tdb: consolidate tdb allocation code - re-use dead records at hash top.

[Samba/wip.git] / lib / tdb / common / lock.c

 /*
   Unix SMB/CIFS implementation.

   trivial database library

   Copyright (C) Andrew Tridgell              1999-2005
   Copyright (C) Paul `Rusty' Russell              2000
   Copyright (C) Jeremy Allison                    2000-2003

     ** NOTE! The following LGPL license applies to the tdb
     ** library. This does NOT imply that all of Samba is released
     ** under the LGPL

   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 "tdb_private.h"

_PUBLIC_ void tdb_setalarm_sigptr(struct tdb_context *tdb, volatile sig_atomic_t *ptr)
{
        tdb->interrupt_sig_ptr = ptr;
}

static int fcntl_lock(struct tdb_context *tdb,
                      int rw, off_t off, off_t len, bool waitflag)
{
        struct flock fl;
        int cmd;

        fl.l_type = rw;
        fl.l_whence = SEEK_SET;
        fl.l_start = off;
        fl.l_len = len;
        fl.l_pid = 0;

        cmd = waitflag ? F_SETLKW : F_SETLK;

        return fcntl(tdb->fd, cmd, &fl);
}

static int fcntl_unlock(struct tdb_context *tdb, int rw, off_t off, off_t len)
{
        struct flock fl;
#if 0 /* Check they matched up locks and unlocks correctly. */
        char line[80];
        FILE *locks;
        bool found = false;

        locks = fopen("/proc/locks", "r");

        while (fgets(line, 80, locks)) {
                char *p;
                int type, start, l;

                /* eg. 1: FLOCK  ADVISORY  WRITE 2440 08:01:2180826 0 EOF */
                p = strchr(line, ':') + 1;
                if (strncmp(p, " POSIX  ADVISORY  ", strlen(" POSIX  ADVISORY  ")))
                        continue;
                p += strlen(" FLOCK  ADVISORY  ");
                if (strncmp(p, "READ  ", strlen("READ  ")) == 0)
                        type = F_RDLCK;
                else if (strncmp(p, "WRITE ", strlen("WRITE ")) == 0)
                        type = F_WRLCK;
                else
                        abort();
                p += 6;
                if (atoi(p) != getpid())
                        continue;
                p = strchr(strchr(p, ' ') + 1, ' ') + 1;
                start = atoi(p);
                p = strchr(p, ' ') + 1;
                if (strncmp(p, "EOF", 3) == 0)
                        l = 0;
                else
                        l = atoi(p) - start + 1;

                if (off == start) {
                        if (len != l) {
                                fprintf(stderr, "Len %u should be %u: %s",
                                        (int)len, l, line);
                                abort();
                        }
                        if (type != rw) {
                                fprintf(stderr, "Type %s wrong: %s",
                                        rw == F_RDLCK ? "READ" : "WRITE", line);
                                abort();
                        }
                        found = true;
                        break;
                }
        }

        if (!found) {
                fprintf(stderr, "Unlock on %u@%u not found!\n",
                        (int)off, (int)len);
                abort();
        }

        fclose(locks);
#endif

        fl.l_type = F_UNLCK;
        fl.l_whence = SEEK_SET;
        fl.l_start = off;
        fl.l_len = len;
        fl.l_pid = 0;

        return fcntl(tdb->fd, F_SETLKW, &fl);
}

/* list -1 is the alloc list, otherwise a hash chain. */
static tdb_off_t lock_offset(int list)
{
        return FREELIST_TOP + 4*list;
}

/* a byte range locking function - return 0 on success
   this functions locks/unlocks "len" byte at the specified offset.

   On error, errno is also set so that errors are passed back properly
   through tdb_open().

