posix: Do not use WNOHANG in waitpid call for Linux posix_spawn

As shown in some buildbot issues on aarch64 and powerpc, calling
clone (VFORK) and waitpid (WNOHANG) does not guarantee the child
is ready to be collected.  This patch changes the call back to 0
as before fe05e1cb6d64 fix.

This change can lead to the scenario 4.3 described in the commit,
where the waitpid call can hang undefinitely on the call.  However
this is also a very unlikely and also undefinied situation where
both the caller is trying to terminate a pid before posix_spawn
returns and the race pid reuse is triggered.  I don't see how to
correct handle this specific situation within posix_spawn.

Checked on x86_64-linux-gnu, aarch64-linux-gnu and
powerpc64-linux-gnu.

	* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Use 0 instead of
	WNOHANG in waitpid call.

2 files changed, 10 insertions, 5 deletions

diff --git a/ChangeLog b/ChangeLog
index b756f97..4448c3d 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2017-10-23  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
+
+	* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Use 0 instead of
+	WNOHANG in waitpid call.
+
 2017-10-23  Siddhesh Poyarekar  <siddhesh@sourceware.org>
 
 	* manual/conf.texi (_SC_LEVEL1_DCACHE_LINESIZE,
diff --git a/sysdeps/unix/sysv/linux/spawni.c b/sysdeps/unix/sysv/linux/spawni.c
index d15fbb1..fb83c2e 100644
--- a/sysdeps/unix/sysv/linux/spawni.c
+++ b/sysdeps/unix/sysv/linux/spawni.c
@@ -374,12 +374,12 @@ __spawnix (pid_t * pid, const char *file,
       ec = args.err;
       if (ec > 0)
 	/* There still an unlikely case where the child is cancelled after
-	   setting args.err, due to a positive error value.  Also due a
+	   setting args.err, due to a positive error value.  Also there is
 	   possible pid reuse race (where the kernel allocated the same pid
-	   to unrelated process) we need not to undefinitely hang expecting
-	   an invalid pid.  In both cases an error is returned to the
-	   caller.  */
-	__waitpid (new_pid, NULL, WNOHANG);
+	   to an unrelated process).  Unfortunately due synchronization
+	   issues where the kernel might not have the process collected
+	   the waitpid below can not use WNOHANG.  */
+	__waitpid (new_pid, NULL, 0);
     }
   else
     ec = -new_pid;