From 7cd79ce0519964bf52a3f88d6fd8a5cc5dff5c6c Mon Sep 17 00:00:00 2001
From: Erik Johnston <erikj@matrix.org>
Date: Wed, 23 Aug 2023 13:45:19 +0100
Subject: [PATCH] Reduce DB contention on worker locks (#16160)

---
 changelog.d/16160.misc                        |  1 +
 .../03_read_write_locks_triggers.sql.postgres | 37 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 changelog.d/16160.misc
 create mode 100644 synapse/storage/schema/main/delta/80/03_read_write_locks_triggers.sql.postgres

diff --git a/changelog.d/16160.misc b/changelog.d/16160.misc
new file mode 100644
index 0000000000..78803b7bcd
--- /dev/null
+++ b/changelog.d/16160.misc
@@ -0,0 +1 @@
+Reduce DB contention on worker locks.
diff --git a/synapse/storage/schema/main/delta/80/03_read_write_locks_triggers.sql.postgres b/synapse/storage/schema/main/delta/80/03_read_write_locks_triggers.sql.postgres
new file mode 100644
index 0000000000..31de5bfa18
--- /dev/null
+++ b/synapse/storage/schema/main/delta/80/03_read_write_locks_triggers.sql.postgres
@@ -0,0 +1,37 @@
+/* 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.
+ */
+
+-- Fix up the triggers that were in `78/04_read_write_locks_triggers.sql`
+
+-- Reduce the number of writes we do on this table.
+--
+-- Note: that we still want to lock the row here (i.e. still do a `DO UPDATE
+-- SET`) so that we serialize updates.
+CREATE OR REPLACE FUNCTION upsert_read_write_lock_parent() RETURNS trigger AS $$
+BEGIN
+    INSERT INTO worker_read_write_locks_mode (lock_name, lock_key, write_lock, token)
+        VALUES (NEW.lock_name, NEW.lock_key, NEW.write_lock, NEW.token)
+        ON CONFLICT (lock_name, lock_key)
+        DO UPDATE SET write_lock = NEW.write_lock
+            WHERE OLD.write_lock != NEW.write_lock;
+    RETURN NEW;
+END
+$$
+LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS upsert_read_write_lock_parent_trigger ON worker_read_write_locks;
+CREATE TRIGGER upsert_read_write_lock_parent_trigger BEFORE INSERT ON worker_read_write_locks
+    FOR EACH ROW
+    EXECUTE PROCEDURE upsert_read_write_lock_parent();