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

[Adam Yi <ayi@janestreet.com>]

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.
---
 stdlib/tst-system.c       | 27 +++++++++++++++++++++++++++
 support/shell-container.c | 10 ++++++++++
 sysdeps/posix/system.c    |  6 +++---
 3 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/stdlib/tst-system.c b/stdlib/tst-system.c
index 634acfe264..be95c2de44 100644
--- a/stdlib/tst-system.c
+++ b/stdlib/tst-system.c
@@ -20,6 +20,8 @@
 #include <string.h>
 #include <signal.h>
 #include <paths.h>
+#include <pthread.h>
+#include <inttypes.h>

 #include <support/capture_subprocess.h>
 #include <support/check.h>
@@ -71,6 +73,19 @@ call_system (void *closure)
     }
 }

+static void *
+sleep_and_check_sigchld (void *seconds)
+{
+  char cmd[namemax];
+  sprintf(cmd, "sleep %" PRIdPTR, (intptr_t) seconds);
+  TEST_COMPARE (system (cmd), 0);
+
+  sigset_t blocked = {0};
+  TEST_COMPARE (sigprocmask (SIG_BLOCK, NULL, &blocked), 0);
+  TEST_COMPARE (sigismember (&blocked, SIGCHLD), 0);
+  return NULL;
+}
+
 static int
 do_test (void)
 {
@@ -154,6 +169,18 @@ do_test (void)
     xchmod (_PATH_BSHELL, st.st_mode);
   }

+  {
+    pthread_t long_sleep_thread, short_sleep_thread;
+
+    TEST_COMPARE (pthread_create (&long_sleep_thread, NULL,
+                  sleep_and_check_sigchld, (void *) 2), 0);
+    TEST_COMPARE (pthread_create (&short_sleep_thread, NULL,
+                  sleep_and_check_sigchld, (void *) 1), 0);
+
+    TEST_COMPARE (pthread_join (short_sleep_thread, NULL), 0);
+    TEST_COMPARE (pthread_join (long_sleep_thread, NULL), 0);
+  }
+
   TEST_COMPARE (system (""), 0);

   return 0;
diff --git a/support/shell-container.c b/support/shell-container.c
index e9ac9b6d04..083426550b 100644
--- a/support/shell-container.c
+++ b/support/shell-container.c
@@ -171,6 +171,15 @@ kill_func (char **argv)
   return 0;
 }

+/* Emulate the "/bin/sleep" command.  Options are ignored.  */
+static int
+sleep_func (char **argv)
+{
+  int secs = atoi (argv[0]);
+  sleep (secs);
+  return 0;
+}
+
 /* This is a list of all the built-in commands we understand.  */
 static struct {
   const char *name;
@@ -181,6 +190,7 @@ static struct {
   { "cp", copy_func },
   { "exit", exit_func },
   { "kill", kill_func },
+  { "sleep", sleep_func },
   { NULL, NULL }
 };

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);
-- 
2.31.1