   note that a len of zero means lock to end of file
*/
int tdb_brlock(struct tdb_context *tdb,
               int rw_type, tdb_off_t offset, size_t len,
               enum tdb_lock_flags flags)
{
        int ret;

        if (tdb->flags & TDB_NOLOCK) {
                return 0;
        }

        if (flags & TDB_LOCK_MARK_ONLY) {
                return 0;
        }

        if ((rw_type == F_WRLCK) && (tdb->read_only || tdb->traverse_read)) {
                tdb->ecode = TDB_ERR_RDONLY;
                return -1;
        }

        do {
                ret = fcntl_lock(tdb, rw_type, offset, len,
                                 flags & TDB_LOCK_WAIT);
                /* Check for a sigalarm break. */
                if (ret == -1 && errno == EINTR &&
                                tdb->interrupt_sig_ptr &&
                                *tdb->interrupt_sig_ptr) {
                        break;
                }
        } while (ret == -1 && errno == EINTR);

        if (ret == -1) {
                tdb->ecode = TDB_ERR_LOCK;
                /* Generic lock error. errno set by fcntl.
                 * EAGAIN is an expected return from non-blocking
                 * locks. */
                if (!(flags & TDB_LOCK_PROBE) && errno != EAGAIN) {
                        TDB_LOG((tdb, TDB_DEBUG_TRACE,"tdb_brlock failed (fd=%d) at offset %u rw_type=%d flags=%d len=%zu\n",
                                 tdb->fd, offset, rw_type, flags, len));
                }
                return -1;
        }
        return 0;
}

int tdb_brunlock(struct tdb_context *tdb,
                 int rw_type, tdb_off_t offset, size_t len)
{
        int ret;

        if (tdb->flags & TDB_NOLOCK) {
                return 0;
        }

        do {
                ret = fcntl_unlock(tdb, rw_type, offset, len);
        } while (ret == -1 && errno == EINTR);

        if (ret == -1) {
                TDB_LOG((tdb, TDB_DEBUG_TRACE,"tdb_brunlock failed (fd=%d) at offset %u rw_type=%u len=%zu\n",
                         tdb->fd, offset, rw_type, len));
        }
        return ret;
}

/*
 * Do a tdb_brlock in a loop. Some OSes (such as solaris) have too
 * conservative deadlock detection and claim a deadlock when progress can be
 * made. For those OSes we may loop for a while.
 */

static int tdb_brlock_retry(struct tdb_context *tdb,
                            int rw_type, tdb_off_t offset, size_t len,
                            enum tdb_lock_flags flags)
{
        int count = 1000;

        while (count--) {
                struct timeval tv;
                int ret;

                ret = tdb_brlock(tdb, rw_type, offset, len, flags);
                if (ret == 0) {
                        return 0;
                }
                if (errno != EDEADLK) {
                        break;
                }
                /* sleep for as short a time as we can - more portable than usleep() */
                tv.tv_sec = 0;
                tv.tv_usec = 1;
                select(0, NULL, NULL, NULL, &tv);
        }
        return -1;
}

/*
  upgrade a read lock to a write lock.
*/
int tdb_allrecord_upgrade(struct tdb_context *tdb)
{
        int ret;

        if (tdb->allrecord_lock.count != 1) {
                TDB_LOG((tdb, TDB_DEBUG_ERROR,
                         "tdb_allrecord_upgrade failed: count %u too high\n",
                         tdb->allrecord_lock.count));
                return -1;
        }

        if (tdb->allrecord_lock.off != 1) {
                TDB_LOG((tdb, TDB_DEBUG_ERROR,
                         "tdb_allrecord_upgrade failed: already upgraded?\n"));
                return -1;
        }

        ret = tdb_brlock_retry(tdb, F_WRLCK, FREELIST_TOP, 0,
                               TDB_LOCK_WAIT|TDB_LOCK_PROBE);
        if (ret == 0) {
                tdb->allrecord_lock.ltype = F_WRLCK;
                tdb->allrecord_lock.off = 0;
                return 0;
        }
        TDB_LOG((tdb, TDB_DEBUG_TRACE,"tdb_allrecord_upgrade failed\n"));
        return -1;
}

static struct tdb_lock_type *find_nestlock(struct tdb_context *tdb,
                                           tdb_off_t offset)
{
        unsigned int i;

        for (i=0; i<tdb->num_lockrecs; i++) {
                if (tdb->lockrecs[i].off == offset) {
                        return &tdb->lockrecs[i];
                }
        }
        return NULL;
}

