Fix #8518 (sync requests being cached wrongly on timeout)

changelog.d/9358.misc

@@ -0,0 +1 @@
+Added a fix that invalidates cache for empty timed-out sync responses.

synapse/handlers/sync.py

@@ -40,7 +40,7 @@
 from synapse.util.async_helpers import concurrently_execute
 from synapse.util.caches.expiringcache import ExpiringCache
 from synapse.util.caches.lrucache import LruCache
-from synapse.util.caches.response_cache import ResponseCache
+from synapse.util.caches.response_cache import NoTimedCache, ResponseCache
 from synapse.util.metrics import Measure, measure_func
 from synapse.visibility import filter_events_for_client
 
@@ -331,6 +331,11 @@ def current_sync_callback(before_token, after_token):
                 lazy_loaded = "false"
             non_empty_sync_counter.labels(sync_type, lazy_loaded).inc()
 
+        # fixme hack: sanity check to invalidate cache, prevent a self-referral loop
+        #  replace with contextvars approach once it gets working on twisted (twisted/twisted#1262)
+        if since_token == result.next_batch:
+            result = NoTimedCache(result)  # type: ignore # because it is unwrapped in ResponseCache.set.<remove>()
+
         return result
 
     async def current_sync_for_user(

synapse/util/caches/response_cache.py

@@ -15,6 +15,8 @@
 import logging
 from typing import TYPE_CHECKING, Any, Callable, Dict, Generic, Optional, TypeVar
 
+import attr
+
 from twisted.internet import defer
 
 from synapse.logging.context import make_deferred_yieldable, run_in_background
@@ -29,6 +31,11 @@
 T = TypeVar("T")
 
 
+@attr.s(slots=True, frozen=True)
+class NoTimedCache:
+    obj = attr.ib()
+
+
 class ResponseCache(Generic[T]):
     """
     This caches a deferred response. Until the deferred completes it will be
@@ -101,13 +108,13 @@ def set(self, key: T, deferred: defer.Deferred) -> defer.Deferred:
         self.pending_result_cache[key] = result
 
         def remove(r):
-            if self.timeout_sec:
+            if self.timeout_sec and not isinstance(r, NoTimedCache):
                 self.clock.call_later(
                     self.timeout_sec, self.pending_result_cache.pop, key, None
                 )
             else:
                 self.pending_result_cache.pop(key, None)
-            return r
+            return r.obj if isinstance(r, NoTimedCache) else r
 
         result.addBoth(remove)
         return result.observe()