return ilog64(val);
}

/* In which bucket would we find a particular record size? (ignoring header) */
unsigned int size_to_bucket(tdb_len_t data_len)
{
        unsigned int bucket;

        /* We can't have records smaller than this. */
        assert(data_len >= TDB_MIN_DATA_LEN);

        /* Ignoring the header... */
        if (data_len - TDB_MIN_DATA_LEN <= 64) {
                /* 0 in bucket 0, 8 in bucket 1... 64 in bucket 8. */
                bucket = (data_len - TDB_MIN_DATA_LEN) / 8;
        } else {
                /* After that we go power of 2. */
                bucket = fls64(data_len - TDB_MIN_DATA_LEN) + 2;
        }

        if (unlikely(bucket >= TDB_FREE_BUCKETS))
                bucket = TDB_FREE_BUCKETS - 1;
        return bucket;
}

tdb_off_t first_ftable(struct tdb_context *tdb)
{
        return tdb_read_off(tdb, offsetof(struct tdb_header, free_table));
}

tdb_off_t next_ftable(struct tdb_context *tdb, tdb_off_t ftable)
{
        return tdb_read_off(tdb, ftable + offsetof(struct tdb_freetable,next));
}

enum TDB_ERROR tdb_ftable_init(struct tdb_context *tdb)
{
        /* Use reservoir sampling algorithm to select a free list at random. */
        unsigned int rnd, max = 0, count = 0;
        tdb_off_t off;

        tdb->tdb2.ftable_off = off = first_ftable(tdb);
        tdb->tdb2.ftable = 0;

        while (off) {
                if (TDB_OFF_IS_ERR(off)) {
                        return TDB_OFF_TO_ERR(off);
                }

                rnd = random();
                if (rnd >= max) {
                        tdb->tdb2.ftable_off = off;
                        tdb->tdb2.ftable = count;
                        max = rnd;
                }

                off = next_ftable(tdb, off);
                count++;
        }
        return TDB_SUCCESS;
}

/* Offset of a given bucket. */
tdb_off_t bucket_off(tdb_off_t ftable_off, unsigned bucket)
{
        return ftable_off + offsetof(struct tdb_freetable, buckets)
                + bucket * sizeof(tdb_off_t);
}

/* Returns free_buckets + 1, or list number to search, or -ve error. */
static tdb_off_t find_free_head(struct tdb_context *tdb,
                                tdb_off_t ftable_off,
                                tdb_off_t bucket)
{
        /* Speculatively search for a non-zero bucket. */
        return tdb_find_nonzero_off(tdb, bucket_off(ftable_off, 0),
                                    bucket, TDB_FREE_BUCKETS);
}

static void check_list(struct tdb_context *tdb, tdb_off_t b_off)
{
#ifdef CCAN_TDB2_DEBUG
        tdb_off_t off, prev = 0, first;
        struct tdb_free_record r;

        first = off = (tdb_read_off(tdb, b_off) & TDB_OFF_MASK);
        while (off != 0) {
                tdb_read_convert(tdb, off, &r, sizeof(r));
                if (frec_magic(&r) != TDB_FREE_MAGIC)
                        abort();
                if (prev && frec_prev(&r) != prev)
                        abort();
                prev = off;
                off = r.next;
        }

        if (first) {
                tdb_read_convert(tdb, first, &r, sizeof(r));
                if (frec_prev(&r) != prev)
                        abort();
        }
#endif
}

/* Remove from free bucket. */
static enum TDB_ERROR remove_from_list(struct tdb_context *tdb,
                                       tdb_off_t b_off, tdb_off_t r_off,
                                       const struct tdb_free_record *r)
{
        tdb_off_t off, prev_next, head;
        enum TDB_ERROR ecode;

        /* Is this only element in list?  Zero out bucket, and we're done. */
        if (frec_prev(r) == r_off)
                return tdb_write_off(tdb, b_off, 0);

        /* off = &r->prev->next */
        off = frec_prev(r) + offsetof(struct tdb_free_record, next);

        /* Get prev->next */
        prev_next = tdb_read_off(tdb, off);
        if (TDB_OFF_IS_ERR(prev_next))
                return TDB_OFF_TO_ERR(prev_next);