/* lock an offset in the database. */
int tdb_nest_lock(struct tdb_context *tdb, uint32_t offset, int ltype,
                  enum tdb_lock_flags flags)
{
        struct tdb_lock_type *new_lck;

        if (offset >= lock_offset(tdb->hash_size)) {
                tdb->ecode = TDB_ERR_LOCK;
                TDB_LOG((tdb, TDB_DEBUG_ERROR,"tdb_lock: invalid offset %u for ltype=%d\n",
                         offset, ltype));
                return -1;
        }
        if (tdb->flags & TDB_NOLOCK)
                return 0;

        new_lck = find_nestlock(tdb, offset);
        if (new_lck) {
                /*
                 * Just increment the in-memory struct, posix locks
                 * don't stack.
                 */
                new_lck->count++;
                return 0;
        }

        if (tdb->num_lockrecs == tdb->lockrecs_array_length) {
                new_lck = (struct tdb_lock_type *)realloc(
                        tdb->lockrecs,
                        sizeof(*tdb->lockrecs) * (tdb->num_lockrecs+1));
                if (new_lck == NULL) {
                        errno = ENOMEM;
                        return -1;
                }
                tdb->lockrecs_array_length = tdb->num_lockrecs+1;
                tdb->lockrecs = new_lck;
        }

        /* Since fcntl locks don't nest, we do a lock for the first one,
           and simply bump the count for future ones */
        if (tdb_brlock(tdb, ltype, offset, 1, flags)) {
                return -1;
        }

        new_lck = &tdb->lockrecs[tdb->num_lockrecs];

        new_lck->off = offset;
        new_lck->count = 1;
        new_lck->ltype = ltype;
        tdb->num_lockrecs++;

        return 0;
}

static int tdb_lock_and_recover(struct tdb_context *tdb)
{
        int ret;

        /* We need to match locking order in transaction commit. */
        if (tdb_brlock(tdb, F_WRLCK, FREELIST_TOP, 0, TDB_LOCK_WAIT)) {
                return -1;
        }

        if (tdb_brlock(tdb, F_WRLCK, OPEN_LOCK, 1, TDB_LOCK_WAIT)) {
                tdb_brunlock(tdb, F_WRLCK, FREELIST_TOP, 0);
                return -1;
        }

        ret = tdb_transaction_recover(tdb);

        tdb_brunlock(tdb, F_WRLCK, OPEN_LOCK, 1);
        tdb_brunlock(tdb, F_WRLCK, FREELIST_TOP, 0);

        return ret;
}

static bool have_data_locks(const struct tdb_context *tdb)
{
        unsigned int i;

        for (i = 0; i < tdb->num_lockrecs; i++) {
                if (tdb->lockrecs[i].off >= lock_offset(-1))
                        return true;
        }
        return false;
}

/*
 * A allrecord lock allows us to avoid per chain locks. Check if the allrecord
 * lock is strong enough.
 */
static int tdb_lock_covered_by_allrecord_lock(struct tdb_context *tdb,
                                              int ltype)
{
        if (ltype == F_RDLCK) {
                /*
                 * The allrecord_lock is equal (F_RDLCK) or stronger
                 * (F_WRLCK). Pass.
                 */
                return 0;
        }

        if (tdb->allrecord_lock.ltype == F_RDLCK) {
                /*
                 * We ask for ltype==F_WRLCK, but the allrecord_lock
                 * is too weak. We can't upgrade here, so fail.
                 */
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        /*
         * Asking for F_WRLCK, allrecord is F_WRLCK as well. Pass.
         */
        return 0;
}

static int tdb_lock_list(struct tdb_context *tdb, int list, int ltype,
                         enum tdb_lock_flags waitflag)
{
        int ret;
        bool check = false;

        if (tdb->allrecord_lock.count) {
                return tdb_lock_covered_by_allrecord_lock(tdb, ltype);
        }

        /*
         * Check for recoveries: Someone might have kill -9'ed a process
         * during a commit.
         */
        check = !have_data_locks(tdb);
        ret = tdb_nest_lock(tdb, lock_offset(list), ltype, waitflag);

        if (ret == 0 && check && tdb_needs_recovery(tdb)) {
                tdb_nest_unlock(tdb, lock_offset(list), ltype, false);

                if (tdb_lock_and_recover(tdb) == -1) {
                        return -1;
                }
                return tdb_lock_list(tdb, list, ltype, waitflag);
        }
        return ret;
}

/* lock a list in the database. list -1 is the alloc list */
int tdb_lock(struct tdb_context *tdb, int list, int ltype)
{
        int ret;

        ret = tdb_lock_list(tdb, list, ltype, TDB_LOCK_WAIT);
        if (ret) {
                TDB_LOG((tdb, TDB_DEBUG_ERROR, "tdb_lock failed on list %d "
                         "ltype=%d (%s)\n",  list, ltype, strerror(errno)));
        }
        return ret;
}

