Cygwin: posix timers: fix a deadlock

Canceling the timer thread runs under lock.  The thread uses the same
lock to guard its timer_tracker struct access.  If the timing is bad,
timer_settime or timer_delete grab the lock at the same time, the timer
expires.  In the end, cancel waits for the thread sync while the thread
waits for ther lock to be released.

Fix this by not waiting for the thread sync under lock.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/posix_timer.cc

@@ -31,7 +31,9 @@ timer_tracker::cancel ()
   SetEvent (cancel_evt);
   if (sync_thr)
     {
+      ReleaseSRWLockExclusive (&srwlock);
       res = WaitForSingleObject (sync_thr, INFINITE);
+      AcquireSRWLockExclusive (&srwlock);
       if (res != WAIT_OBJECT_0)
 	debug_printf ("WFSO returned unexpected value %u, %E", res);
     }

winsup/cygwin/release/3.0.3

@@ -11,3 +11,6 @@ Bug Fixes
 
 - Fix a resource leak in posix timers.
   Addresses: https://cygwin.com/ml/cygwin/2019-03/msg00120.html
+
+- Fix a deadlock in posix timers
+  Addresses: https://cygwin.com/ml/cygwin/2019-03/msg00158.html