        /* If prev->next == 0, we were head: update bucket to point to next. */
        if (prev_next == 0) {
                /* We must preserve upper bits. */
                head = tdb_read_off(tdb, b_off);
                if (TDB_OFF_IS_ERR(head))
                        return TDB_OFF_TO_ERR(head);

                if ((head & TDB_OFF_MASK) != r_off) {
                        return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
                                          "remove_from_list:"
                                          " %llu head %llu on list %llu",
                                          (long long)r_off,
                                          (long long)head,
                                          (long long)b_off);
                }
                head = ((head & ~TDB_OFF_MASK) | r->next);
                ecode = tdb_write_off(tdb, b_off, head);
                if (ecode != TDB_SUCCESS)
                        return ecode;
        } else {
                /* r->prev->next = r->next */
                ecode = tdb_write_off(tdb, off, r->next);
                if (ecode != TDB_SUCCESS)
                        return ecode;
        }

        /* If we were the tail, off = &head->prev. */
        if (r->next == 0) {
                head = tdb_read_off(tdb, b_off);
                if (TDB_OFF_IS_ERR(head))
                        return TDB_OFF_TO_ERR(head);
                head &= TDB_OFF_MASK;
                off = head + offsetof(struct tdb_free_record, magic_and_prev);
        } else {
                /* off = &r->next->prev */
                off = r->next + offsetof(struct tdb_free_record,
                                         magic_and_prev);
        }

#ifdef CCAN_TDB2_DEBUG
        /* *off == r */
        if ((tdb_read_off(tdb, off) & TDB_OFF_MASK) != r_off) {
                return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
                                  "remove_from_list:"
                                  " %llu bad prev in list %llu",
                                  (long long)r_off, (long long)b_off);
        }
#endif
        /* r->next->prev = r->prev */
        return tdb_write_off(tdb, off, r->magic_and_prev);
}

/* Enqueue in this free bucket: sets coalesce if we've added 128
 * entries to it. */
static enum TDB_ERROR enqueue_in_free(struct tdb_context *tdb,
                                      tdb_off_t b_off,
                                      tdb_off_t off,
                                      tdb_len_t len,
                                      bool *coalesce)
{
        struct tdb_free_record new;
        enum TDB_ERROR ecode;
        tdb_off_t prev, head;
        uint64_t magic = (TDB_FREE_MAGIC << (64 - TDB_OFF_UPPER_STEAL));

        head = tdb_read_off(tdb, b_off);
        if (TDB_OFF_IS_ERR(head))
                return TDB_OFF_TO_ERR(head);

        /* We only need to set ftable_and_len; rest is set in enqueue_in_free */
        new.ftable_and_len = ((uint64_t)tdb->tdb2.ftable << (64 - TDB_OFF_UPPER_STEAL))
                | len;

        /* new->next = head. */
        new.next = (head & TDB_OFF_MASK);

        /* First element?  Prev points to ourselves. */
        if (!new.next) {
                new.magic_and_prev = (magic | off);
        } else {
                /* new->prev = next->prev */
                prev = tdb_read_off(tdb,
                                    new.next + offsetof(struct tdb_free_record,
                                                        magic_and_prev));
                new.magic_and_prev = prev;
                if (frec_magic(&new) != TDB_FREE_MAGIC) {
                        return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
                                          "enqueue_in_free: %llu bad head"
                                          " prev %llu",
                                          (long long)new.next,
                                          (long long)prev);
                }
                /* next->prev = new. */
                ecode = tdb_write_off(tdb, new.next
                                      + offsetof(struct tdb_free_record,
                                                 magic_and_prev),
                                      off | magic);
                if (ecode != TDB_SUCCESS) {
                        return ecode;
                }

#ifdef CCAN_TDB2_DEBUG
                prev = tdb_read_off(tdb, frec_prev(&new)
                                    + offsetof(struct tdb_free_record, next));
                if (prev != 0) {
                        return tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
                                          "enqueue_in_free:"
                                          " %llu bad tail next ptr %llu",
                                          (long long)frec_prev(&new)
                                          + offsetof(struct tdb_free_record,
                                                     next),
                                          (long long)prev);
                }
#endif
        }

