Mark sync as limited if there is a gap in the timeline (#16485)

This splits thinsg into two queries, but most of the time we won't have
new event backwards extremities so this shouldn't actually add an extra
RTT for the majority of cases.

Note this removes the check for events with no prev events, but that was
part of MSC2716 work that has since been removed.

1 files changed, 25 insertions, 0 deletions

diff --git a/synapse/storage/schema/main/delta/82/05gaps.sql b/synapse/storage/schema/main/delta/82/05gaps.sql
new file mode 100644
index 0000000000..6813b488ca
--- /dev/null
+++ b/synapse/storage/schema/main/delta/82/05gaps.sql
@@ -0,0 +1,25 @@
+/* Copyright 2023 The Matrix.org Foundation C.I.C
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- Records when we see a "gap in the timeline", due to missing events over
+-- federation. We record this so that we can tell clients there is a gap (by
+-- marking the timeline section of a sync request as limited).
+CREATE TABLE IF NOT EXISTS timeline_gaps (
+    room_id TEXT NOT NULL,
+    instance_name TEXT NOT NULL,
+    stream_ordering BIGINT NOT NULL
+);
+
+CREATE INDEX timeline_gaps_room_id ON timeline_gaps(room_id, stream_ordering);