Failover logical slots (#1820)

Effectively, this PR consists of a few changes:

1. The easy part:
  In case of permanent logical slots are defined in the global configuration, Patroni on the primary will not only create them, but also periodically update DCS with the current values of `confirmed_flush_lsn` for all these slots.
  In order to reduce the number of interactions with DCS the new `/status` key was introduced. It will contain the json object with `optime` and `slots` keys. For backward compatibility the `/optime/leader` will be updated if there are members with old Patroni in the cluster.

2. The tricky part:
  On replicas that are eligible for a failover, Patroni creates the logical replication slot by copying the slot file from the primary and restarting the replica. In order to copy the slot file Patroni opens a connection to the primary with `rewind` or `superuser` credentials and calls `pg_read_binary_file()`  function.
  When the logical slot already exists on the replica Patroni periodically calls `pg_replication_slot_advance()` function, which allows moving the slot forward.

3. Additional requirements:
  In order to ensure that primary doesn't cleanup tuples from pg_catalog that are required for logical decoding, Patroni enables `hot_standby_feedback` on replicas with logical slots and on cascading replicas if they are used for streaming by replicas with logical slots.

4. When logical slots are copied from to the replica there is a timeframe when it could be not safe to use them after promotion. Right now there is no protection from promoting such a replica. But, Patroni will show the warning with names of the slots that might be not safe to use.

Compatibility.
The `pg_replication_slot_advance()` function is only available starting from PostgreSQL 11. For older Postgres versions Patroni will refuse to create the logical slot on the primary.

The old "permanent slots" feature, which creates logical slots right after promotion and before allowing connections, was removed.

Close: https://github.com/zalando/patroni/issues/1749

tests/test_postgresql.py

@@ -1,5 +1,4 @@
 import datetime
-import mock  # for the mock.call method, importing it without a namespace breaks python3
 import os
 import psutil
 import psycopg2
@@ -9,11 +8,10 @@ import time
 
 from mock import Mock, MagicMock, PropertyMock, patch, mock_open
 from patroni.async_executor import CriticalTask
-from patroni.dcs import Cluster, ClusterConfig, Member, RemoteMember, SyncState
+from patroni.dcs import Cluster, RemoteMember, SyncState
 from patroni.exceptions import PostgresConnectionException, PatroniException
 from patroni.postgresql import Postgresql, STATE_REJECT, STATE_NO_RESPONSE
 from patroni.postgresql.postmaster import PostmasterProcess
-from patroni.postgresql.slots import SlotsHandler
 from patroni.utils import RetryFailedError
 from six.moves import builtins
 from threading import Thread, current_thread
@@ -303,29 +301,6 @@ class TestPostgresql(BaseTestPostgresql):
         m = RemoteMember('1', {'restore_command': '2', 'primary_slot_name': 'foo', 'conn_kwargs': {'host': 'bar'}})
         self.p.follow(m)
 
-    @patch.object(Postgresql, 'is_running', Mock(return_value=True))
-    def test_sync_replication_slots(self):
-        self.p.start()
-        config = ClusterConfig(1, {'slots': {'test_3': {'database': 'a', 'plugin': 'b'},
-                                             'A': 0, 'ls': 0, 'b': {'type': 'logical', 'plugin': '1'}},
-                                   'ignore_slots': [{'name': 'blabla'}]}, 1)
-        cluster = Cluster(True, config, self.leader, 0, [self.me, self.other, self.leadermem], None, None, None)
-        with mock.patch('patroni.postgresql.Postgresql._query', Mock(side_effect=psycopg2.OperationalError)):
-            self.p.slots_handler.sync_replication_slots(cluster)
-        self.p.slots_handler.sync_replication_slots(cluster)
-        with mock.patch('patroni.postgresql.Postgresql.role', new_callable=PropertyMock(return_value='replica')):
-            self.p.slots_handler.sync_replication_slots(cluster)
-        with patch.object(SlotsHandler, 'drop_replication_slot', Mock(return_value=True)),\
-                patch('patroni.dcs.logger.error', new_callable=Mock()) as errorlog_mock:
-            alias1 = Member(0, 'test-3', 28, {'conn_url': 'postgres://replicator:[email protected]:5436/postgres'})
-            alias2 = Member(0, 'test.3', 28, {'conn_url': 'postgres://replicator:[email protected]:5436/postgres'})
-            cluster.members.extend([alias1, alias2])
-            self.p.slots_handler.sync_replication_slots(cluster)
-            self.assertEqual(errorlog_mock.call_count, 5)
-            ca = errorlog_mock.call_args_list[0][0][1]
-            self.assertTrue("test-3" in ca, "non matching {0}".format(ca))
-            self.assertTrue("test.3" in ca, "non matching {0}".format(ca))
-
     @patch.object(MockCursor, 'execute', Mock(side_effect=psycopg2.OperationalError))
     def test__query(self):
         self.assertRaises(PostgresConnectionException, self.p._query, 'blabla')
@@ -340,7 +315,7 @@ class TestPostgresql(BaseTestPostgresql):
     @patch.object(Postgresql, 'pg_isready', Mock(return_value=STATE_REJECT))
     def test_is_leader(self):
         self.assertTrue(self.p.is_leader())
-        self.p.reset_cluster_info_state()
+        self.p.reset_cluster_info_state(None)
         with patch.object(Postgresql, '_query', Mock(side_effect=RetryFailedError(''))):
             self.assertRaises(PostgresConnectionException, self.p.is_leader)
 
@@ -611,7 +586,7 @@ class TestPostgresql(BaseTestPostgresql):
 
     def test_pick_sync_standby(self):
         cluster = Cluster(True, None, self.leader, 0, [self.me, self.other, self.leadermem], None,
-                          SyncState(0, self.me.name, self.leadermem.name), None)
+                          SyncState(0, self.me.name, self.leadermem.name), None, None)
         mock_cursor = Mock()
         mock_cursor.fetchone.return_value = ('remote_apply',)
 
@@ -727,10 +702,14 @@ class TestPostgresql(BaseTestPostgresql):
     @patch.object(Postgresql, '_query', Mock(side_effect=RetryFailedError('')))
     def test_received_timeline(self):
         self.p.set_role('standby_leader')
-        self.p.reset_cluster_info_state()
+        self.p.reset_cluster_info_state(None)
         self.assertRaises(PostgresConnectionException, self.p.received_timeline)
 
     def test__write_recovery_params(self):
         self.p.config._write_recovery_params(Mock(), {'pause_at_recovery_target': 'false'})
         with patch.object(Postgresql, 'major_version', PropertyMock(return_value=90400)):
             self.p.config._write_recovery_params(Mock(), {'recovery_target_action': 'PROMOTE'})
+
+    @patch.object(Postgresql, 'is_running', Mock(return_value=True))
+    def test_set_enforce_hot_standby_feedback(self):
+        self.p.set_enforce_hot_standby_feedback(True)