        /* Update enqueue count, but don't set high bit: see TDB_OFF_IS_ERR */
        if (*coalesce)
                head += (1ULL << (64 - TDB_OFF_UPPER_STEAL));
        head &= ~(TDB_OFF_MASK | (1ULL << 63));
        head |= off;

        ecode = tdb_write_off(tdb, b_off, head);
        if (ecode != TDB_SUCCESS) {
                return ecode;
        }

        /* It's time to coalesce if counter wrapped. */
        if (*coalesce)
                *coalesce = ((head & ~TDB_OFF_MASK) == 0);

        return tdb_write_convert(tdb, off, &new, sizeof(new));
}

static tdb_off_t ftable_offset(struct tdb_context *tdb, unsigned int ftable)
{
        tdb_off_t off;
        unsigned int i;

        if (likely(tdb->tdb2.ftable == ftable))
                return tdb->tdb2.ftable_off;

        off = first_ftable(tdb);
        for (i = 0; i < ftable; i++) {
                if (TDB_OFF_IS_ERR(off)) {
                        break;
                }
                off = next_ftable(tdb, off);
        }
        return off;
}

/* Note: we unlock the current bucket if fail (-ve), or coalesce (+ve) and
 * need to blatt the *protect record (which is set to an error). */
static tdb_len_t coalesce(struct tdb_context *tdb,
                          tdb_off_t off, tdb_off_t b_off,
                          tdb_len_t data_len,
                          tdb_off_t *protect)
{
        tdb_off_t end;
        struct tdb_free_record rec;
        enum TDB_ERROR ecode;

        tdb->stats.alloc_coalesce_tried++;
        end = off + sizeof(struct tdb_used_record) + data_len;

        while (end < tdb->file->map_size) {
                const struct tdb_free_record *r;
                tdb_off_t nb_off;
                unsigned ftable, bucket;

                r = tdb_access_read(tdb, end, sizeof(*r), true);
                if (TDB_PTR_IS_ERR(r)) {
                        ecode = TDB_PTR_ERR(r);
                        goto err;
                }

                if (frec_magic(r) != TDB_FREE_MAGIC
                    || frec_ftable(r) == TDB_FTABLE_NONE) {
                        tdb_access_release(tdb, r);
                        break;
                }

                ftable = frec_ftable(r);
                bucket = size_to_bucket(frec_len(r));
                nb_off = ftable_offset(tdb, ftable);
                if (TDB_OFF_IS_ERR(nb_off)) {
                        tdb_access_release(tdb, r);
                        ecode = TDB_OFF_TO_ERR(nb_off);
                        goto err;
                }
                nb_off = bucket_off(nb_off, bucket);
                tdb_access_release(tdb, r);

                /* We may be violating lock order here, so best effort. */
                if (tdb_lock_free_bucket(tdb, nb_off, TDB_LOCK_NOWAIT)
                    != TDB_SUCCESS) {
                        tdb->stats.alloc_coalesce_lockfail++;
                        break;
                }

                /* Now we have lock, re-check. */
                ecode = tdb_read_convert(tdb, end, &rec, sizeof(rec));
                if (ecode != TDB_SUCCESS) {
                        tdb_unlock_free_bucket(tdb, nb_off);
                        goto err;
                }

                if (unlikely(frec_magic(&rec) != TDB_FREE_MAGIC)) {
                        tdb->stats.alloc_coalesce_race++;
                        tdb_unlock_free_bucket(tdb, nb_off);
                        break;
                }

                if (unlikely(frec_ftable(&rec) != ftable)
                    || unlikely(size_to_bucket(frec_len(&rec)) != bucket)) {
                        tdb->stats.alloc_coalesce_race++;
                        tdb_unlock_free_bucket(tdb, nb_off);
                        break;
                }

