Safe async event cache (#13308)

Fix race conditions in the async cache invalidation logic, by separating
the async & local invalidation calls and ensuring any async call i
executed first.

Signed off by Nick @ Beeper (@Fizzadar).

1 files changed, 8 insertions, 1 deletions

diff --git a/synapse/storage/_base.py b/synapse/storage/_base.py
index b8c8dcd76b..a2f8310388 100644
--- a/synapse/storage/_base.py
+++ b/synapse/storage/_base.py
@@ -96,6 +96,10 @@ class SQLBaseStore(metaclass=ABCMeta):
         cache doesn't exist. Mainly used for invalidating caches on workers,
         where they may not have the cache.
 
+        Note that this function does not invalidate any remote caches, only the
+        local in-memory ones. Any remote invalidation must be performed before
+        calling this.
+
         Args:
             cache_name
             key: Entry to invalidate. If None then invalidates the entire
@@ -112,7 +116,10 @@ class SQLBaseStore(metaclass=ABCMeta):
         if key is None:
             cache.invalidate_all()
         else:
-            cache.invalidate(tuple(key))
+            # Prefer any local-only invalidation method. Invalidating any non-local
+            # cache must be be done before this.
+            invalidate_method = getattr(cache, "invalidate_local", cache.invalidate)
+            invalidate_method(tuple(key))
 
 
 def db_to_json(db_content: Union[memoryview, bytes, bytearray, str]) -> Any: