bpo-40550: fix time-of-check/time-of-action issue in multiprocessing

Signed-off-by: Filipe Laíns <lains@archlinux.org>

Author: FFY00

Lib/subprocess.py

@@ -2061,7 +2061,13 @@ def send_signal(self, sig):
             # The race condition can still happen if the race condition
             # described above happens between the returncode test
             # and the kill() call.
-            os.kill(self.pid, sig)
+            try:
+                os.kill(self.pid, sig)
+            except ProcessLookupError as e:
+                # supress the process not found error in case the race
+                # condition happens, see bpo-40550
+                if e.errno != errno.ESRCH:
+                    raise e
 
         def terminate(self):
             """Terminate the process with SIGTERM

Lib/test/test_subprocess.py

@@ -3145,6 +3145,19 @@ def test_send_signal_race(self):
         # so Popen failed to read it and uses a default returncode instead.
         self.assertIsNotNone(proc.returncode)
 
+    def test_send_signal_race2(self):
+        # bpo-40550: the process might exist between the returncode check and
+        # the kill operation
+        p = subprocess.Popen([sys.executable, '-c', 'exit(1)'])
+
+        # wait for process to exit
+        while not p.returncode:
+            p.poll()
+
+        with mock.patch.object(p, 'poll', new=lambda: None):
+            p.returncode = None
+            p.send_signal(signal.SIGTERM)
+
     def test_communicate_repeated_call_after_stdout_close(self):
         proc = subprocess.Popen([sys.executable, '-c',
                                  'import os, time; os.close(1), time.sleep(2)'],

Misc/NEWS.d/next/Library/2020-05-08-21-30-54.bpo-40550.i7GWkb.rst

@@ -0,0 +1 @@
+Fix time-of-check/time-of-action issue in subprocess.Popen.send_signal.