Fix rare deadlock when using read/write locks (#16169)

release-v1.91.0
Erik Johnston 2023-08-23 16:24:30 +01:00 committed by GitHub
parent 85118420a2
commit 18279631e9
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3 changed files with 72 additions and 37 deletions

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changelog.d/16169.bugfix Normal file
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Fix a rare race that could block new events from being sent for up to two minutes. Introduced in v1.90.0.

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/* 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.
*/
-- To avoid the possibility of a deadlock, lock the
-- `worker_read_write_locks_mode` table so that we serialize inserts/deletes
-- for a specific lock name/key.
CREATE OR REPLACE FUNCTION delete_read_write_lock_parent_before() RETURNS trigger AS $$
BEGIN
-- `PERFORM` is a `SELECT` which discards the rows.
PERFORM * FROM worker_read_write_locks_mode
WHERE
lock_name = OLD.lock_name
AND lock_key = OLD.lock_key
FOR UPDATE;
RETURN OLD;
END
$$
LANGUAGE plpgsql;
DROP TRIGGER IF EXISTS delete_read_write_lock_parent_before_trigger ON worker_read_write_locks;
CREATE TRIGGER delete_read_write_lock_parent_before_trigger BEFORE DELETE ON worker_read_write_locks
FOR EACH ROW
EXECUTE PROCEDURE delete_read_write_lock_parent_before();

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/* 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.
*/
-- Remove a previous attempt to avoid deadlocks
DROP TRIGGER IF EXISTS delete_read_write_lock_parent_before_trigger ON worker_read_write_locks;
DROP FUNCTION IF EXISTS delete_read_write_lock_parent_before;
-- Ensure that we keep `worker_read_write_locks_mode` up to date whenever a lock
-- is released (i.e. a row deleted from `worker_read_write_locks`). Either we
-- update the `worker_read_write_locks_mode.token` to match another instance
-- that has currently acquired the lock, or we delete the row if nobody has
-- currently acquired a lock.
CREATE OR REPLACE FUNCTION delete_read_write_lock_parent() RETURNS trigger AS $$
DECLARE
new_token TEXT;
mode_row_token TEXT;
BEGIN
-- Only update the token in `_mode` if its our token. This prevents
-- deadlocks.
--
-- We shove the token into `mode_row_token`, as otherwise postgres complains
-- we're not using the returned data.
SELECT token INTO mode_row_token FROM worker_read_write_locks_mode
WHERE
lock_name = OLD.lock_name
AND lock_key = OLD.lock_key
AND token = OLD.token
FOR UPDATE;
IF NOT FOUND THEN
RETURN NEW;
END IF;
SELECT token INTO new_token FROM worker_read_write_locks
WHERE
lock_name = OLD.lock_name
AND lock_key = OLD.lock_key
LIMIT 1 FOR UPDATE SKIP LOCKED;
IF NOT FOUND THEN
DELETE FROM worker_read_write_locks_mode
WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key AND token = OLD.token;
ELSE
UPDATE worker_read_write_locks_mode
SET token = new_token
WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key;
END IF;
RETURN NEW;
END
$$
LANGUAGE plpgsql;
DROP TRIGGER IF EXISTS delete_read_write_lock_parent_trigger ON worker_read_write_locks;
CREATE TRIGGER delete_read_write_lock_parent_trigger AFTER DELETE ON worker_read_write_locks
FOR EACH ROW
EXECUTE PROCEDURE delete_read_write_lock_parent();