diff --git a/synapse/rest/client/sync.py b/synapse/rest/client/sync.py
index 2b103ca6a8..c857c658ce 100644
--- a/synapse/rest/client/sync.py
+++ b/synapse/rest/client/sync.py
@@ -45,7 +45,7 @@ from synapse.http.server import HttpServer
from synapse.http.servlet import RestServlet, parse_boolean, parse_integer, parse_string
from synapse.http.site import SynapseRequest
from synapse.logging.opentracing import trace_with_opname
-from synapse.types import JsonDict, Requester, StreamToken
+from synapse.types import JsonDict, Requester, RoomStreamToken, StreamToken
from synapse.util import json_decoder
from ._base import client_patterns, set_timeline_upper_limit
@@ -238,9 +238,52 @@ class SyncRestServlet(RestServlet):
time_now, sync_result, requester, filter_collection
)
+ if (
+ since_token is not None
+ and sync_result.next_batch is not None
+ and self._sync_token_went_backwards(
+ since_token.room_key, sync_result.next_batch.room_key
+ )
+ ):
+ since_str = await since_token.to_string(self.store)
+ next_str = await sync_result.next_batch.to_string(self.store)
+ logger.warning(
+ "sync token went backwards! from %s to %s", since_str, next_str
+ )
+
logger.debug("Event formatting complete")
return 200, response_content
+ def _sync_token_went_backwards(
+ self, since: RoomStreamToken, next: RoomStreamToken
+ ) -> bool:
+ """
+ Returns true if and only if the given token went backwards.
+ """
+ if next.stream < since.stream:
+ return True
+
+ if since.instance_map:
+ for instance, pos in since.instance_map.items():
+ if (
+ next.instance_map
+ and next.instance_map.get(instance, next.stream) < pos
+ ):
+ return True
+ elif next.stream < pos:
+ return True
+
+ if next.instance_map:
+ for instance, pos in next.instance_map.items():
+ if since.instance_map and pos < since.instance_map.get(
+ instance, since.stream
+ ):
+ return True
+ elif pos < since.stream:
+ return True
+
+ return False
+
@trace_with_opname("sync.encode_response")
async def encode_response(
self,
|
