nptl: pthread_kill, pthread_cancel should not fail after exit (bug 19193)

This closes one remaining race condition related to bug 12889: if
the thread already exited on the kernel side, returning ESRCH
is not correct because that error is reserved for the thread IDs
(pthread_t values) whose lifetime has ended.  In case of a
kernel-side exit and a valid thread ID, no signal needs to be sent
and cancellation does not have an effect, so just return 0.

sysdeps/pthread/tst-kill4.c triggers undefined behavior and is
removed with this commit.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/nptl/pthread_kill.c b/nptl/pthread_kill.c
index f79a2b26fc..5d4c86f920 100644
--- a/nptl/pthread_kill.c
+++ b/nptl/pthread_kill.c
@@ -46,7 +46,12 @@ __pthread_kill_internal (pthread_t threadid, int signo)
 	    ? INTERNAL_SYSCALL_ERRNO (val) : 0);
     }
   else
-    val = ESRCH;
+    /* The kernel reports that the thread has exited.  POSIX specifies
+       the ESRCH error only for the case when the lifetime of a thread
+       ID has ended, but calling pthread_kill on such a thread ID is
+       undefined in glibc.  Therefore, do not treat kernel thread exit
+       as an error.  */
+    val = 0;
 
   return val;
 }