lib: tdb: Use sigaction when testing for robust mutexes.

[Samba.git] / lib / tdb / common / mutex.c

/*
   Unix SMB/CIFS implementation.

   trivial database library

   Copyright (C) Volker Lendecke 2012,2013
   Copyright (C) Stefan Metzmacher 2013,2014
   Copyright (C) Michael Adam 2014

     ** 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"
#include "system/threads.h"

#ifdef USE_TDB_MUTEX_LOCKING

/*
 * If we run with mutexes, we store the "struct tdb_mutexes" at the
 * beginning of the file. We store an additional tdb_header right
 * beyond the mutex area, page aligned. All the offsets within the tdb
 * are relative to the area behind the mutex area. tdb->map_ptr points
 * behind the mmap area as well, so the read and write path in the
 * mutex case can remain unchanged.
 *
 * Early in the mutex development the mutexes were placed between the hash
 * chain pointers and the real tdb data. This had two drawbacks: First, it
 * made pointer calculations more complex. Second, we had to mmap the mutex
 * area twice. One was the normal map_ptr in the tdb. This frequently changed
 * from within tdb_oob. At least the Linux glibc robust mutex code assumes
 * constant pointers in memory, so a constantly changing mmap area destroys
 * the mutex list. So we had to mmap the first bytes of the file with a second
 * mmap call. With that scheme, very weird errors happened that could be
 * easily fixed by doing the mutex mmap in a second file. It seemed that
 * mapping the same memory area twice does not end up in accessing the same
 * physical page, looking at the mutexes in gdb it seemed that old data showed
 * up after some re-mapping. To avoid a separate mutex file, the code now puts
 * the real content of the tdb file after the mutex area. This way we do not
 * have overlapping mmap areas, the mutex area is mmapped once and not
 * changed, the tdb data area's mmap is constantly changed but does not
 * overlap.
 */

struct tdb_mutexes {
        struct tdb_header hdr;

        /* protect allrecord_lock */
        pthread_mutex_t allrecord_mutex;

        /*
         * F_UNLCK: free,
         * F_RDLCK: shared,
         * F_WRLCK: exclusive
         */
        short int allrecord_lock;

        /*
         * Index 0 is the freelist mutex, followed by
         * one mutex per hashchain.
         */
        pthread_mutex_t hashchains[1];
};

bool tdb_have_mutexes(struct tdb_context *tdb)
{
        return ((tdb->feature_flags & TDB_FEATURE_FLAG_MUTEX) != 0);
}

size_t tdb_mutex_size(struct tdb_context *tdb)
{
        size_t mutex_size;

        if (!tdb_have_mutexes(tdb)) {
                return 0;
        }

        mutex_size = sizeof(struct tdb_mutexes);
        mutex_size += tdb->hash_size * sizeof(pthread_mutex_t);

        return TDB_ALIGN(mutex_size, tdb->page_size);
}

/*
 * Get the index for a chain mutex
 */
static bool tdb_mutex_index(struct tdb_context *tdb, off_t off, off_t len,
                            unsigned *idx)
{
        /*
         * Weird but true: We fcntl lock 1 byte at an offset 4 bytes before
         * the 4 bytes of the freelist start and the hash chain that is about
         * to be locked. See lock_offset() where the freelist is -1 vs the
         * "+1" in TDB_HASH_TOP(). Because the mutex array is represented in
         * the tdb file itself as data, we need to adjust the offset here.
         */
        const off_t freelist_lock_ofs = FREELIST_TOP - sizeof(tdb_off_t);

        if (!tdb_have_mutexes(tdb)) {
                return false;
        }
        if (len != 1) {
                /* Possibly the allrecord lock */
                return false;
        }
        if (off < freelist_lock_ofs) {
                /* One of the special locks */
                return false;
        }
        if (tdb->hash_size == 0) {
                /* tdb not initialized yet, called from tdb_open_ex() */
                return false;
        }
        if (off >= TDB_DATA_START(tdb->hash_size)) {
                /* Single record lock from traverses */
                return false;
        }

