Fix logic flaw preventing tracking of MSC2716 events in existing room versions (#10962)

We correctly allowed using the MSC2716 batch endpoint for
the room creator in existing room versions but accidentally didn't track
the events because of a logic flaw.

This prevented you from connecting subsequent chunks together because it would
throw the unknown batch ID error.

We only want to process MSC2716 events when:

 - The room version supports MSC2716
 - Any room where the homeserver has the `msc2716_enabled` experimental feature enabled and the event is from the room creator

3 files changed, 7 insertions, 9 deletions

diff --git a/changelog.d/10962.bugfix b/changelog.d/10962.bugfix
new file mode 100644

index 0000000000..9b0760d731
--- /dev/null
+++ b/changelog.d/10962.bugfix
@@ -0,0 +1 @@
+Fix [MSC2716](https://github.com/matrix-org/matrix-doc/pull/2716) `/batch_send` endpoint rejecting subsequent batches with unknown batch ID error in existing room versions from the room creator.
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py

index 9269cb444d..243be46267 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1015,9 +1015,8 @@ class FederationEventHandler:
         room_version = await self._store.get_room_version(marker_event.room_id)
         create_event = await self._store.get_create_event_for_room(marker_event.room_id)
         room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
-        if (
-            not room_version.msc2716_historical
-            or not self._config.experimental.msc2716_enabled
+        if not room_version.msc2716_historical and (
+            not self._config.experimental.msc2716_enabled
             or marker_event.sender != room_creator
         ):
             return
diff --git a/synapse/storage/databases/main/events.py b/synapse/storage/databases/main/events.py

index bc7d213fe2..19f55c19c5 100644
--- a/synapse/storage/databases/main/events.py
+++ b/synapse/storage/databases/main/events.py
@@ -1763,9 +1763,8 @@ class PersistEventsStore:
             retcol="creator",
             allow_none=True,
         )
-        if (
-            not room_version.msc2716_historical
-            or not self.hs.config.experimental.msc2716_enabled
+        if not room_version.msc2716_historical and (
+            not self.hs.config.experimental.msc2716_enabled
             or event.sender != room_creator
         ):
             return
@@ -1825,9 +1824,8 @@ class PersistEventsStore:
             retcol="creator",
             allow_none=True,
         )
-        if (
-            not room_version.msc2716_historical
-            or not self.hs.config.experimental.msc2716_enabled
+        if not room_version.msc2716_historical and (
+            not self.hs.config.experimental.msc2716_enabled
             or event.sender != room_creator
         ):
             return