Ensure we always drop the federation inbound lock (#10336)

changelog.d/10336.bugfix

@@ -0,0 +1 @@
+Fix bug where inbound federation in a room could be delayed due to not correctly dropping a lock. Introduced in v1.37.1.

synapse/federation/federation_server.py

@@ -949,6 +949,7 @@ async def _process_incoming_pdus_in_room_inner(
                 room_id, room_version
             )
             if not next:
+                await lock.release()
                 return
 
             origin, event = next

synapse/storage/databases/main/lock.py

@@ -310,14 +310,25 @@ async def __aexit__(
         _excinst: Optional[BaseException],
         _exctb: Optional[TracebackType],
     ) -> bool:
+        await self.release()
+
+        return False
+
+    async def release(self) -> None:
+        """Release the lock.
+
+        This is automatically called when using the lock as a context manager.
+        """
+
+        if self._dropped:
+            return
+
         if self._looping_call.running:
             self._looping_call.stop()
 
         await self._store._drop_lock(self._lock_name, self._lock_key, self._token)
         self._dropped = True
 
-        return False
-
     def __del__(self) -> None:
         if not self._dropped:
             # We should not be dropped without the lock being released (unless