Start thread that will handle SIGCHLD for on_reload callback (#2898)

Close #2897

patroni/postgresql/callback_executor.py

@@ -30,7 +30,9 @@ class OnReloadExecutor(CancellableSubprocess):
         self.cancel(kill=True)
         self._kill_children()
         with self._lock:
-            self._start_process(cmd, close_fds=True)
+            started = self._start_process(cmd, close_fds=True)
+        if started and self._process is not None:
+            Thread(target=self._process.wait).start()
 
 
 class CallbackExecutor(CancellableExecutor, Thread):

tests/test_callback_executor.py

@@ -35,5 +35,6 @@ class TestCallbackExecutor(unittest.TestCase):
         ce._invoke_excepthook = Mock()
         self.assertIsNone(ce.call(callback))
 
+        mock_popen.side_effect = [Mock()]
         self.assertIsNone(ce.call(['test.sh', 'on_reload', 'replica', 'foo']))
         ce.join()