Adding upstream version 2:4.17.12+dfsg.upstream/2%4.17.12+dfsgupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1078 insertions, 0 deletions

diff --git a/lib/tdb/common/mutex.c b/lib/tdb/common/mutex.c
new file mode 100644
index 0000000..78926db
--- /dev/null
+++ b/lib/tdb/common/mutex.c
@@ -0,0 +1,1078 @@
+/*
+   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)
+{
+	int 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;
+	uint32_t i;
+	int 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:
+
+	if (ret == 0) {
+		return 0;
+	}
+
+	tdb_mutex_munmap(tdb);
+
+	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;
+	}
+
+	if (tdb->mutexes != NULL) {
+		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;
+	int ret;
+
+	len = tdb_mutex_size(tdb);
+	if (len == 0) {
+		return 0;
+	}
+
+	ret = munmap(tdb->mutexes, len);
+	if (ret == -1) {
+		return -1;
+	}
+	tdb->mutexes = NULL;
+
+	return 0;
+}
+
+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 bool tdb_robust_mutex_setup_sigchild(void (*handler)(int),
+			void (**p_old_handler)(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);
+	if (p_old_handler) {
+		*p_old_handler = oldact.sa_handler;
+	}
+	return true;
+#else /* !HAVE_SIGACTION */
+	return false;
+#endif
+}
+
+static void tdb_robust_mutex_handler(int sig)
+{
+	pid_t child_pid = tdb_robust_mutex_pid;
+
+	if (child_pid != -1) {
+		pid_t pid;
+
+		pid = waitpid(child_pid, NULL, WNOHANG);
+		if (pid == -1) {
+			switch (errno) {
+			case ECHILD:
+				tdb_robust_mutex_pid = -1;
+				return;
+
+			default:
+				return;
+			}
+		}
+		if (pid == child_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);
+}
+
+static void tdb_robust_mutex_wait_for_child(pid_t *child_pid)
+{
+	int options = WNOHANG;
+
+	if (*child_pid == -1) {
+		return;
+	}
+
+	while (tdb_robust_mutex_pid > 0) {
+		pid_t pid;
+
+		/*
+		 * First we try with WNOHANG, as the process might not exist
+		 * anymore. Once we've sent SIGKILL we block waiting for the
+		 * exit.
+		 */
+		pid = waitpid(*child_pid, NULL, options);
+		if (pid == -1) {
+			if (errno == EINTR) {
+				continue;
+			} else if (errno == ECHILD) {
+				break;
+			} else {
+				abort();
+			}
+		}
+		if (pid == *child_pid) {
+			break;
+		}
+
+		kill(*child_pid, SIGKILL);
+		options = 0;
+	}
+
+	tdb_robust_mutex_pid = -1;
+	*child_pid = -1;
+}
+
+_PUBLIC_ bool tdb_runtime_check_for_robust_mutexes(void)
+{
+	void *ptr = NULL;
+	pthread_mutex_t *m = NULL;
+	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;
+	static bool initialized;
+	pid_t saved_child_pid = -1;
+	bool cleanup_ma = false;
+
+	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;
+	}
+
+	ret = pipe(pipe_down);
+	if (ret != 0) {
+		goto cleanup;
+	}
+	ret = pipe(pipe_up);
+	if (ret != 0) {
+		goto cleanup;
+	}
+
+	ret = pthread_mutexattr_init(&ma);
+	if (ret != 0) {
+		goto cleanup;
+	}
+	cleanup_ma = true;
+	ret = pthread_mutexattr_settype(&ma, PTHREAD_MUTEX_ERRORCHECK);
+	if (ret != 0) {
+		goto cleanup;
+	}
+	ret = pthread_mutexattr_setpshared(&ma, PTHREAD_PROCESS_SHARED);
+	if (ret != 0) {
+		goto cleanup;
+	}
+	ret = pthread_mutexattr_setrobust(&ma, PTHREAD_MUTEX_ROBUST);
+	if (ret != 0) {
+		goto cleanup;
+	}
+	ret = pthread_mutex_init(ptr, &ma);
+	if (ret != 0) {
+		goto cleanup;
+	}
+	m = (pthread_mutex_t *)ptr;
+
+	if (tdb_robust_mutex_setup_sigchild(tdb_robust_mutex_handler,
+			&tdb_robust_mutext_old_handler) == false) {
+		goto cleanup;
+	}
+
+	tdb_robust_mutex_pid = fork();
+	saved_child_pid = tdb_robust_mutex_pid;
+	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;
+	}
+	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;
+	}
+
+	ret = pthread_mutex_trylock(m);
+	if (ret != EBUSY) {
+		if (ret == 0) {
+			pthread_mutex_unlock(m);
+		}
+		goto cleanup;
+	}
+
+	if (write(pipe_down[1], &c, 1) != 1) {
+		goto cleanup;
+	}
+
+	nread = read(pipe_up[0], &c, 1);
+	if (nread != 0) {
+		goto cleanup;
+	}
+
+	tdb_robust_mutex_wait_for_child(&saved_child_pid);
+
+	ret = pthread_mutex_trylock(m);
+	if (ret != EOWNERDEAD) {
+		if (ret == 0) {
+			pthread_mutex_unlock(m);
+		}
+		goto cleanup;
+	}
+
+	ret = pthread_mutex_consistent(m);
+	if (ret != 0) {
+		goto cleanup;
+	}
+
+	ret = pthread_mutex_trylock(m);
+	if (ret != EDEADLK && ret != EBUSY) {
+		pthread_mutex_unlock(m);
+		goto cleanup;
+	}
+
+	ret = pthread_mutex_unlock(m);
+	if (ret != 0) {
+		goto cleanup;
+	}
+
+	tdb_mutex_locking_cached = true;
+
+cleanup:
+	/*
+	 * Note that we don't reset the signal handler we just reset
+	 * tdb_robust_mutex_pid to -1. This is ok as this code path is only
+	 * called once per process.
+	 *
+	 * Leaving our signal handler avoids races with other threads potentialy
+	 * setting up their SIGCHLD handlers.
+	 *
+	 * The worst thing that can happen is that the other newer signal
+	 * handler will get the SIGCHLD signal for our child and/or reap the
+	 * child with a wait() function. tdb_robust_mutex_wait_for_child()
+	 * handles the case where waitpid returns ECHILD.
+	 */
+	tdb_robust_mutex_wait_for_child(&saved_child_pid);
+
+	if (m != NULL) {
+		pthread_mutex_destroy(m);
+	}
+	if (cleanup_ma) {
+		pthread_mutexattr_destroy(&ma);
+	}
+	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]);
+	}
+	if (ptr != NULL) {
+		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