/* lock a list in the database. list -1 is the alloc list. non-blocking lock */
_PUBLIC_ int tdb_lock_nonblock(struct tdb_context *tdb, int list, int ltype)
{
        return tdb_lock_list(tdb, list, ltype, TDB_LOCK_NOWAIT);
}


int tdb_nest_unlock(struct tdb_context *tdb, uint32_t offset, int ltype,
                    bool mark_lock)
{
        int ret = -1;
        struct tdb_lock_type *lck;

        if (tdb->flags & TDB_NOLOCK)
                return 0;

        /* Sanity checks */
        if (offset >= lock_offset(tdb->hash_size)) {
                TDB_LOG((tdb, TDB_DEBUG_ERROR, "tdb_unlock: offset %u invalid (%d)\n", offset, tdb->hash_size));
                return ret;
        }

        lck = find_nestlock(tdb, offset);
        if ((lck == NULL) || (lck->count == 0)) {
                TDB_LOG((tdb, TDB_DEBUG_ERROR, "tdb_unlock: count is 0\n"));
                return -1;
        }

        if (lck->count > 1) {
                lck->count--;
                return 0;
        }

        /*
         * This lock has count==1 left, so we need to unlock it in the
         * kernel. We don't bother with decrementing the in-memory array
         * element, we're about to overwrite it with the last array element
         * anyway.
         */

        if (mark_lock) {
                ret = 0;
        } else {
                ret = tdb_brunlock(tdb, ltype, offset, 1);
        }

        /*
         * Shrink the array by overwriting the element just unlocked with the
         * last array element.
         */
        *lck = tdb->lockrecs[--tdb->num_lockrecs];

        /*
         * We don't bother with realloc when the array shrinks, but if we have
         * a completely idle tdb we should get rid of the locked array.
         */

        if (ret)
                TDB_LOG((tdb, TDB_DEBUG_ERROR, "tdb_unlock: An error occurred unlocking!\n"));
        return ret;
}

_PUBLIC_ int tdb_unlock(struct tdb_context *tdb, int list, int ltype)
{
        /* a global lock allows us to avoid per chain locks */
        if (tdb->allrecord_lock.count) {
                return tdb_lock_covered_by_allrecord_lock(tdb, ltype);
        }

        return tdb_nest_unlock(tdb, lock_offset(list), ltype, false);
}

/*
  get the transaction lock
 */
int tdb_transaction_lock(struct tdb_context *tdb, int ltype,
                         enum tdb_lock_flags lockflags)
{
        return tdb_nest_lock(tdb, TRANSACTION_LOCK, ltype, lockflags);
}

/*
  release the transaction lock
 */
int tdb_transaction_unlock(struct tdb_context *tdb, int ltype)
{
        return tdb_nest_unlock(tdb, TRANSACTION_LOCK, ltype, false);
}

/* Returns 0 if all done, -1 if error, 1 if ok. */
static int tdb_allrecord_check(struct tdb_context *tdb, int ltype,
                               enum tdb_lock_flags flags, bool upgradable)
{
        /* There are no locks on read-only dbs */
        if (tdb->read_only || tdb->traverse_read) {
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        if (tdb->allrecord_lock.count && tdb->allrecord_lock.ltype == ltype) {
                tdb->allrecord_lock.count++;
                return 0;
        }

        if (tdb->allrecord_lock.count) {
                /* a global lock of a different type exists */
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        if (tdb_have_extra_locks(tdb)) {
                /* can't combine global and chain locks */
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        if (upgradable && ltype != F_RDLCK) {
                /* tdb error: you can't upgrade a write lock! */
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }
        return 1;
}

/* We only need to lock individual bytes, but Linux merges consecutive locks
 * so we lock in contiguous ranges. */
static int tdb_chainlock_gradual(struct tdb_context *tdb,
                                 int ltype, enum tdb_lock_flags flags,
                                 size_t off, size_t len)
{
        int ret;
        enum tdb_lock_flags nb_flags = (flags & ~TDB_LOCK_WAIT);

        if (len <= 4) {
                /* Single record.  Just do blocking lock. */
                return tdb_brlock(tdb, ltype, off, len, flags);
        }

        /* First we try non-blocking. */
        ret = tdb_brlock(tdb, ltype, off, len, nb_flags);
        if (ret == 0) {
                return 0;
        }

        /* Try locking first half, then second. */
        ret = tdb_chainlock_gradual(tdb, ltype, flags, off, len / 2);
        if (ret == -1)
                return -1;

        ret = tdb_chainlock_gradual(tdb, ltype, flags,
                                    off + len / 2, len - len / 2);
        if (ret == -1) {
                tdb_brunlock(tdb, ltype, off, len / 2);
                return -1;
        }
        return 0;
}

/* lock/unlock entire database.  It can only be upgradable if you have some
 * other way of guaranteeing exclusivity (ie. transaction write lock).
 * We do the locking gradually to avoid being starved by smaller locks. */
int tdb_allrecord_lock(struct tdb_context *tdb, int ltype,
                       enum tdb_lock_flags flags, bool upgradable)
{
        switch (tdb_allrecord_check(tdb, ltype, flags, upgradable)) {
        case -1:
                return -1;
        case 0:
                return 0;
        }

        /* We cover two kinds of locks:
         * 1) Normal chain locks.  Taken for almost all operations.
         * 2) Individual records locks.  Taken after normal or free
         *    chain locks.
         *
         * It is (1) which cause the starvation problem, so we're only
         * gradual for that. */
        if (tdb_chainlock_gradual(tdb, ltype, flags, FREELIST_TOP,
                                  tdb->hash_size * 4) == -1) {
                return -1;
        }

        /* Grab individual record locks. */
        if (tdb_brlock(tdb, ltype, lock_offset(tdb->hash_size), 0,
                       flags) == -1) {
                tdb_brunlock(tdb, ltype, FREELIST_TOP,
                             tdb->hash_size * 4);
                return -1;
        }

        tdb->allrecord_lock.count = 1;
        /* If it's upgradable, it's actually exclusive so we can treat
         * it as a write lock. */
        tdb->allrecord_lock.ltype = upgradable ? F_WRLCK : ltype;
        tdb->allrecord_lock.off = upgradable;

        if (tdb_needs_recovery(tdb)) {
                bool mark = flags & TDB_LOCK_MARK_ONLY;
                tdb_allrecord_unlock(tdb, ltype, mark);
                if (mark) {
                        tdb->ecode = TDB_ERR_LOCK;
                        TDB_LOG((tdb, TDB_DEBUG_ERROR,
                                 "tdb_lockall_mark cannot do recovery\n"));
                        return -1;
                }
                if (tdb_lock_and_recover(tdb) == -1) {
                        return -1;
                }
                return tdb_allrecord_lock(tdb, ltype, flags, upgradable);
        }

        return 0;
}



/* unlock entire db */
int tdb_allrecord_unlock(struct tdb_context *tdb, int ltype, bool mark_lock)
{
        /* There are no locks on read-only dbs */
        if (tdb->read_only || tdb->traverse_read) {
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        if (tdb->allrecord_lock.count == 0) {
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        /* Upgradable locks are marked as write locks. */
        if (tdb->allrecord_lock.ltype != ltype
            && (!tdb->allrecord_lock.off || ltype != F_RDLCK)) {
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        if (tdb->allrecord_lock.count > 1) {
                tdb->allrecord_lock.count--;
                return 0;
        }

        if (!mark_lock && tdb_brunlock(tdb, ltype, FREELIST_TOP, 0)) {
                TDB_LOG((tdb, TDB_DEBUG_ERROR, "tdb_unlockall failed (%s)\n", strerror(errno)));
                return -1;
        }

        tdb->allrecord_lock.count = 0;
        tdb->allrecord_lock.ltype = 0;

        return 0;
}

/* lock entire database with write lock */
_PUBLIC_ int tdb_lockall(struct tdb_context *tdb)
{
        tdb_trace(tdb, "tdb_lockall");
        return tdb_allrecord_lock(tdb, F_WRLCK, TDB_LOCK_WAIT, false);
}

/* lock entire database with write lock - mark only */
_PUBLIC_ int tdb_lockall_mark(struct tdb_context *tdb)
{
        tdb_trace(tdb, "tdb_lockall_mark");
        return tdb_allrecord_lock(tdb, F_WRLCK, TDB_LOCK_MARK_ONLY, false);
}

