posix: Fix system blocks SIGCHLD erroneously [BZ #30163]

Fix bug that SIGCHLD is erroneously blocked forever in the following
scenario:

1. Thread A calls system but hasn't returned yet
2. Thread B calls another system but returns

SIGCHLD would be blocked forever in thread B after its system() returns,
even after the system() in thread A returns.

Although POSIX does not require, glibc system implementation aims to be
thread and cancellation safe. This bug was introduced in
5fb7fc96350575c9adb1316833e48ca11553be49 when we moved reverting signal
mask to happen when the last concurrently running system returns,
despite that signal mask is per thread. This commit reverts this logic
and adds a test.

Signed-off-by: Adam Yi <ayi@janestreet.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/sysdeps/posix/system.c b/sysdeps/posix/system.c
index 2335a99184..d77720a625 100644
--- a/sysdeps/posix/system.c
+++ b/sysdeps/posix/system.c
@@ -179,16 +179,16 @@ do_system (const char *line)
       as if the shell had terminated using _exit(127).  */
    status = W_EXITCODE (127, 0);
 
+  /* sigaction can not fail with SIGINT/SIGQUIT used with old
+     disposition.  Same applies for sigprocmask.  */
   DO_LOCK ();
   if (SUB_REF () == 0)
     {
-      /* sigaction can not fail with SIGINT/SIGQUIT used with old
-	 disposition.  Same applies for sigprocmask.  */
       __sigaction (SIGINT, &intr, NULL);
       __sigaction (SIGQUIT, &quit, NULL);
-      __sigprocmask (SIG_SETMASK, &omask, NULL);
     }
   DO_UNLOCK ();
+  __sigprocmask (SIG_SETMASK, &omask, NULL);
 
   if (ret != 0)
     __set_errno (ret);