Explicitly enable synchronous mode (#2820)

Close https://github.com/zalando/patroni/issues/2819

Co-authored-by: Polina Bungina <[email protected]>

patroni/ha.py

@@ -656,11 +656,20 @@ class Ha(object):
         promoting standbys that were guaranteed to be replicating synchronously.
         """
         if self.is_synchronous_mode():
-            current = CaseInsensitiveSet(self.cluster.sync.members)
+            sync = self.cluster.sync
+            if sync.is_empty:
+                # corner case: we need to explicitly enable synchronous mode by updating the
+                # ``/sync`` key with the current leader name and empty members. In opposite case
+                # it will never be automatically enabled if there are not eligible candidates.
+                sync = self.dcs.write_sync_state(self.state_handler.name, None, version=sync.version)
+                if not sync:
+                    return logger.warning("Updating sync state failed")
+                logger.info("Enabled synchronous replication")
+
+            current = CaseInsensitiveSet(sync.members)
             picked, allow_promote = self.state_handler.sync_handler.current_state(self.cluster)
 
             if picked != current:
-                sync = self.cluster.sync
                 # update synchronous standby list in dcs temporarily to point to common nodes in current and picked
                 sync_common = current & allow_promote
                 if sync_common != current:

tests/test_ha.py

@@ -1294,6 +1294,19 @@ class TestHa(PostgresInit):
         mock_restart.assert_called_once()
         self.ha.dcs.get_cluster.assert_not_called()
 
+    def test_enable_synchronous_mode(self):
+        self.ha.is_synchronous_mode = true
+        self.ha.has_lock = true
+        self.p.name = 'leader'
+        self.ha.dcs.write_sync_state = Mock(return_value=SyncState.empty())
+        with patch('patroni.ha.logger.info') as mock_logger:
+            self.ha.run_cycle()
+            self.assertEqual(mock_logger.call_args[0][0], 'Enabled synchronous replication')
+        self.ha.dcs.write_sync_state = Mock(return_value=None)
+        with patch('patroni.ha.logger.warning') as mock_logger:
+            self.ha.run_cycle()
+            self.assertEqual(mock_logger.call_args[0][0], 'Updating sync state failed')
+
     def test_effective_tags(self):
         self.ha._disable_sync = True
         self.assertEqual(self.ha.get_effective_tags(), {'foo': 'bar', 'nosync': True})