Fix bug in device list caching when remote users leave rooms (#13749)

When a remote user leaves the last room shared with the homeserver, we
have to mark their device list as unsubscribed, otherwise we would hold
on to a stale device list in our cache. Crucially, the device list would
remain cached even after the remote user rejoined the room, which could
lead to E2EE failures until the next change to the remote user's device
list.

Fixes #13651.

Signed-off-by: Sean Quah <seanq@matrix.org>

synapse/handlers/e2e_keys.py

@@ -175,6 +175,32 @@ class E2eKeysHandler:
                     user_ids_not_in_cache,
                     remote_results,
                 ) = await self.store.get_user_devices_from_cache(query_list)
+
+                # Check that the homeserver still shares a room with all cached users.
+                # Note that this check may be slightly racy when a remote user leaves a
+                # room after we have fetched their cached device list. In the worst case
+                # we will do extra federation queries for devices that we had cached.
+                cached_users = set(remote_results.keys())
+                valid_cached_users = (
+                    await self.store.get_users_server_still_shares_room_with(
+                        remote_results.keys()
+                    )
+                )
+                invalid_cached_users = cached_users - valid_cached_users
+                if invalid_cached_users:
+                    # Fix up results. If we get here, there is either a bug in device
+                    # list tracking, or we hit the race mentioned above.
+                    user_ids_not_in_cache.update(invalid_cached_users)
+                    for invalid_user_id in invalid_cached_users:
+                        remote_results.pop(invalid_user_id)
+                    # This log message may be removed if it turns out it's almost
+                    # entirely triggered by races.
+                    logger.error(
+                        "Devices for %s were cached, but the server no longer shares "
+                        "any rooms with them. The cached device lists are stale.",
+                        invalid_cached_users,
+                    )
+
                 for user_id, devices in remote_results.items():
                     user_devices = results.setdefault(user_id, {})
                     for device_id, device in devices.items():