Fix race in tst-mqueue5

The check is done on line 117 by a thread spawned
from do_child(), forked from do_test().  This test
generates a signal in the forked process.

Either thread may handle the signal, and on ppc,
it happens to be done on do_child, on the thread
which is not doing the check on line 117.

This exposes a race condition whereby the test
incorrectly fails as the signal is caught during
or after the check.

This is mitigated by ensuring the signal is blocked
in the child thread while thread is running.

ChangeLog

@@ -1,3 +1,8 @@
+2016-01-15  Paul E. Murphy  <murphyp@linux.vnet.ibm.com>
+
+	* rt/tst-mqueue5.c (thr): Cleanup misleading comment.
+	(do_child): Mask SIGRTMIN while thr is running.
+
 2016-01-15  Martin Sebor  <msebor@redhat.com>
 
 	[BZ #19432]

rt/tst-mqueue5.c

@@ -116,7 +116,7 @@ thr (void *arg)
 
   if (rtmin_cnt != 2)
     {
-      puts ("SIGRTMIN signal in child did not arrive");
+      puts ("SIGRTMIN signal in thread did not arrive");
       result = 1;
     }
   else if (rtmin_pid != getppid ()
@@ -403,6 +403,16 @@ do_child (const char *name, pthread_barrier_t *b2, pthread_barrier_t *b3,
       result = 1;
     }
 
+  /* Ensure the thr thread gets the signal, not us.  */
+  sigset_t set;
+  sigemptyset (&set);
+  sigaddset (&set, SIGRTMIN);
+  if (pthread_sigmask (SIG_BLOCK, &set, NULL))
+    {
+      printf ("Failed to block SIGRTMIN in child: %m\n");
+      result = 1;
+    }
+
   (void) pthread_barrier_wait (b2);
 
   /* Parent calls mqsend (q), which should wake up mqrecv (q)
@@ -514,7 +524,14 @@ do_child (const char *name, pthread_barrier_t *b2, pthread_barrier_t *b3,
       result = 1;
     }
 
- void *thr_ret;
+  /* Reenable test signals before cleaning up the thread.  */
+  if (pthread_sigmask (SIG_UNBLOCK, &set, NULL))
+    {
+      printf ("Failed to unblock SIGRTMIN in child: %m\n");
+      result = 1;
+    }
+
+  void *thr_ret;
   ret = pthread_join (th, &thr_ret);
   if (ret)
     {