        /*
         * Now we know it's a freelist or hash chain lock. Those are always 4
         * byte aligned. Paranoia check.
         */
        if ((off % sizeof(tdb_off_t)) != 0) {
                abort();
        }

        /*
         * Re-index the fcntl offset into an offset into the mutex array
         */
        off -= freelist_lock_ofs; /* rebase to index 0 */
        off /= sizeof(tdb_off_t); /* 0 for freelist 1-n for hashchain */

        *idx = off;
        return true;
}

static bool tdb_have_mutex_chainlocks(struct tdb_context *tdb)
{
        size_t i;

        for (i=0; i < tdb->num_lockrecs; i++) {
                bool ret;
                unsigned idx;

                ret = tdb_mutex_index(tdb,
                                      tdb->lockrecs[i].off,
                                      tdb->lockrecs[i].count,
                                      &idx);
                if (!ret) {
                        continue;
                }

                if (idx == 0) {
                        /* this is the freelist mutex */
                        continue;
                }

                return true;
        }

        return false;
}

static int chain_mutex_lock(pthread_mutex_t *m, bool waitflag)
{
        int ret;

        if (waitflag) {
                ret = pthread_mutex_lock(m);
        } else {
                ret = pthread_mutex_trylock(m);
        }
        if (ret != EOWNERDEAD) {
                return ret;
        }

        /*
         * For chainlocks, we don't do any cleanup (yet?)
         */
        return pthread_mutex_consistent(m);
}

static int allrecord_mutex_lock(struct tdb_mutexes *m, bool waitflag)
{
        int ret;

        if (waitflag) {
                ret = pthread_mutex_lock(&m->allrecord_mutex);
        } else {
                ret = pthread_mutex_trylock(&m->allrecord_mutex);
        }
        if (ret != EOWNERDEAD) {
                return ret;
        }

        /*
         * The allrecord lock holder died. We need to reset the allrecord_lock
         * to F_UNLCK. This should also be the indication for
         * tdb_needs_recovery.
         */
        m->allrecord_lock = F_UNLCK;

        return pthread_mutex_consistent(&m->allrecord_mutex);
}

bool tdb_mutex_lock(struct tdb_context *tdb, int rw, off_t off, off_t len,
                    bool waitflag, int *pret)
{
        struct tdb_mutexes *m = tdb->mutexes;
        pthread_mutex_t *chain;
        int ret;
        unsigned idx;
        bool allrecord_ok;

        if (!tdb_mutex_index(tdb, off, len, &idx)) {
                return false;
        }
        chain = &m->hashchains[idx];

again:
        ret = chain_mutex_lock(chain, waitflag);
        if (ret == EBUSY) {
                ret = EAGAIN;
        }
        if (ret != 0) {
                errno = ret;
                goto fail;
        }

        if (idx == 0) {
                /*
                 * This is a freelist lock, which is independent to
                 * the allrecord lock. So we're done once we got the
                 * freelist mutex.
                 */
                *pret = 0;
                return true;
        }

        if (tdb_have_mutex_chainlocks(tdb)) {
                /*
                 * We can only check the allrecord lock once. If we do it with
                 * one chain mutex locked, we will deadlock with the allrecord
                 * locker process in the following way: We lock the first hash
                 * chain, we check for the allrecord lock. We keep the hash
                 * chain locked. Then the allrecord locker locks the
                 * allrecord_mutex. It walks the list of chain mutexes,
                 * locking them all in sequence. Meanwhile, we have the chain
                 * mutex locked, so the allrecord locker blocks trying to lock
                 * our chain mutex. Then we come in and try to lock the second
                 * chain lock, which in most cases will be the freelist. We
                 * see that the allrecord lock is locked and put ourselves on
                 * the allrecord_mutex. This will never be signalled though
                 * because the allrecord locker waits for us to give up the
                 * chain lock.
                 */