                /* Did we just mess up a record you were hoping to use? */
                if (end == *protect) {
                        tdb->stats.alloc_coalesce_iterate_clash++;
                        *protect = TDB_ERR_TO_OFF(TDB_ERR_NOEXIST);
                }

                ecode = remove_from_list(tdb, nb_off, end, &rec);
                check_list(tdb, nb_off);
                if (ecode != TDB_SUCCESS) {
                        tdb_unlock_free_bucket(tdb, nb_off);
                        goto err;
                }

                end += sizeof(struct tdb_used_record) + frec_len(&rec);
                tdb_unlock_free_bucket(tdb, nb_off);
                tdb->stats.alloc_coalesce_num_merged++;
        }

        /* Didn't find any adjacent free? */
        if (end == off + sizeof(struct tdb_used_record) + data_len)
                return 0;

        /* Before we expand, check this isn't one you wanted protected? */
        if (off == *protect) {
                *protect = TDB_ERR_TO_OFF(TDB_ERR_EXISTS);
                tdb->stats.alloc_coalesce_iterate_clash++;
        }

        /* OK, expand initial record */
        ecode = tdb_read_convert(tdb, off, &rec, sizeof(rec));
        if (ecode != TDB_SUCCESS) {
                goto err;
        }

        if (frec_len(&rec) != data_len) {
                ecode = tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
                                   "coalesce: expected data len %zu not %zu",
                                   (size_t)data_len, (size_t)frec_len(&rec));
                goto err;
        }

        ecode = remove_from_list(tdb, b_off, off, &rec);
        check_list(tdb, b_off);
        if (ecode != TDB_SUCCESS) {
                goto err;
        }

        /* Try locking violation first.  We don't allow coalesce recursion! */
        ecode = add_free_record(tdb, off, end - off, TDB_LOCK_NOWAIT, false);
        if (ecode != TDB_SUCCESS) {
                /* Need to drop lock.  Can't rely on anything stable. */
                tdb->stats.alloc_coalesce_lockfail++;
                *protect = TDB_ERR_TO_OFF(TDB_ERR_CORRUPT);

                /* We have to drop this to avoid deadlocks, so make sure record
                 * doesn't get coalesced by someone else! */
                rec.ftable_and_len = (TDB_FTABLE_NONE
                                      << (64 - TDB_OFF_UPPER_STEAL))
                        | (end - off - sizeof(struct tdb_used_record));
                ecode = tdb_write_off(tdb,
                                      off + offsetof(struct tdb_free_record,
                                                     ftable_and_len),
                                      rec.ftable_and_len);
                if (ecode != TDB_SUCCESS) {
                        goto err;
                }

                tdb_unlock_free_bucket(tdb, b_off);

                ecode = add_free_record(tdb, off, end - off, TDB_LOCK_WAIT,
                                        false);
                if (ecode != TDB_SUCCESS) {
                        return TDB_ERR_TO_OFF(ecode);
                }
        } else if (TDB_OFF_IS_ERR(*protect)) {
                /* For simplicity, we always drop lock if they can't continue */
                tdb_unlock_free_bucket(tdb, b_off);
        }
        tdb->stats.alloc_coalesce_succeeded++;

        /* Return usable length. */
        return end - off - sizeof(struct tdb_used_record);

err:
        /* To unify error paths, we *always* unlock bucket on error. */
        tdb_unlock_free_bucket(tdb, b_off);
        return TDB_ERR_TO_OFF(ecode);
}

/* List is locked: we unlock it. */
static enum TDB_ERROR coalesce_list(struct tdb_context *tdb,
                                    tdb_off_t ftable_off,
                                    tdb_off_t b_off,
                                    unsigned int limit)
{
        enum TDB_ERROR ecode;
        tdb_off_t off;

        off = tdb_read_off(tdb, b_off);
        if (TDB_OFF_IS_ERR(off)) {
                ecode = TDB_OFF_TO_ERR(off);
                goto unlock_err;
        }
        /* A little bit of paranoia: counter should be 0. */
        off &= TDB_OFF_MASK;

        while (off && limit--) {
                struct tdb_free_record rec;
                tdb_len_t coal;
                tdb_off_t next;

                ecode = tdb_read_convert(tdb, off, &rec, sizeof(rec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                next = rec.next;
                coal = coalesce(tdb, off, b_off, frec_len(&rec), &next);
                if (TDB_OFF_IS_ERR(coal)) {
                        /* This has already unlocked on error. */
                        return TDB_OFF_TO_ERR(coal);
                }
                if (TDB_OFF_IS_ERR(next)) {
                        /* Coalescing had to unlock, so stop. */
                        return TDB_SUCCESS;
                }
                /* Keep going if we're doing well... */
                limit += size_to_bucket(coal / 16 + TDB_MIN_DATA_LEN);
                off = next;
        }

