Add a distributed lock (#10269)

This adds a simple best effort locking mechanism that works cross workers.

1 files changed, 37 insertions, 0 deletions

diff --git a/synapse/storage/schema/main/delta/59/15locks.sql b/synapse/storage/schema/main/delta/59/15locks.sql
new file mode 100644
index 0000000000..8b2999ff3e
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+++ b/synapse/storage/schema/main/delta/59/15locks.sql
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+/* Copyright 2021 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.
+ */
+
+
+-- A noddy implementation of a distributed lock across workers. While a worker
+-- has taken a lock out they should regularly update the `last_renewed_ts`
+-- column, a lock will be considered dropped if `last_renewed_ts` is from ages
+-- ago.
+CREATE TABLE worker_locks (
+    lock_name TEXT NOT NULL,
+    lock_key TEXT NOT NULL,
+    -- We write the instance name to ease manual debugging, we don't ever read
+    -- from it.
+    -- Note: instance names aren't guarenteed to be unique.
+    instance_name TEXT NOT NULL,
+    -- A random string generated each time an instance takes out a lock. Used by
+    -- the instance to tell whether the lock is still held by it (e.g. in the
+    -- case where the process stalls for a long time the lock may time out and
+    -- be taken out by another instance, at which point the original instance
+    -- can tell it no longer holds the lock as the tokens no longer match).
+    token TEXT NOT NULL,
+    last_renewed_ts BIGINT NOT NULL
+);
+
+CREATE UNIQUE INDEX worker_locks_key ON worker_locks (lock_name, lock_key);