                *pret = 0;
                return true;
        }

        /*
         * Check if someone is has the allrecord lock: queue if so.
         */

        allrecord_ok = false;

        if (m->allrecord_lock == F_UNLCK) {
                /*
                 * allrecord lock not taken
                 */
                allrecord_ok = true;
        }

        if ((m->allrecord_lock == F_RDLCK) && (rw == F_RDLCK)) {
                /*
                 * allrecord shared lock taken, but we only want to read
                 */
                allrecord_ok = true;
        }

        if (allrecord_ok) {
                *pret = 0;
                return true;
        }

        ret = pthread_mutex_unlock(chain);
        if (ret != 0) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_unlock"
                         "(chain_mutex) failed: %s\n", strerror(ret)));
                errno = ret;
                goto fail;
        }
        ret = allrecord_mutex_lock(m, waitflag);
        if (ret == EBUSY) {
                ret = EAGAIN;
        }
        if (ret != 0) {
                if (waitflag || (ret != EAGAIN)) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_%slock"
                                 "(allrecord_mutex) failed: %s\n",
                                 waitflag ? "" : "try_",  strerror(ret)));
                }
                errno = ret;
                goto fail;
        }
        ret = pthread_mutex_unlock(&m->allrecord_mutex);
        if (ret != 0) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_unlock"
                         "(allrecord_mutex) failed: %s\n", strerror(ret)));
                errno = ret;
                goto fail;
        }
        goto again;

fail:
        *pret = -1;
        return true;
}

bool tdb_mutex_unlock(struct tdb_context *tdb, int rw, off_t off, off_t len,
                      int *pret)
{
        struct tdb_mutexes *m = tdb->mutexes;
        pthread_mutex_t *chain;
        int ret;
        unsigned idx;

        if (!tdb_mutex_index(tdb, off, len, &idx)) {
                return false;
        }
        chain = &m->hashchains[idx];

        ret = pthread_mutex_unlock(chain);
        if (ret == 0) {
                *pret = 0;
                return true;
        }
        errno = ret;
        *pret = -1;
        return true;
}

int tdb_mutex_allrecord_lock(struct tdb_context *tdb, int ltype,
                             enum tdb_lock_flags flags)
{
        struct tdb_mutexes *m = tdb->mutexes;
        int ret;
        uint32_t i;
        bool waitflag = (flags & TDB_LOCK_WAIT);
        int saved_errno;

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

        if (flags & TDB_LOCK_MARK_ONLY) {
                return 0;
        }

        ret = allrecord_mutex_lock(m, waitflag);
        if (!waitflag && (ret == EBUSY)) {
                errno = EAGAIN;
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }
        if (ret != 0) {
                if (!(flags & TDB_LOCK_PROBE)) {
                        TDB_LOG((tdb, TDB_DEBUG_TRACE,
                                 "allrecord_mutex_lock() failed: %s\n",
                                 strerror(ret)));
                }
                tdb->ecode = TDB_ERR_LOCK;
                return -1;
        }

        if (m->allrecord_lock != F_UNLCK) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "allrecord_lock == %d\n",
                         (int)m->allrecord_lock));
                goto fail_unlock_allrecord_mutex;
        }
        m->allrecord_lock = (ltype == F_RDLCK) ? F_RDLCK : F_WRLCK;

        for (i=0; i<tdb->hash_size; i++) {

                /* ignore hashchains[0], the freelist */
                pthread_mutex_t *chain = &m->hashchains[i+1];

                ret = chain_mutex_lock(chain, waitflag);
                if (!waitflag && (ret == EBUSY)) {
                        errno = EAGAIN;
                        goto fail_unroll_allrecord_lock;
                }
                if (ret != 0) {
                        if (!(flags & TDB_LOCK_PROBE)) {
                                TDB_LOG((tdb, TDB_DEBUG_TRACE,
                                         "chain_mutex_lock() failed: %s\n",
                                         strerror(ret)));
                        }
                        errno = ret;
                        goto fail_unroll_allrecord_lock;
                }

                ret = pthread_mutex_unlock(chain);
                if (ret != 0) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_unlock"
                                 "(chainlock) failed: %s\n", strerror(ret)));
                        errno = ret;
                        goto fail_unroll_allrecord_lock;
                }
        }
        /*
         * We leave this routine with m->allrecord_mutex locked
         */
        return 0;

fail_unroll_allrecord_lock:
        m->allrecord_lock = F_UNLCK;

fail_unlock_allrecord_mutex:
        saved_errno = errno;
        ret = pthread_mutex_unlock(&m->allrecord_mutex);
        if (ret != 0) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_unlock"
                         "(allrecord_mutex) failed: %s\n", strerror(ret)));
        }
        errno = saved_errno;
        tdb->ecode = TDB_ERR_LOCK;
        return -1;
}

int tdb_mutex_allrecord_upgrade(struct tdb_context *tdb)
{
        struct tdb_mutexes *m = tdb->mutexes;
        int ret;
        uint32_t i;

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

        /*
         * Our only caller tdb_allrecord_upgrade()
         * garantees that we already own the allrecord lock.
         *
         * Which means m->allrecord_mutex is still locked by us.
         */

        if (m->allrecord_lock != F_RDLCK) {
                tdb->ecode = TDB_ERR_LOCK;
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "allrecord_lock == %d\n",
                         (int)m->allrecord_lock));
                return -1;
        }

        m->allrecord_lock = F_WRLCK;

        for (i=0; i<tdb->hash_size; i++) {

                /* ignore hashchains[0], the freelist */
                pthread_mutex_t *chain = &m->hashchains[i+1];

                ret = chain_mutex_lock(chain, true);
                if (ret != 0) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_lock"
                                 "(chainlock) failed: %s\n", strerror(ret)));
                        goto fail_unroll_allrecord_lock;
                }

                ret = pthread_mutex_unlock(chain);
                if (ret != 0) {
                        TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_unlock"
                                 "(chainlock) failed: %s\n", strerror(ret)));
                        goto fail_unroll_allrecord_lock;
                }
        }

        return 0;

fail_unroll_allrecord_lock:
        m->allrecord_lock = F_RDLCK;
        tdb->ecode = TDB_ERR_LOCK;
        return -1;
}

void tdb_mutex_allrecord_downgrade(struct tdb_context *tdb)
{
        struct tdb_mutexes *m = tdb->mutexes;

        /*
         * Our only caller tdb_allrecord_upgrade() (in the error case)
         * garantees that we already own the allrecord lock.
         *
         * Which means m->allrecord_mutex is still locked by us.
         */

        if (m->allrecord_lock != F_WRLCK) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "allrecord_lock == %d\n",
                         (int)m->allrecord_lock));
                return;
        }

        m->allrecord_lock = F_RDLCK;
        return;
}


int tdb_mutex_allrecord_unlock(struct tdb_context *tdb)
{
        struct tdb_mutexes *m = tdb->mutexes;
        short old;
        int ret;

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

        /*
         * Our only callers tdb_allrecord_unlock() and
         * tdb_allrecord_lock() (in the error path)
         * garantee that we already own the allrecord lock.
         *
         * Which means m->allrecord_mutex is still locked by us.
         */

        if ((m->allrecord_lock != F_RDLCK) && (m->allrecord_lock != F_WRLCK)) {
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "allrecord_lock == %d\n",
                         (int)m->allrecord_lock));
                return -1;
        }

        old = m->allrecord_lock;
        m->allrecord_lock = F_UNLCK;

        ret = pthread_mutex_unlock(&m->allrecord_mutex);
        if (ret != 0) {
                m->allrecord_lock = old;
                TDB_LOG((tdb, TDB_DEBUG_FATAL, "pthread_mutex_unlock"
                         "(allrecord_mutex) failed: %s\n", strerror(ret)));
                return -1;
        }
        return 0;
}

int tdb_mutex_init(struct tdb_context *tdb)
{
        struct tdb_mutexes *m;
        pthread_mutexattr_t ma;
        int i, ret;

