bpo-33613, test_semaphore_tracker_sigint: fix race condition (#7850)

Fail `test_semaphore_tracker_sigint` if no warnings are expected and one is received.

Fix race condition when the child receives SIGINT before it can register signal handlers for it.

The race condition occurs when the parent calls
`_semaphore_tracker.ensure_running()` (which in turn spawns the
semaphore_tracker using `_posixsubprocess.fork_exec`), the child
registers the signal handlers and the parent tries to kill the child.
What seem to happen is that in some slow systems, the parent sends the
signal to kill the child before the child protects against the signal.

Lib/multiprocessing/semaphore_tracker.py

@@ -23,6 +23,9 @@ from . import util
 
 __all__ = ['ensure_running', 'register', 'unregister']
 
+_HAVE_SIGMASK = hasattr(signal, 'pthread_sigmask')
+_IGNORED_SIGNALS = (signal.SIGINT, signal.SIGTERM)
+
 
 class SemaphoreTracker(object):
 
@@ -43,10 +46,16 @@ class SemaphoreTracker(object):
         with self._lock:
             if self._pid is not None:
                 # semaphore tracker was launched before, is it still running?
-                pid, status = os.waitpid(self._pid, os.WNOHANG)
+                try:
+                    pid, _ = os.waitpid(self._pid, os.WNOHANG)
+                except ChildProcessError:
+                    # The process terminated
+                    pass
+                else:
                     if not pid:
                         # => still alive
                         return
+
                 # => dead, launch it again
                 os.close(self._fd)
                 self._fd = None
@@ -68,7 +77,19 @@ class SemaphoreTracker(object):
                 exe = spawn.get_executable()
                 args = [exe] + util._args_from_interpreter_flags()
                 args += ['-c', cmd % r]
+                # bpo-33613: Register a signal mask that will block the signals.
+                # This signal mask will be inherited by the child that is going
+                # to be spawned and will protect the child from a race condition
+                # that can make the child die before it registers signal handlers
+                # for SIGINT and SIGTERM. The mask is unregistered after spawning
+                # the child.
+                try:
+                    if _HAVE_SIGMASK:
+                        signal.pthread_sigmask(signal.SIG_BLOCK, _IGNORED_SIGNALS)
                     pid = util.spawnv_passfds(exe, args, fds_to_pass)
+                finally:
+                    if _HAVE_SIGMASK:
+                        signal.pthread_sigmask(signal.SIG_UNBLOCK, _IGNORED_SIGNALS)
             except:
                 os.close(w)
                 raise
@@ -104,12 +125,13 @@ register = _semaphore_tracker.register
 unregister = _semaphore_tracker.unregister
 getfd = _semaphore_tracker.getfd
 
-
 def main(fd):
     '''Run semaphore tracker.'''
     # protect the process from ^C and "killall python" etc
     signal.signal(signal.SIGINT, signal.SIG_IGN)
     signal.signal(signal.SIGTERM, signal.SIG_IGN)
+    if _HAVE_SIGMASK:
+        signal.pthread_sigmask(signal.SIG_UNBLOCK, _IGNORED_SIGNALS)
 
     for f in (sys.stdin, sys.stdout):
         try:

Lib/test/_test_multiprocessing.py

@@ -20,6 +20,7 @@ import logging
 import struct
 import operator
 import weakref
+import warnings
 import test.support
 import test.support.script_helper
 from test import support
@@ -4517,17 +4518,19 @@ class TestSemaphoreTracker(unittest.TestCase):
         # bpo-31310: if the semaphore tracker process has died, it should
         # be restarted implicitly.
         from multiprocessing.semaphore_tracker import _semaphore_tracker
+        pid = _semaphore_tracker._pid
+        if pid is not None:
+            os.kill(pid, signal.SIGKILL)
+            os.waitpid(pid, 0)
+        with warnings.catch_warnings(record=True) as all_warn:
             _semaphore_tracker.ensure_running()
         pid = _semaphore_tracker._pid
+
         os.kill(pid, signum)
         time.sleep(1.0)  # give it time to die
 
         ctx = multiprocessing.get_context("spawn")
-        with contextlib.ExitStack() as stack:
+        with warnings.catch_warnings(record=True) as all_warn:
-            if should_die:
-                stack.enter_context(self.assertWarnsRegex(
-                    UserWarning,
-                    "semaphore_tracker: process died"))
             sem = ctx.Semaphore()
             sem.acquire()
             sem.release()
@@ -4537,11 +4540,23 @@ class TestSemaphoreTracker(unittest.TestCase):
             del sem
             gc.collect()
             self.assertIsNone(wr())
+            if should_die:
+                self.assertEqual(len(all_warn), 1)
+                the_warn = all_warn[0]
+                issubclass(the_warn.category, UserWarning)
+                self.assertTrue("semaphore_tracker: process died"
+                                in str(the_warn.message))
+            else:
+                self.assertEqual(len(all_warn), 0)
 
     def test_semaphore_tracker_sigint(self):
         # Catchable signal (ignored by semaphore tracker)
         self.check_semaphore_tracker_death(signal.SIGINT, False)
 
+    def test_semaphore_tracker_sigterm(self):
+        # Catchable signal (ignored by semaphore tracker)
+        self.check_semaphore_tracker_death(signal.SIGTERM, False)
+
     def test_semaphore_tracker_sigkill(self):
         # Uncatchable signal.
         self.check_semaphore_tracker_death(signal.SIGKILL, True)

Misc/NEWS.d/next/Library/2018-07-31-23-33-06.bpo-33613.Cdnt0i.rst

@@ -0,0 +1,3 @@
+Fix a race condition in ``multiprocessing.semaphore_tracker`` when the
+tracker receives SIGINT before it can register signal handlers for ignoring
+it.