        /* Now, move those elements to the tail of the list so we get something
         * else next time. */
        if (off) {
                struct tdb_free_record oldhrec, newhrec, oldtrec, newtrec;
                tdb_off_t oldhoff, oldtoff, newtoff;

                /* The record we were up to is the new head. */
                ecode = tdb_read_convert(tdb, off, &newhrec, sizeof(newhrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                /* Get the new tail. */
                newtoff = frec_prev(&newhrec);
                ecode = tdb_read_convert(tdb, newtoff, &newtrec,
                                         sizeof(newtrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                /* Get the old head. */
                oldhoff = tdb_read_off(tdb, b_off);
                if (TDB_OFF_IS_ERR(oldhoff)) {
                        ecode = TDB_OFF_TO_ERR(oldhoff);
                        goto unlock_err;
                }

                /* This could happen if they all coalesced away. */
                if (oldhoff == off)
                        goto out;

                ecode = tdb_read_convert(tdb, oldhoff, &oldhrec,
                                         sizeof(oldhrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                /* Get the old tail. */
                oldtoff = frec_prev(&oldhrec);
                ecode = tdb_read_convert(tdb, oldtoff, &oldtrec,
                                         sizeof(oldtrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                /* Old tail's next points to old head. */
                oldtrec.next = oldhoff;

                /* Old head's prev points to old tail. */
                oldhrec.magic_and_prev
                        = (TDB_FREE_MAGIC << (64 - TDB_OFF_UPPER_STEAL))
                        | oldtoff;

                /* New tail's next is 0. */
                newtrec.next = 0;

                /* Write out the modified versions. */
                ecode = tdb_write_convert(tdb, oldtoff, &oldtrec,
                                          sizeof(oldtrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                ecode = tdb_write_convert(tdb, oldhoff, &oldhrec,
                                          sizeof(oldhrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                ecode = tdb_write_convert(tdb, newtoff, &newtrec,
                                          sizeof(newtrec));
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;

                /* And finally link in new head. */
                ecode = tdb_write_off(tdb, b_off, off);
                if (ecode != TDB_SUCCESS)
                        goto unlock_err;
        }
out:
        tdb_unlock_free_bucket(tdb, b_off);
        return TDB_SUCCESS;

unlock_err:
        tdb_unlock_free_bucket(tdb, b_off);
        return ecode;
}

/* List must not be locked if coalesce_ok is set. */
enum TDB_ERROR add_free_record(struct tdb_context *tdb,
                               tdb_off_t off, tdb_len_t len_with_header,
                               enum tdb_lock_flags waitflag,
                               bool coalesce_ok)
{
        tdb_off_t b_off;
        tdb_len_t len;
        enum TDB_ERROR ecode;

        assert(len_with_header >= sizeof(struct tdb_free_record));

        len = len_with_header - sizeof(struct tdb_used_record);

        b_off = bucket_off(tdb->tdb2.ftable_off, size_to_bucket(len));
        ecode = tdb_lock_free_bucket(tdb, b_off, waitflag);
        if (ecode != TDB_SUCCESS) {
                return ecode;
        }

        ecode = enqueue_in_free(tdb, b_off, off, len, &coalesce_ok);
        check_list(tdb, b_off);

        /* Coalescing unlocks free list. */
        if (!ecode && coalesce_ok)
                ecode = coalesce_list(tdb, tdb->tdb2.ftable_off, b_off, 2);
        else
                tdb_unlock_free_bucket(tdb, b_off);
        return ecode;
}

static size_t adjust_size(size_t keylen, size_t datalen)
{
        size_t size = keylen + datalen;

        if (size < TDB_MIN_DATA_LEN)
                size = TDB_MIN_DATA_LEN;

