K8s: reset watchers if PATCH fails with 409 (#2283)

High CPU load on Etcd nodes and K8s API servers created a very strange situation. A few clusters were running without a leader and the pod which is ahead of others was failing to take a leader lock because updates were failing with HTTP response code `409` (`resource_version` mismatch).

Effectively that means that TCP connections to K8s master nodes were alive (in the opposite case tcp keepalives would have resolved it), but no `UPDATE` events were arriving via these connections, resulting in the stale cache of the cluster in memory.

The only good way to prevent this situation is to intercept 409 HTTP responses and terminate existing TCP connections used for watches.

Now a few words about implementation. Unfortunately, watch threads are waiting in the read() call most of the time and there is no good way to interrupt them. But, the `socket.shutdown()` seems to do this job. We already used this trick in the Etcd3 implementation.

This approach will help to mitigate the issue of not having a leader, but at the same time replicas might still end up with the stale cluster state cached and in the worst case will not stream from the leader. Non-streaming replicas are less dangerous and could be covered by monitoring and partially mitigated by correctly configured `archive_command` and `restore_command`.

tests/test_kubernetes.py

@@ -4,7 +4,7 @@ import socket
 import time
 import unittest
 
-from mock import Mock, mock_open, patch
+from mock import Mock, PropertyMock, mock_open, patch
 from patroni.dcs.kubernetes import k8s_client, k8s_config, K8sConfig, K8sConnectionFailed,\
         K8sException, K8sObject, Kubernetes, KubernetesError, KubernetesRetriableException,\
         Retry, RetryFailedError, SERVICE_HOST_ENV_NAME, SERVICE_PORT_ENV_NAME
@@ -235,7 +235,9 @@ class TestKubernetesConfigMaps(BaseTestKubernetes):
             self.k.manual_failover('foo', 'bar')
 
     def test_set_config_value(self):
-        self.k.set_config_value('{}')
+        with patch.object(k8s_client.CoreV1Api, 'patch_namespaced_config_map',
+                          Mock(side_effect=k8s_client.rest.ApiException(409, '')), create=True):
+            self.k.set_config_value('{}', 1)
 
     @patch.object(k8s_client.CoreV1Api, 'patch_namespaced_pod', create=True)
     def test_touch_member(self, mock_patch_namespaced_pod):
@@ -345,3 +347,18 @@ class TestCacheBuilder(BaseTestKubernetes):
     def test__list(self):
         self.k._pods._func = Mock(side_effect=Exception)
         self.assertRaises(Exception, self.k._pods._list)
+
+    @patch('patroni.dcs.kubernetes.ObjectCache._watch', Mock(return_value=None))
+    def test__do_watch(self):
+        self.assertRaises(AttributeError, self.k._kinds._do_watch, '1')
+
+    @patch.object(k8s_client.CoreV1Api, 'list_namespaced_config_map', mock_list_namespaced_config_map, create=True)
+    @patch('patroni.dcs.kubernetes.ObjectCache._watch')
+    def test_kill_stream(self, mock_watch):
+        self.k._kinds.kill_stream()
+        mock_watch.return_value.read_chunked.return_value = []
+        mock_watch.return_value.connection.sock.close.side_effect = Exception
+        self.k._kinds._do_watch('1')
+        self.k._kinds.kill_stream()
+        type(mock_watch.return_value).connection = PropertyMock(side_effect=Exception)
+        self.k._kinds.kill_stream()