Disable calculating unread counts unless the config flag is enabled.

changelog.d/13694.bugfix

@@ -0,0 +1 @@
+Fix a bug introduced in Synapse v1.20.0 that would cause the unstable unread counts from [MSC2654](https://github.com/matrix-org/matrix-spec-proposals/pull/2654) to be calculated even if the feature is disabled.

synapse/config/experimental.py

@@ -71,6 +71,9 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         self.msc3720_enabled: bool = experimental.get("msc3720_enabled", False)
 
         # MSC2654: Unread counts
+        #
+        # Note that enabling this will result in an incorrect unread count for
+        # previously calculated push actions.
         self.msc2654_enabled: bool = experimental.get("msc2654_enabled", False)
 
         # MSC2815 (allow room moderators to view redacted event content)

synapse/push/bulk_push_rule_evaluator.py

@@ -262,7 +262,12 @@ async def action_for_event_by_user(
             # This can happen due to out of band memberships
             return
 
-        count_as_unread = _should_count_as_unread(event, context)
+        # Disable counting as unread unless the experimental configuration is
+        # enabled, as it can cause additional (unwanted) rows to be added to the
+        # event_push_actions table.
+        count_as_unread = False
+        if self.hs.config.experimental.msc2654_enabled:
+            count_as_unread = _should_count_as_unread(event, context)
 
         rules_by_user = await self._get_rules_for_event(event)
         actions_by_user: Dict[str, Collection[Union[Mapping, str]]] = {}