        /* Round to next uint64_t boundary. */
        return (size + (sizeof(uint64_t) - 1ULL)) & ~(sizeof(uint64_t) - 1ULL);
}

/* If we have enough left over to be useful, split that off. */
static size_t record_leftover(size_t keylen, size_t datalen,
                              bool want_extra, size_t total_len)
{
        ssize_t leftover;

        if (want_extra)
                datalen += datalen / 2;
        leftover = total_len - adjust_size(keylen, datalen);

        if (leftover < (ssize_t)sizeof(struct tdb_free_record))
                return 0;

        return leftover;
}

/* We need size bytes to put our key and data in. */
static tdb_off_t lock_and_alloc(struct tdb_context *tdb,
                                tdb_off_t ftable_off,
                                tdb_off_t bucket,
                                size_t keylen, size_t datalen,
                                bool want_extra,
                                unsigned magic,
                                unsigned hashlow)
{
        tdb_off_t off, b_off,best_off;
        struct tdb_free_record best = { 0 };
        double multiplier;
        size_t size = adjust_size(keylen, datalen);
        enum TDB_ERROR ecode;

        tdb->stats.allocs++;
        b_off = bucket_off(ftable_off, bucket);

        /* FIXME: Try non-blocking wait first, to measure contention. */
        /* Lock this bucket. */
        ecode = tdb_lock_free_bucket(tdb, b_off, TDB_LOCK_WAIT);
        if (ecode != TDB_SUCCESS) {
                return TDB_ERR_TO_OFF(ecode);
        }

        best.ftable_and_len = -1ULL;
        best_off = 0;

        /* Get slack if we're after extra. */
        if (want_extra)
                multiplier = 1.5;
        else
                multiplier = 1.0;

        /* Walk the list to see if any are large enough, getting less fussy
         * as we go. */
        off = tdb_read_off(tdb, b_off);
        if (TDB_OFF_IS_ERR(off)) {
                ecode = TDB_OFF_TO_ERR(off);
                goto unlock_err;
        }
        off &= TDB_OFF_MASK;

        while (off) {
                const struct tdb_free_record *r;
                tdb_len_t len;
                tdb_off_t next;

                r = tdb_access_read(tdb, off, sizeof(*r), true);
                if (TDB_PTR_IS_ERR(r)) {
                        ecode = TDB_PTR_ERR(r);
                        goto unlock_err;
                }

                if (frec_magic(r) != TDB_FREE_MAGIC) {
                        ecode = tdb_logerr(tdb, TDB_ERR_CORRUPT, TDB_LOG_ERROR,
                                           "lock_and_alloc:"
                                           " %llu non-free 0x%llx",
                                           (long long)off,
                                           (long long)r->magic_and_prev);
                        tdb_access_release(tdb, r);
                        goto unlock_err;
                }

                if (frec_len(r) >= size && frec_len(r) < frec_len(&best)) {
                        best_off = off;
                        best = *r;
                }

                if (frec_len(&best) <= size * multiplier && best_off) {
                        tdb_access_release(tdb, r);
                        break;
                }

                multiplier *= 1.01;

                next = r->next;
                len = frec_len(r);
                tdb_access_release(tdb, r);
                off = next;
        }

        /* If we found anything at all, use it. */
        if (best_off) {
                struct tdb_used_record rec;
                size_t leftover;

                /* We're happy with this size: take it. */
                ecode = remove_from_list(tdb, b_off, best_off, &best);
                check_list(tdb, b_off);
                if (ecode != TDB_SUCCESS) {
                        goto unlock_err;
                }

                leftover = record_leftover(keylen, datalen, want_extra,
                                           frec_len(&best));

                assert(keylen + datalen + leftover <= frec_len(&best));
                /* We need to mark non-free before we drop lock, otherwise
                 * coalesce() could try to merge it! */
                ecode = set_header(tdb, &rec, magic, keylen, datalen,
                                   frec_len(&best) - leftover, hashlow);
                if (ecode != TDB_SUCCESS) {
                        goto unlock_err;
                }

                ecode = tdb_write_convert(tdb, best_off, &rec, sizeof(rec));
                if (ecode != TDB_SUCCESS) {
                        goto unlock_err;
                }

                /* For futureproofing, we put a 0 in any unused space. */
                if (rec_extra_padding(&rec)) {
                        ecode = tdb->tdb2.io->twrite(tdb, best_off + sizeof(rec)
                                                     + keylen + datalen, "", 1);
                        if (ecode != TDB_SUCCESS) {
                                goto unlock_err;
                        }
                }

                /* Bucket of leftover will be <= current bucket, so nested
                 * locking is allowed. */
                if (leftover) {
                        tdb->stats.alloc_leftover++;
                        ecode = add_free_record(tdb,
                                                best_off + sizeof(rec)
                                                + frec_len(&best) - leftover,
                                                leftover, TDB_LOCK_WAIT, false);
                        if (ecode != TDB_SUCCESS) {
                                best_off = TDB_ERR_TO_OFF(ecode);
                        }
                }
                tdb_unlock_free_bucket(tdb, b_off);

                return best_off;
        }

        tdb_unlock_free_bucket(tdb, b_off);
        return 0;

unlock_err:
        tdb_unlock_free_bucket(tdb, b_off);
        return TDB_ERR_TO_OFF(ecode);
}

/* Get a free block from current free list, or 0 if none, -ve on error. */
static tdb_off_t get_free(struct tdb_context *tdb,
                          size_t keylen, size_t datalen, bool want_extra,
                          unsigned magic, unsigned hashlow)
{
        tdb_off_t off, ftable_off;
        tdb_off_t start_b, b, ftable;
        bool wrapped = false;

        /* If they are growing, add 50% to get to higher bucket. */
        if (want_extra)
                start_b = size_to_bucket(adjust_size(keylen,
                                                     datalen + datalen / 2));
        else
                start_b = size_to_bucket(adjust_size(keylen, datalen));

        ftable_off = tdb->tdb2.ftable_off;
        ftable = tdb->tdb2.ftable;
        while (!wrapped || ftable_off != tdb->tdb2.ftable_off) {
                /* Start at exact size bucket, and search up... */
                for (b = find_free_head(tdb, ftable_off, start_b);
                     b < TDB_FREE_BUCKETS;
                     b = find_free_head(tdb, ftable_off, b + 1)) {
                        /* Try getting one from list. */
                        off = lock_and_alloc(tdb, ftable_off,
                                             b, keylen, datalen, want_extra,
                                             magic, hashlow);
                        if (TDB_OFF_IS_ERR(off))
                                return off;
                        if (off != 0) {
                                if (b == start_b)
                                        tdb->stats.alloc_bucket_exact++;
                                if (b == TDB_FREE_BUCKETS - 1)
                                        tdb->stats.alloc_bucket_max++;
                                /* Worked?  Stay using this list. */
                                tdb->tdb2.ftable_off = ftable_off;
                                tdb->tdb2.ftable = ftable;
                                return off;
                        }
                        /* Didn't work.  Try next bucket. */
                }

                if (TDB_OFF_IS_ERR(b)) {
                        return b;
                }

                /* Hmm, try next table. */
                ftable_off = next_ftable(tdb, ftable_off);
                if (TDB_OFF_IS_ERR(ftable_off)) {
                        return ftable_off;
                }
                ftable++;

                if (ftable_off == 0) {
                        wrapped = true;
                        ftable_off = first_ftable(tdb);
                        if (TDB_OFF_IS_ERR(ftable_off)) {
                                return ftable_off;
                        }
                        ftable = 0;
                }
        }

        return 0;
}

enum TDB_ERROR set_header(struct tdb_context *tdb,
                          struct tdb_used_record *rec,
                          unsigned magic, uint64_t keylen, uint64_t datalen,
                          uint64_t actuallen, unsigned hashlow)
{
        uint64_t keybits = (fls64(keylen) + 1) / 2;

        /* Use bottom bits of hash, so it's independent of hash table size. */
        rec->magic_and_meta = (hashlow & ((1 << 11)-1))
                | ((actuallen - (keylen + datalen)) << 11)
                | (keybits << 43)
                | ((uint64_t)magic << 48);
        rec->key_and_data_len = (keylen | (datalen << (keybits*2)));

        /* Encoding can fail on big values. */
        if (rec_key_length(rec) != keylen
            || rec_data_length(rec) != datalen
            || rec_extra_padding(rec) != actuallen - (keylen + datalen)) {
                return tdb_logerr(tdb, TDB_ERR_IO, TDB_LOG_ERROR,
                                  "Could not encode k=%llu,d=%llu,a=%llu",
                                  (long long)keylen, (long long)datalen,
                                  (long long)actuallen);
        }
        return TDB_SUCCESS;
}

/* You need 'size', this tells you how much you should expand by. */
tdb_off_t tdb_expand_adjust(tdb_off_t map_size, tdb_off_t size)
{
        tdb_off_t new_size, top_size;

        /* limit size in order to avoid using up huge amounts of memory for
         * in memory tdbs if an oddball huge record creeps in */
        if (size > 100 * 1024) {
                top_size = map_size + size * 2;
        } else {
                top_size = map_size + size * 100;
        }

        /* always make room for at least top_size more records, and at
           least 25% more space. if the DB is smaller than 100MiB,
           otherwise grow it by 10% only. */
        if (map_size > 100 * 1024 * 1024) {
                new_size = map_size * 1.10;
        } else {
                new_size = map_size * 1.25;
        }

        /* Round the database up to a multiple of the page size */
        if (new_size < top_size)
                new_size = top_size;
        return new_size - map_size;
}

/* Expand the database. */
static enum TDB_ERROR tdb_expand(struct tdb_context *tdb, tdb_len_t size)
{
        uint64_t old_size;
        tdb_len_t wanted;
        enum TDB_ERROR ecode;

        /* Need to hold a hash lock to expand DB: transactions rely on it. */
        if (!(tdb->flags & TDB_NOLOCK)
            && !tdb->file->allrecord_lock.count && !tdb_has_hash_locks(tdb)) {
                return tdb_logerr(tdb, TDB_ERR_LOCK, TDB_LOG_ERROR,
                                  "tdb_expand: must hold lock during expand");
        }

        /* Only one person can expand file at a time. */
        ecode = tdb_lock_expand(tdb, F_WRLCK);
        if (ecode != TDB_SUCCESS) {
                return ecode;
        }

        /* Someone else may have expanded the file, so retry. */
        old_size = tdb->file->map_size;
        tdb->tdb2.io->oob(tdb, tdb->file->map_size, 1, true);
        if (tdb->file->map_size != old_size) {
                tdb_unlock_expand(tdb, F_WRLCK);
                return TDB_SUCCESS;
        }

        /* Overallocate. */
        wanted = tdb_expand_adjust(old_size, size);
        /* We need room for the record header too. */
        wanted = adjust_size(0, sizeof(struct tdb_used_record) + wanted);

        ecode = tdb->tdb2.io->expand_file(tdb, wanted);
        if (ecode != TDB_SUCCESS) {
                tdb_unlock_expand(tdb, F_WRLCK);
                return ecode;
        }

        /* We need to drop this lock before adding free record. */
        tdb_unlock_expand(tdb, F_WRLCK);

        tdb->stats.expands++;
        return add_free_record(tdb, old_size, wanted, TDB_LOCK_WAIT, true);
}

/* This won't fail: it will expand the database if it has to. */
tdb_off_t alloc(struct tdb_context *tdb, size_t keylen, size_t datalen,
                uint64_t hash, unsigned magic, bool growing)
{
        tdb_off_t off;

        /* We can't hold pointers during this: we could unmap! */
        assert(!tdb->tdb2.direct_access);

        for (;;) {
                enum TDB_ERROR ecode;
                off = get_free(tdb, keylen, datalen, growing, magic, hash);
                if (likely(off != 0))
                        break;

                ecode = tdb_expand(tdb, adjust_size(keylen, datalen));
                if (ecode != TDB_SUCCESS) {
                        return TDB_ERR_TO_OFF(ecode);
                }
        }

        return off;
}