        ret = tdb_mutex_mmap(tdb);
        if (ret == -1) {
                return -1;
        }
        m = tdb->mutexes;

        ret = pthread_mutexattr_init(&ma);
        if (ret != 0) {
                goto fail_munmap;
        }
        ret = pthread_mutexattr_settype(&ma, PTHREAD_MUTEX_ERRORCHECK);
        if (ret != 0) {
                goto fail;
        }
        ret = pthread_mutexattr_setpshared(&ma, PTHREAD_PROCESS_SHARED);
        if (ret != 0) {
                goto fail;
        }
        ret = pthread_mutexattr_setrobust(&ma, PTHREAD_MUTEX_ROBUST);
        if (ret != 0) {
                goto fail;
        }

        for (i=0; i<tdb->hash_size+1; i++) {
                pthread_mutex_t *chain = &m->hashchains[i];

                ret = pthread_mutex_init(chain, &ma);
                if (ret != 0) {
                        goto fail;
                }
        }

        m->allrecord_lock = F_UNLCK;

        ret = pthread_mutex_init(&m->allrecord_mutex, &ma);
        if (ret != 0) {
                goto fail;
        }
        ret = 0;
fail:
        pthread_mutexattr_destroy(&ma);
fail_munmap:
        tdb_mutex_munmap(tdb);

        if (ret == 0) {
                return 0;
        }

        errno = ret;
        return -1;
}

int tdb_mutex_mmap(struct tdb_context *tdb)
{
        size_t len;
        void *ptr;

        len = tdb_mutex_size(tdb);
        if (len == 0) {
                return 0;
        }

        ptr = mmap(NULL, len, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FILE,
                   tdb->fd, 0);
        if (ptr == MAP_FAILED) {
                return -1;
        }
        tdb->mutexes = (struct tdb_mutexes *)ptr;

        return 0;
}

int tdb_mutex_munmap(struct tdb_context *tdb)
{
        size_t len;

        len = tdb_mutex_size(tdb);
        if (len == 0) {
                return 0;
        }

        return munmap(tdb->mutexes, len);
}

static bool tdb_mutex_locking_cached;

static bool tdb_mutex_locking_supported(void)
{
        pthread_mutexattr_t ma;
        pthread_mutex_t m;
        int ret;
        static bool initialized;

        if (initialized) {
                return tdb_mutex_locking_cached;
        }

        initialized = true;

        ret = pthread_mutexattr_init(&ma);
        if (ret != 0) {
                return false;
        }
        ret = pthread_mutexattr_settype(&ma, PTHREAD_MUTEX_ERRORCHECK);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutexattr_setpshared(&ma, PTHREAD_PROCESS_SHARED);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutexattr_setrobust(&ma, PTHREAD_MUTEX_ROBUST);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutex_init(&m, &ma);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutex_lock(&m);
        if (ret != 0) {
                goto cleanup_m;
        }
        /*
         * This makes sure we have real mutexes
         * from a threading library instead of just
         * stubs from libc.
         */
        ret = pthread_mutex_lock(&m);
        if (ret != EDEADLK) {
                goto cleanup_lock;
        }
        ret = pthread_mutex_unlock(&m);
        if (ret != 0) {
                goto cleanup_m;
        }

        tdb_mutex_locking_cached = true;
        goto cleanup_m;

cleanup_lock:
        pthread_mutex_unlock(&m);
cleanup_m:
        pthread_mutex_destroy(&m);
cleanup_ma:
        pthread_mutexattr_destroy(&ma);
        return tdb_mutex_locking_cached;
}

static void (*tdb_robust_mutext_old_handler)(int) = SIG_ERR;
static pid_t tdb_robust_mutex_pid = -1;

static void (*tdb_robust_mutex_setup_sigchild(void (*handler)(int)))(int)
{
#ifdef HAVE_SIGACTION
        struct sigaction act;
        struct sigaction oldact;

        memset(&act, '\0', sizeof(act));

        act.sa_handler = handler;
#ifdef SA_RESTART
        act.sa_flags = SA_RESTART;
#endif
        sigemptyset(&act.sa_mask);
        sigaddset(&act.sa_mask, SIGCHLD);
        sigaction(SIGCHLD, &act, &oldact);
        return oldact.sa_handler;
#else /* !HAVE_SIGACTION */
        return NULL;
#endif
}

static void tdb_robust_mutex_handler(int sig)
{
        if (tdb_robust_mutex_pid != -1) {
                pid_t pid;
                int status;

                pid = waitpid(tdb_robust_mutex_pid, &status, WNOHANG);
                if (pid == tdb_robust_mutex_pid) {
                        tdb_robust_mutex_pid = -1;
                        return;
                }
        }

        if (tdb_robust_mutext_old_handler == SIG_DFL) {
                return;
        }
        if (tdb_robust_mutext_old_handler == SIG_IGN) {
                return;
        }
        if (tdb_robust_mutext_old_handler == SIG_ERR) {
                return;
        }

        tdb_robust_mutext_old_handler(sig);
}

_PUBLIC_ bool tdb_runtime_check_for_robust_mutexes(void)
{
        void *ptr;
        pthread_mutex_t *m;
        pthread_mutexattr_t ma;
        int ret = 1;
        int pipe_down[2] = { -1, -1 };
        int pipe_up[2] = { -1, -1 };
        ssize_t nread;
        char c = 0;
        bool ok;
        int status;
        static bool initialized;

        if (initialized) {
                return tdb_mutex_locking_cached;
        }

        initialized = true;

        ok = tdb_mutex_locking_supported();
        if (!ok) {
                return false;
        }

        tdb_mutex_locking_cached = false;

        ptr = mmap(NULL, sizeof(pthread_mutex_t), PROT_READ|PROT_WRITE,
                   MAP_SHARED|MAP_ANON, -1 /* fd */, 0);
        if (ptr == MAP_FAILED) {
                return false;
        }
        m = (pthread_mutex_t *)ptr;

        ret = pipe(pipe_down);
        if (ret != 0) {
                goto cleanup_mmap;
        }
        ret = pipe(pipe_up);
        if (ret != 0) {
                goto cleanup_pipe;
        }

        ret = pthread_mutexattr_init(&ma);
        if (ret != 0) {
                goto cleanup_pipe;
        }
        ret = pthread_mutexattr_settype(&ma, PTHREAD_MUTEX_ERRORCHECK);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutexattr_setpshared(&ma, PTHREAD_PROCESS_SHARED);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutexattr_setrobust(&ma, PTHREAD_MUTEX_ROBUST);
        if (ret != 0) {
                goto cleanup_ma;
        }
        ret = pthread_mutex_init(m, &ma);
        if (ret != 0) {
                goto cleanup_ma;
        }

        tdb_robust_mutext_old_handler = tdb_robust_mutex_setup_sigchild(
                                                tdb_robust_mutex_handler);
        if (tdb_robust_mutext_old_handler == NULL) {
                goto cleanup_ma;
        }

        tdb_robust_mutex_pid = fork();
        if (tdb_robust_mutex_pid == 0) {
                size_t nwritten;
                close(pipe_down[1]);
                close(pipe_up[0]);
                ret = pthread_mutex_lock(m);
                nwritten = write(pipe_up[1], &ret, sizeof(ret));
                if (nwritten != sizeof(ret)) {
                        _exit(1);
                }
                if (ret != 0) {
                        _exit(1);
                }
                nread = read(pipe_down[0], &c, 1);
                if (nread != 1) {
                        _exit(1);
                }
                /* leave locked */
                _exit(0);
        }
        if (tdb_robust_mutex_pid == -1) {
                goto cleanup_sig_child;
        }
        close(pipe_down[0]);
        pipe_down[0] = -1;
        close(pipe_up[1]);
        pipe_up[1] = -1;

        nread = read(pipe_up[0], &ret, sizeof(ret));
        if (nread != sizeof(ret)) {
                goto cleanup_child;
        }

