Postpone writing postgresql.conf when joining running Postgres 12+ (#1956)

When joining already running Postgres, Patroni ensures that config files are set according to expectations.
With recovery parameters converted to GUCs in Postgres v12 it became a little problem, because when the `Postgresql` object is being created it is not yet known where the given replica is supposed to stream from.
It resulted in postgresql.conf first being written without recovery parameters, and on the next run of HA loop Patroni noticing inconsistencies and updating the config one more time.

For Postgres v12 it is not a big issue, but for v13+ it resulted in interruption of streaming replication.

patroni/postgresql/__init__.py

@@ -109,10 +109,12 @@ class Postgresql(object):
         # Last known running process
         self._postmaster_proc = None
 
-        if self.is_running():
+        if self.is_running():  # we are "joining" already running postgres
             self.set_state('running')
             self.set_role('master' if self.is_leader() else 'replica')
-            self.config.write_postgresql_conf()  # we are "joining" already running postgres
+            # postpone writing postgresql.conf for 12+ because recovery parameters are not yet known
+            if self.major_version < 120000 or self.is_leader():
+                self.config.write_postgresql_conf()
             hba_saved = self.config.replace_pg_hba()
             ident_saved = self.config.replace_pg_ident()
             if hba_saved or ident_saved:

patroni/postgresql/config.py

@@ -702,7 +702,9 @@ class ConfigHandler(object):
         if wal_receiver_primary_slot_name is not None:
             self._current_recovery_params['primary_slot_name'][0] = wal_receiver_primary_slot_name
 
-        required = {'restart': 0, 'reload': 0}
+        # Increment the 'reload' to enforce write of postgresql.conf when joining the running postgres
+        required = {'restart': 0,
+                    'reload': int(not self._postgresql.cb_called and self._postgresql.major_version >= 120000)}
 
         def record_missmatch(mtype):
             required['restart' if mtype else 'reload'] += 1

tests/test_postgresql.py

@@ -224,6 +224,7 @@ class TestPostgresql(BaseTestPostgresql):
     @patch('patroni.postgresql.config.mtime', mock_mtime)
     @patch('patroni.postgresql.config.ConfigHandler._get_pg_settings')
     def test_check_recovery_conf(self, mock_get_pg_settings):
+        self.p.call_nowait('on_start')
         mock_get_pg_settings.return_value = {
             'primary_conninfo': ['primary_conninfo', 'foo=', None, 'string', 'postmaster', self.p.config._auto_conf],
             'recovery_min_apply_delay': ['recovery_min_apply_delay', '0', 'ms', 'integer', 'sighup', 'foo']
@@ -259,6 +260,7 @@ class TestPostgresql(BaseTestPostgresql):
     @patch.object(MockPostmaster, 'create_time', Mock(return_value=1234567), create=True)
     @patch('patroni.postgresql.config.ConfigHandler._get_pg_settings')
     def test__read_recovery_params(self, mock_get_pg_settings):
+        self.p.call_nowait('on_start')
         mock_get_pg_settings.return_value = {'primary_conninfo': ['primary_conninfo', '', None, 'string',
                                                                   'postmaster', self.p.config._postgresql_conf]}
         self.p.config.write_recovery_conf({'standby_mode': 'on', 'primary_conninfo': {'password': 'foo'}})