Shorten and beautify history log output (#1526)

when Patroni is trying to figure out the necessity of pg_rewind it could write the content history file from the primary into the log. The history file is growing with every failover/switchover and eventually starts taking too many lines in the log, most of them are not so much useful.
Instead of showing the raw data, we will show only 3 lines before the current replica timeline and 2 lines after.

tests/__init__.py

@@ -106,7 +106,7 @@ class MockCursor(object):
                             ('autovacuum', 'on', None, 'bool', 'sighup'),
                             ('unix_socket_directories', '/tmp', None, 'string', 'postmaster')]
         elif sql.startswith('IDENTIFY_SYSTEM'):
-            self.results = [('1', 2, '0/402EEC0', '')]
+            self.results = [('1', 3, '0/402EEC0', '')]
         elif sql.startswith('SELECT isdir, modification'):
             self.results = [(False, datetime.datetime.now())]
         elif sql.startswith('SELECT pg_catalog.pg_read_file'):

tests/test_postgresql.py

@@ -711,7 +711,7 @@ class TestPostgresql(BaseTestPostgresql):
         self.assertTrue(self.p.fix_cluster_state())
 
     def test_replica_cached_timeline(self):
-        self.assertEqual(self.p.replica_cached_timeline(1), 2)
+        self.assertEqual(self.p.replica_cached_timeline(2), 3)
 
     def test_get_master_timeline(self):
         self.assertEqual(self.p.get_master_timeline(), 1)

tests/test_rewind.py

@@ -85,6 +85,13 @@ class TestRewind(BaseTestPostgresql):
         with patch.object(Postgresql, 'is_running', Mock(return_value=True)):
             self.r.execute(self.leader)
 
+    @patch('patroni.postgresql.rewind.logger.info')
+    def test__log_master_history(self, mock_logger):
+        history = [[n, n, ''] for n in range(1, 10)]
+        self.r._log_master_history(history, 1)
+        expected = '\n'.join(['{0}\t0/{0}\t'.format(n) for n in range(1, 4)] + ['...', '9\t0/9\t'])
+        self.assertEqual(mock_logger.call_args[0][1], expected)
+
     @patch.object(Postgresql, 'start', Mock())
     @patch.object(Rewind, 'can_rewind', PropertyMock(return_value=True))
     @patch.object(Rewind, '_get_local_timeline_lsn', Mock(return_value=(2, '40159C1')))
@@ -101,7 +108,7 @@ class TestRewind(BaseTestPostgresql):
         with patch('psycopg2.connect', Mock(side_effect=Exception)):
             self.assertFalse(self.r.rewind_or_reinitialize_needed_and_possible(self.leader))
         self.r.trigger_check_diverged_lsn()
-        with patch.object(MockCursor, 'fetchone', Mock(side_effect=[('', 2, '0/0'), ('', b'3\t0/40159C0\tn\n')])):
+        with patch.object(MockCursor, 'fetchone', Mock(side_effect=[('', 3, '0/0'), ('', b'3\t0/40159C0\tn\n')])):
             self.assertFalse(self.r.rewind_or_reinitialize_needed_and_possible(self.leader))
         self.r.trigger_check_diverged_lsn()
         with patch.object(MockCursor, 'fetchone', Mock(return_value=('', 1, '0/0'))):