        ret = pthread_mutex_trylock(m);
        if (ret != EBUSY) {
                if (ret == 0) {
                        pthread_mutex_unlock(m);
                }
                goto cleanup_child;
        }

        if (write(pipe_down[1], &c, 1) != 1) {
                goto cleanup_child;
        }

        nread = read(pipe_up[0], &c, 1);
        if (nread != 0) {
                goto cleanup_child;
        }

        while (tdb_robust_mutex_pid > 0) {
                pid_t pid;

                errno = 0;
                pid = waitpid(tdb_robust_mutex_pid, &status, 0);
                if (pid == tdb_robust_mutex_pid) {
                        tdb_robust_mutex_pid = -1;
                        break;
                }
                if (pid == -1 && errno != EINTR) {
                        goto cleanup_child;
                }
        }
        tdb_robust_mutex_setup_sigchild(tdb_robust_mutext_old_handler);

        ret = pthread_mutex_trylock(m);
        if (ret != EOWNERDEAD) {
                if (ret == 0) {
                        pthread_mutex_unlock(m);
                }
                goto cleanup_m;
        }

        ret = pthread_mutex_consistent(m);
        if (ret != 0) {
                goto cleanup_m;
        }

        ret = pthread_mutex_trylock(m);
        if (ret != EDEADLK) {
                pthread_mutex_unlock(m);
                goto cleanup_m;
        }

        ret = pthread_mutex_unlock(m);
        if (ret != 0) {
                goto cleanup_m;
        }

        tdb_mutex_locking_cached = true;
        goto cleanup_m;

cleanup_child:
        while (tdb_robust_mutex_pid > 0) {
                pid_t pid;

                kill(tdb_robust_mutex_pid, SIGKILL);

                errno = 0;
                pid = waitpid(tdb_robust_mutex_pid, &status, 0);
                if (pid == tdb_robust_mutex_pid) {
                        tdb_robust_mutex_pid = -1;
                        break;
                }
                if (pid == -1 && errno != EINTR) {
                        break;
                }
        }
cleanup_sig_child:
        tdb_robust_mutex_setup_sigchild(tdb_robust_mutext_old_handler);
cleanup_m:
        pthread_mutex_destroy(m);
cleanup_ma:
        pthread_mutexattr_destroy(&ma);
cleanup_pipe:
        if (pipe_down[0] != -1) {
                close(pipe_down[0]);
        }
        if (pipe_down[1] != -1) {
                close(pipe_down[1]);
        }
        if (pipe_up[0] != -1) {
                close(pipe_up[0]);
        }
        if (pipe_up[1] != -1) {
                close(pipe_up[1]);
        }
cleanup_mmap:
        munmap(ptr, sizeof(pthread_mutex_t));

        return tdb_mutex_locking_cached;
}

#else

size_t tdb_mutex_size(struct tdb_context *tdb)
{
        return 0;
}

bool tdb_have_mutexes(struct tdb_context *tdb)
{
        return false;
}

int tdb_mutex_allrecord_lock(struct tdb_context *tdb, int ltype,
                             enum tdb_lock_flags flags)
{
        tdb->ecode = TDB_ERR_LOCK;
        return -1;
}

int tdb_mutex_allrecord_unlock(struct tdb_context *tdb)
{
        return -1;
}

int tdb_mutex_allrecord_upgrade(struct tdb_context *tdb)
{
        tdb->ecode = TDB_ERR_LOCK;
        return -1;
}

void tdb_mutex_allrecord_downgrade(struct tdb_context *tdb)
{
        return;
}

int tdb_mutex_mmap(struct tdb_context *tdb)
{
        errno = ENOSYS;
        return -1;
}

int tdb_mutex_munmap(struct tdb_context *tdb)
{
        errno = ENOSYS;
        return -1;
}

int tdb_mutex_init(struct tdb_context *tdb)
{
        errno = ENOSYS;
        return -1;
}

_PUBLIC_ bool tdb_runtime_check_for_robust_mutexes(void)
{
        return false;
}

#endif