Make sequence `cache_invalidation_stream_seq` begin at `2` (#13766)

Signed-off-by: Mathieu Velten <mathieuv@matrix.org>
Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

changelog.d/13766.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where the `cache_invalidation_stream_seq` sequence would begin at 1 instead of 2.

synapse/storage/schema/__init__.py

@@ -76,6 +76,7 @@ Changes in SCHEMA_VERSION = 72:
     - event_edges.(room_id, is_state) are no longer written to.
     - Tables related to groups are dropped.
     - Unused column application_services_state.last_txn is dropped
+    - Cache invalidation stream id sequence now begins at 2 to match code expectation.
 """
 
 

synapse/storage/schema/main/delta/72/08begin_cache_invalidation_seq_at_2.sql.postgres

@@ -0,0 +1,23 @@
+/* Copyright 2022 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.
+ */
+ 
+
+-- The sequence needs to begin at 2 because a bunch of code assumes that
+-- get_next_id_txn will return values >= 2, cf this comment:
+-- https://github.com/matrix-org/synapse/blob/b93bd95e8ab64d27ae26841020f62ee61272a5f2/synapse/storage/util/id_generators.py#L344
+
+SELECT setval('cache_invalidation_stream_seq', (
+    SELECT COALESCE(MAX(last_value), 1) FROM cache_invalidation_stream_seq
+));