[PATCH COMMITTED] tst-mallocfork2: Fix race condition, use fewer resources

[fweimer at redhat dot com (Florian Weimer)]

The first SIGUSR1 signal could arrive when sigusr1_sender_pid
was still 0.  As a result, kill would send SIGSTOP to the
entire process group.  This would cause the test to hang before
printing any output.

This commit also adds a sched_yield to the signal source, so that
it does not flood the parent process with signals it has never a
chance to handle.

Even with these changes, tst-mallocfork2 still fails reliably
after the fix in commit commit 56290d6e762c1194547e73ff0b948cd79d3a1e03
(Increase fork signal safety for single-threaded processes) is
backed out.

2016-05-13  Florian Weimer  <fweimer@redhat.com>

	Fix race condition in tst-mallocfork2, use fewer resources.
	* malloc/tst-mallocfork2.c (sigusr1_handler): Do not send SIGSTOP
	to current process group.
	(signal_sender): Yield in the non-sleeping case.

diff --git a/malloc/tst-mallocfork2.c b/malloc/tst-mallocfork2.c
index a9e3e94..4939938 100644
--- a/malloc/tst-mallocfork2.c
+++ b/malloc/tst-mallocfork2.c
@@ -25,6 +25,7 @@
    still make fork unsafe, even in single-threaded processes.  */
 
 #include <errno.h>
+#include <sched.h>
 #include <signal.h>
 #include <stdbool.h>
 #include <stdio.h>
@@ -70,7 +71,9 @@ sigusr1_handler (int signo)
      signals from the subprocess.  */
   if (sigusr1_received)
     return;
-  if (kill (sigusr1_sender_pid, SIGSTOP) != 0)
+  /* sigusr1_sender_pid might not be initialized in the parent when
+     the first SIGUSR1 signal arrives.  */
+  if (sigusr1_sender_pid > 0 && kill (sigusr1_sender_pid, SIGSTOP) != 0)
     {
       write_message ("error: kill (SIGSTOP)\n");
       abort ();
@@ -123,6 +126,9 @@ signal_sender (int signo, bool sleep)
         }
       if (sleep)
         usleep (1 * 1000 * 1000);
+      else
+        /* Reduce the rate at which we send signals.  */
+        sched_yield ();
     }
 }