Reported by Daniel Richard G. <skunk@iskunk.org>.
* modules/lock-tests (configure.ac): Test for <semaphore.h>.
* tests/test-lock.c (USE_SEMAPHORE): Don't set if <semaphore.h> does not
exist.
The rationale is: 1) Read-preferring read-write locks are prone to
writer starvation if the number of reader threads multiplied by the
percentage of time they have the lock held is too high. 2) Write-
preferring read-write locks are the only reliable way to avoid this.
3) There have been reports of 'test-lock' hanging on glibc systems
http://lists.gnu.org/archive/html/bug-gnulib/2017-01/msg00009.html,
and glibc indeed implements read-preferring rwlocks by default, see
http://man7.org/linux/man-pages/man3/pthread_rwlockattr_setkind_np.3.html
and https://sourceware.org/bugzilla/show_bug.cgi?id=13701 .
* m4/pthread_rwlock_rdlock.m4: New file.
* m4/lock.m4 (gl_LOCK): Invoke gl_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER.
* lib/glthread/lock.h [USE_POSIX_THREADS]: Test
HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER. Use a different implementation
of rwlock initialization on glibc systems without
HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER. Use a different implementation
of rwlocks altogether on non-glibc systems without
HAVE_PTHREAD_RWLOCK_RDLOCK_PREFER_WRITER.
[USE_PTH_THREADS]: Use a different implementation of rwlocks altogether.
* lib/glthread/lock.c [USE_POSIX_THREADS]
(glthread_rwlock_init_for_glibc): New function.
[USE_POSIX_THREADS] (glthread_rwlock_rdlock_multithreaded): Update
comment.
[USE_PTH_THREADS]: New implementation of rwlocks.
[USE_WINDOWS_THREADS] (glthread_rwlock_rdlock_func): Prefer writers over
readers.
* modules/lock (Files): Add m4/pthread_rwlock_rdlock.m4.
(Depends-on): Add 'extensions'.
* tests/test-rwlock1.c: New file.
* lock-tests (Files): Add it.
(Depends-on): Add usleep.
(Makefile.am): Add test-rwlock1 to the tests.
