Start readonly when holding leader lock

Not starting of postgres was causeing situation when there were no
master running...

patroni/postgresql.py

@@ -729,14 +729,13 @@ class Postgresql(object):
             return True
 
         change_role = self.role == 'master'
-        self._need_rewind = self._need_rewind or change_role and self.can_rewind
+
+        self._need_rewind = (not leader or leader.name != self.name) \
+            and (self._need_rewind or change_role and self.can_rewind)
 
         if self._need_rewind:
             logger.info("set the rewind flag after demote")
 
-            if leader and leader.name == self.name:
-                return logger.info('Can not rewind from myself')
-
             if self.is_running():
                 stopped = self.stop()
                 self.set_role('unknown')

tests/test_postgresql.py

@@ -262,8 +262,6 @@ class TestPostgresql(unittest.TestCase):
             self.p.follow(None, None)  # restart without rewind
             self.p.set_role('master')
 
-        self.p.follow(self.leader, self.me)  # Can not rewind from myself
-
         with patch.object(Postgresql, 'stop', Mock(return_value=False)):
             self.p.follow(self.leader, self.leader)  # failed to stop postgres