[3.6] bpo-31310: multiprocessing's semaphore tracker should be launched again if crashed (GH-3247) (#4254)

* bpo-31310: multiprocessing's semaphore tracker should be launched again if crashed

* Avoid mucking with process state in test.
Add a warning if the semaphore process died, as semaphores may then be leaked.

* Add NEWS entry
(cherry picked from commit cbe1756)

Lib/test/_test_multiprocessing.py

@@ -4,6 +4,7 @@
 
 import unittest
 import queue as pyqueue
+import contextlib
 import time
 import io
 import itertools
@@ -4125,14 +4126,14 @@ class TestStartMethod(unittest.TestCase):
             self.fail("failed spawning forkserver or grandchild")
 
 
-#
-# Check that killing process does not leak named semaphores
-#
-
 @unittest.skipIf(sys.platform == "win32",
                  "test semantics don't make sense on Windows")
 class TestSemaphoreTracker(unittest.TestCase):
+
     def test_semaphore_tracker(self):
+        #
+        # Check that killing process does not leak named semaphores
+        #
         import subprocess
         cmd = '''if 1:
             import multiprocessing as mp, time, os
@@ -4166,6 +4167,40 @@ class TestSemaphoreTracker(unittest.TestCase):
         self.assertRegex(err, expected)
         self.assertRegex(err, r'semaphore_tracker: %r: \[Errno' % name1)
 
+    def check_semaphore_tracker_death(self, signum, should_die):
+        # bpo-31310: if the semaphore tracker process has died, it should
+        # be restarted implicitly.
+        from multiprocessing.semaphore_tracker import _semaphore_tracker
+        _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:
+            if should_die:
+                stack.enter_context(self.assertWarnsRegex(
+                    UserWarning,
+                    "semaphore_tracker: process died"))
+            sem = ctx.Semaphore()
+            sem.acquire()
+            sem.release()
+            wr = weakref.ref(sem)
+            # ensure `sem` gets collected, which triggers communication with
+            # the semaphore tracker
+            del sem
+            gc.collect()
+            self.assertIsNone(wr())
+
+    def test_semaphore_tracker_sigint(self):
+        # Catchable signal (ignored by semaphore tracker)
+        self.check_semaphore_tracker_death(signal.SIGINT, False)
+
+    def test_semaphore_tracker_sigkill(self):
+        # Uncatchable signal.
+        self.check_semaphore_tracker_death(signal.SIGKILL, True)
+
+
 class TestSimpleQueue(unittest.TestCase):
 
     @classmethod