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Fix rare deadlock when using read/write locks (#16169)
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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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synapse/storage/schema/main/delta/80/02_read_write_locks_deadlock.sql.postgres
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synapse/storage/schema/main/delta/80/04_read_write_locks_deadlock.sql.postgres
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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. | ||
*/ | ||
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-- 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; | ||
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-- 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 $$ | ||
DECLARE | ||
new_token TEXT; | ||
mode_row_token TEXT; | ||
BEGIN | ||
-- Only update the token in `_mode` if its our token. This prevents | ||
-- deadlocks. | ||
-- | ||
-- We shove the token into `mode_row_token`, as otherwise postgres complains | ||
-- we're not using the returned data. | ||
SELECT token INTO mode_row_token FROM worker_read_write_locks_mode | ||
WHERE | ||
lock_name = OLD.lock_name | ||
AND lock_key = OLD.lock_key | ||
AND token = OLD.token | ||
FOR UPDATE; | ||
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IF NOT FOUND THEN | ||
RETURN NEW; | ||
END IF; | ||
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SELECT token INTO new_token FROM worker_read_write_locks | ||
WHERE | ||
lock_name = OLD.lock_name | ||
AND lock_key = OLD.lock_key | ||
LIMIT 1 FOR UPDATE SKIP LOCKED; | ||
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IF NOT FOUND THEN | ||
DELETE FROM worker_read_write_locks_mode | ||
WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key AND token = OLD.token; | ||
ELSE | ||
UPDATE worker_read_write_locks_mode | ||
SET token = new_token | ||
WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key; | ||
END IF; | ||
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RETURN NEW; | ||
END | ||
$$ | ||
LANGUAGE plpgsql; | ||
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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(); |