Fix port in use error on certificate replacement (#2185)

When switching certificates there is a race condition with a concurrent API request. If there is one active during the replacement period then the replacement will error out with a port in use error and Patroni gets stuck in a state without an active API server.

Fix is to call server_close after shutdown which will wait for already running requests to complete before returning.

Close #2184

patroni/api.py

@@ -768,6 +768,8 @@ class RestApiServer(ThreadingMixIn, HTTPServer, Thread):
         reloading_config = self.__listen is not None  # changing config in runtime
         if reloading_config:
             self.shutdown()
+            # Rely on ThreadingMixIn.server_close() to have all requests terminate before we continue
+            self.server_close()
 
         self.__listen = listen
         self.__ssl_options = ssl_options

tests/test_api.py

@@ -549,7 +549,8 @@ class TestRestApiServer(unittest.TestCase):
         self.assertRaises(ValueError, MockRestApiServer, None, '', bad_config)
         self.assertRaises(ValueError, self.srv.reload_config, bad_config)
         self.assertRaises(ValueError, self.srv.reload_config, {})
-        with patch.object(socket.socket, 'setsockopt', Mock(side_effect=socket.error)):
+        with patch.object(socket.socket, 'setsockopt', Mock(side_effect=socket.error)), \
+                patch.object(MockRestApiServer, 'server_close', Mock()):
             self.srv.reload_config({'listen': ':8008'})
 
     @patch.object(MockPatroni, 'dcs')