mitogen: Fix subprocess ResourceWarning

Python 3.x emits `ResourceWarning`s if certains resources aren't correctly
closed. Due to the way Mitogen has been terminating child processes this has
been occurring.

```
test_dev_tty_open_succeeds
(create_child_test.TtyCreateChildTest.test_dev_tty_open_succeeds) ...
/opt/hostedtoolcache/Python/3.12.5/x64/lib/python3.12/subprocess.py:1127:
ResourceWarning: subprocess 3313 is still running
  _warn("subprocess %s is still running" % self.pid,
ResourceWarning: Enable tracemalloc to get the object allocation traceback
ok
```

During garbage collection subprocess.Popen() objects emit
ResourceWarning("subprocess 123 is still running")
if proc.returncode hasn't been set. Typically calling proc.wait() does so,
once the sub-process has exited. Calling os.waitpid(proc.pid, 0) also waits
for the sub-process to exit, but it doesn't update proc.returncode, so the
ResourceWarning is still emitted.

This change exposes `subprocess.Popen` methods on
`mitogen.parent.PopenProcess`, so that the returncode can be set.

See https://gist.github.com/moreati/b8d157ff82cb15234bece4033accc5e5

mitogen/parent.py

@@ -2542,7 +2542,7 @@ class Reaper(object):
         # because it is setuid, so this is best-effort only.
         LOG.debug('%r: sending %s', self.proc, SIGNAL_BY_NUM[signum])
         try:
-            os.kill(self.proc.pid, signum)
+            self.proc.send_signal(signum)
         except OSError:
             e = sys.exc_info()[1]
             if e.args[0] != errno.EPERM:
@@ -2662,6 +2662,17 @@ class Process(object):
         """
         raise NotImplementedError()
 
+    def send_signal(self, sig):
+        os.kill(self.pid, sig)
+
+    def terminate(self):
+        "Ask the process to gracefully shutdown."
+        self.send_signal(signal.SIGTERM)
+
+    def kill(self):
+        "Ask the operating system to forcefully destroy the process."
+        self.send_signal(signal.SIGKILL)
+
 
 class PopenProcess(Process):
     """
@@ -2678,6 +2689,9 @@ class PopenProcess(Process):
     def poll(self):
         return self.proc.poll()
 
+    def send_signal(self, sig):
+        self.proc.send_signal(sig)
+
 
 class ModuleForwarder(object):
     """

tests/connection_test.py

@@ -55,7 +55,9 @@ def do_detach(econtext):
 class DetachReapTest(testlib.RouterMixin, testlib.TestCase):
     def test_subprocess_preserved_on_shutdown(self):
         c1 = self.router.local()
+        c1_stream = self.router.stream_by_id(c1.context_id)
         pid = c1.call(os.getpid)
+        self.assertEqual(pid, c1_stream.conn.proc.pid)
 
         l = mitogen.core.Latch()
         mitogen.core.listen(c1, 'disconnect', l.put)
@@ -65,8 +67,8 @@ class DetachReapTest(testlib.RouterMixin, testlib.TestCase):
         self.broker.shutdown()
         self.broker.join()
 
-        os.kill(pid, 0)  # succeeds if process still alive
+        self.assertIsNone(os.kill(pid, 0))  # succeeds if process still alive
 
         # now clean up
-        os.kill(pid, signal.SIGTERM)
-        os.waitpid(pid, 0)
+        c1_stream.conn.proc.terminate()
+        c1_stream.conn.proc.proc.wait()

tests/create_child_test.py

@@ -76,6 +76,7 @@ def close_proc(proc):
     proc.stdout.close()
     if proc.stderr:
         proc.stderr.close()
+    proc.proc.wait()
 
 
 def wait_read(fp, n):

tests/reaper_test.py

@@ -10,8 +10,7 @@ import mitogen.parent
 
 
 class ReaperTest(testlib.TestCase):
-    @mock.patch('os.kill')
-    def test_calc_delay(self, kill):
+    def test_calc_delay(self):
         broker = mock.Mock()
         proc = mock.Mock()
         proc.poll.return_value = None
@@ -24,8 +23,7 @@ class ReaperTest(testlib.TestCase):
         self.assertEqual(752, int(1000 * reaper._calc_delay(5)))
         self.assertEqual(1294, int(1000 * reaper._calc_delay(6)))
 
-    @mock.patch('os.kill')
-    def test_reap_calls(self, kill):
+    def test_reap_calls(self):
         broker = mock.Mock()
         proc = mock.Mock()
         proc.poll.return_value = None
@@ -33,20 +31,20 @@ class ReaperTest(testlib.TestCase):
         reaper = mitogen.parent.Reaper(broker, proc, True, True)
 
         reaper.reap()
-        self.assertEqual(0, kill.call_count)
+        self.assertEqual(0, proc.send_signal.call_count)
 
         reaper.reap()
-        self.assertEqual(1, kill.call_count)
+        self.assertEqual(1, proc.send_signal.call_count)
 
         reaper.reap()
         reaper.reap()
         reaper.reap()
-        self.assertEqual(1, kill.call_count)
+        self.assertEqual(1, proc.send_signal.call_count)
 
         reaper.reap()
-        self.assertEqual(2, kill.call_count)
+        self.assertEqual(2, proc.send_signal.call_count)
 
-        self.assertEqual(kill.mock_calls, [
-            mock.call(proc.pid, signal.SIGTERM),
-            mock.call(proc.pid, signal.SIGKILL),
+        self.assertEqual(proc.send_signal.mock_calls, [
+            mock.call(signal.SIGTERM),
+            mock.call(signal.SIGKILL),
         ])