tdb: runtime check for robust mutexes may hang in threaded programs

tdb: runtime check for robust mutexes may hang in threaded programs

The current runtime check for robust mutexes in
tdb_runtime_check_for_robust_mutexes() is not thread-safe.

When called in a multi-threaded program where any another thread doesn't
have SIGCHLD blocked, we may end up hung in sigsuspend() waiting for a
SIGCHLD of a child procecss and the signal was delivered to another
thread.

Revert to the previous behaviour of waiting for the child instead of
waiting for the SIGCHLD signal.

Ensure the pid we wait for is not reset to -1 in a toctou race with the
signal handler.

Check whether waitpid() returns ECHILD which can happen if the signal
handler is run by more then one thread in parallel (yes, this can
happen) or if tdb_robust_mutex_wait_for_child() and the signal handler
are racing.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12593

Pair-programmed-with: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>