Wrap process in a flag so we don't process whist already processing.

synapse/push/httppusher.py

@@ -48,6 +48,7 @@ class HttpPusher(object):
         self.backoff_delay = HttpPusher.INITIAL_BACKOFF_SEC
         self.failing_since = pusherdict['failing_since']
         self.timed_call = None
+        self.processing = False
 
         # This is the highest stream ordering we know it's safe to process.
         # When new events arrive, we'll be given a window of new events: we
@@ -109,6 +110,14 @@ class HttpPusher(object):
 
     @defer.inlineCallbacks
     def _process(self):
+        try:
+            self.processing = True
+            yield self._unsafe_process()
+        finally:
+            self.processing = False
+
+    @defer.inlineCallbacks
+    def _unsafe_process(self):
         unprocessed = yield self.store.get_unread_push_actions_for_user_in_range(
             self.user_id, self.last_stream_ordering, self.max_stream_ordering
         )