/* unlock entire database with write lock - unmark only */
_PUBLIC_ int tdb_lockall_unmark(struct tdb_context *tdb)
{
        tdb_trace(tdb, "tdb_lockall_unmark");
        return tdb_allrecord_unlock(tdb, F_WRLCK, true);
}

/* lock entire database with write lock - nonblocking varient */
_PUBLIC_ int tdb_lockall_nonblock(struct tdb_context *tdb)
{
        int ret = tdb_allrecord_lock(tdb, F_WRLCK, TDB_LOCK_NOWAIT, false);
        tdb_trace_ret(tdb, "tdb_lockall_nonblock", ret);
        return ret;
}

/* unlock entire database with write lock */
_PUBLIC_ int tdb_unlockall(struct tdb_context *tdb)
{
        tdb_trace(tdb, "tdb_unlockall");
        return tdb_allrecord_unlock(tdb, F_WRLCK, false);
}

/* lock entire database with read lock */
_PUBLIC_ int tdb_lockall_read(struct tdb_context *tdb)
{
        tdb_trace(tdb, "tdb_lockall_read");
        return tdb_allrecord_lock(tdb, F_RDLCK, TDB_LOCK_WAIT, false);
}

/* lock entire database with read lock - nonblock varient */
_PUBLIC_ int tdb_lockall_read_nonblock(struct tdb_context *tdb)
{
        int ret = tdb_allrecord_lock(tdb, F_RDLCK, TDB_LOCK_NOWAIT, false);
        tdb_trace_ret(tdb, "tdb_lockall_read_nonblock", ret);
        return ret;
}

/* unlock entire database with read lock */
_PUBLIC_ int tdb_unlockall_read(struct tdb_context *tdb)
{
        tdb_trace(tdb, "tdb_unlockall_read");
        return tdb_allrecord_unlock(tdb, F_RDLCK, false);
}

/* lock/unlock one hash chain. This is meant to be used to reduce
   contention - it cannot guarantee how many records will be locked */
_PUBLIC_ int tdb_chainlock(struct tdb_context *tdb, TDB_DATA key)
{
        int ret = tdb_lock(tdb, BUCKET(tdb->hash_fn(&key)), F_WRLCK);
        tdb_trace_1rec(tdb, "tdb_chainlock", key);
        return ret;
}

/* lock/unlock one hash chain, non-blocking. This is meant to be used
   to reduce contention - it cannot guarantee how many records will be
   locked */
_PUBLIC_ int tdb_chainlock_nonblock(struct tdb_context *tdb, TDB_DATA key)
{
        int ret = tdb_lock_nonblock(tdb, BUCKET(tdb->hash_fn(&key)), F_WRLCK);
        tdb_trace_1rec_ret(tdb, "tdb_chainlock_nonblock", key, ret);
        return ret;
}

/* mark a chain as locked without actually locking it. Warning! use with great caution! */
_PUBLIC_ int tdb_chainlock_mark(struct tdb_context *tdb, TDB_DATA key)
{
        int ret = tdb_nest_lock(tdb, lock_offset(BUCKET(tdb->hash_fn(&key))),
                                F_WRLCK, TDB_LOCK_MARK_ONLY);
        tdb_trace_1rec(tdb, "tdb_chainlock_mark", key);
        return ret;
}

/* unmark a chain as locked without actually locking it. Warning! use with great caution! */
_PUBLIC_ int tdb_chainlock_unmark(struct tdb_context *tdb, TDB_DATA key)
{
        tdb_trace_1rec(tdb, "tdb_chainlock_unmark", key);
        return tdb_nest_unlock(tdb, lock_offset(BUCKET(tdb->hash_fn(&key))),
                               F_WRLCK, true);
}

_PUBLIC_ int tdb_chainunlock(struct tdb_context *tdb, TDB_DATA key)
{
        tdb_trace_1rec(tdb, "tdb_chainunlock", key);
        return tdb_unlock(tdb, BUCKET(tdb->hash_fn(&key)), F_WRLCK);
}

_PUBLIC_ int tdb_chainlock_read(struct tdb_context *tdb, TDB_DATA key)
{
        int ret;
        ret = tdb_lock(tdb, BUCKET(tdb->hash_fn(&key)), F_RDLCK);
        tdb_trace_1rec(tdb, "tdb_chainlock_read", key);
        return ret;
}

_PUBLIC_ int tdb_chainunlock_read(struct tdb_context *tdb, TDB_DATA key)
{
        tdb_trace_1rec(tdb, "tdb_chainunlock_read", key);
        return tdb_unlock(tdb, BUCKET(tdb->hash_fn(&key)), F_RDLCK);
}

/* record lock stops delete underneath */
int tdb_lock_record(struct tdb_context *tdb, tdb_off_t off)
{
        if (tdb->allrecord_lock.count) {
                return 0;
        }
        return off ? tdb_brlock(tdb, F_RDLCK, off, 1, TDB_LOCK_WAIT) : 0;
}

/*
  Write locks override our own fcntl readlocks, so check it here.
  Note this is meant to be F_SETLK, *not* F_SETLKW, as it's not
  an error to fail to get the lock here.
*/
int tdb_write_lock_record(struct tdb_context *tdb, tdb_off_t off)
{
        struct tdb_traverse_lock *i;
        for (i = &tdb->travlocks; i; i = i->next)
                if (i->off == off)
                        return -1;
        if (tdb->allrecord_lock.count) {
                if (tdb->allrecord_lock.ltype == F_WRLCK) {
                        return 0;
                }
                return -1;
        }
        return tdb_brlock(tdb, F_WRLCK, off, 1, TDB_LOCK_NOWAIT|TDB_LOCK_PROBE);
}

int tdb_write_unlock_record(struct tdb_context *tdb, tdb_off_t off)
{
        if (tdb->allrecord_lock.count) {
                return 0;
        }
        return tdb_brunlock(tdb, F_WRLCK, off, 1);
}

/* fcntl locks don't stack: avoid unlocking someone else's */
int tdb_unlock_record(struct tdb_context *tdb, tdb_off_t off)
{
        struct tdb_traverse_lock *i;
        uint32_t count = 0;

        if (tdb->allrecord_lock.count) {
                return 0;
        }

        if (off == 0)
                return 0;
        for (i = &tdb->travlocks; i; i = i->next)
                if (i->off == off)
                        count++;
        return (count == 1 ? tdb_brunlock(tdb, F_RDLCK, off, 1) : 0);
}

bool tdb_have_extra_locks(struct tdb_context *tdb)
{
        unsigned int extra = tdb->num_lockrecs;

        /* A transaction holds the lock for all records. */
        if (!tdb->transaction && tdb->allrecord_lock.count) {
                return true;
        }

        /* We always hold the active lock if CLEAR_IF_FIRST. */
        if (find_nestlock(tdb, ACTIVE_LOCK)) {
                extra--;
        }

        /* In a transaction, we expect to hold the transaction lock */
        if (tdb->transaction && find_nestlock(tdb, TRANSACTION_LOCK)) {
                extra--;
        }

        return extra;
}

/* The transaction code uses this to remove all locks. */
void tdb_release_transaction_locks(struct tdb_context *tdb)
{
        unsigned int i, active = 0;

        if (tdb->allrecord_lock.count != 0) {
                tdb_allrecord_unlock(tdb, tdb->allrecord_lock.ltype, false);
                tdb->allrecord_lock.count = 0;
        }

        for (i=0;i<tdb->num_lockrecs;i++) {
                struct tdb_lock_type *lck = &tdb->lockrecs[i];

                /* Don't release the active lock!  Copy it to first entry. */
                if (lck->off == ACTIVE_LOCK) {
                        tdb->lockrecs[active++] = *lck;
                } else {
                        tdb_brunlock(tdb, lck->ltype, lck->off, 1);
                }
        }
        tdb->num_lockrecs = active;
}

/* Following functions are added specifically to support CTDB. */

/* Don't do actual fcntl locking, just mark tdb locked */
int tdb_transaction_write_lock_mark(struct tdb_context *tdb);
_PUBLIC_ int tdb_transaction_write_lock_mark(struct tdb_context *tdb)
{
        return tdb_transaction_lock(tdb, F_WRLCK, TDB_LOCK_MARK_ONLY);
}

/* Don't do actual fcntl unlocking, just mark tdb unlocked */
int tdb_transaction_write_lock_unmark(struct tdb_context *tdb);
_PUBLIC_ int tdb_transaction_write_lock_unmark(struct tdb_context *tdb)
{
        return tdb_nest_unlock(tdb, TRANSACTION_LOCK, F_WRLCK, true);
}