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120 lines
5.4 KiB
Plaintext
120 lines
5.4 KiB
Plaintext
/* Copyright 2023 The Matrix.org Foundation C.I.C
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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-- c.f. the postgres version for context. The tables and constraints are the
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-- same, however they need to be defined slightly differently to work around how
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-- each database handles circular foreign key references.
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-- A table to track whether a lock is currently acquired, and if so whether its
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-- in read or write mode.
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CREATE TABLE worker_read_write_locks_mode (
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lock_name TEXT NOT NULL,
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lock_key TEXT NOT NULL,
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-- Whether this lock is in read (false) or write (true) mode
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write_lock BOOLEAN NOT NULL,
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-- A token that has currently acquired the lock. We need this so that we can
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-- add a foreign constraint from this table to `worker_read_write_locks`.
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token TEXT NOT NULL,
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-- Add a foreign key constraint to ensure that if a lock is in
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-- `worker_read_write_locks_mode` then there must be a corresponding row in
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-- `worker_read_write_locks` (i.e. we don't accidentally end up with a row in
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-- `worker_read_write_locks_mode` when the lock is not currently acquired).
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FOREIGN KEY (lock_name, lock_key, token) REFERENCES worker_read_write_locks(lock_name, lock_key, token) DEFERRABLE INITIALLY DEFERRED
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);
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-- Ensure that we can only have one row per lock
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CREATE UNIQUE INDEX worker_read_write_locks_mode_key ON worker_read_write_locks_mode (lock_name, lock_key);
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-- We need this (redundant) constraint so that we can have a foreign key
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-- constraint against this table.
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CREATE UNIQUE INDEX worker_read_write_locks_mode_type ON worker_read_write_locks_mode (lock_name, lock_key, write_lock);
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-- A table to track who has currently acquired a given lock.
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CREATE TABLE worker_read_write_locks (
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lock_name TEXT NOT NULL,
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lock_key TEXT NOT NULL,
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-- We write the instance name to ease manual debugging, we don't ever read
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-- from it.
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-- Note: instance names aren't guarenteed to be unique.
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instance_name TEXT NOT NULL,
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-- Whether the process has taken out a "read" or a "write" lock.
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write_lock BOOLEAN NOT NULL,
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-- A random string generated each time an instance takes out a lock. Used by
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-- the instance to tell whether the lock is still held by it (e.g. in the
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-- case where the process stalls for a long time the lock may time out and
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-- be taken out by another instance, at which point the original instance
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-- can tell it no longer holds the lock as the tokens no longer match).
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token TEXT NOT NULL,
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last_renewed_ts BIGINT NOT NULL,
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-- This constraint ensures that a given lock has only been acquired in read
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-- xor write mode, but not both.
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FOREIGN KEY (lock_name, lock_key, write_lock) REFERENCES worker_read_write_locks_mode (lock_name, lock_key, write_lock)
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);
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CREATE UNIQUE INDEX worker_read_write_locks_key ON worker_read_write_locks (lock_name, lock_key, token);
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-- Ensures that only one instance can acquire a lock in write mode at a time.
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CREATE UNIQUE INDEX worker_read_write_locks_write ON worker_read_write_locks (lock_name, lock_key) WHERE write_lock;
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-- Add a trigger to UPSERT into `worker_read_write_locks_mode` whenever we try
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-- and acquire a lock, i.e. insert into `worker_read_write_locks`,
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CREATE TRIGGER IF NOT EXISTS upsert_read_write_lock_parent_trigger
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BEFORE INSERT ON worker_read_write_locks
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FOR EACH ROW
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BEGIN
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-- First ensure that `worker_read_write_locks_mode` doesn't have stale
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-- entries in it, as on SQLite we don't have the foreign key constraint to
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-- enforce this.
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DELETE FROM worker_read_write_locks_mode
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WHERE lock_name = NEW.lock_name AND lock_key = NEW.lock_key
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AND NOT EXISTS (
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SELECT 1 FROM worker_read_write_locks
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WHERE lock_name = NEW.lock_name AND lock_key = NEW.lock_key
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);
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INSERT INTO worker_read_write_locks_mode (lock_name, lock_key, write_lock, token)
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VALUES (NEW.lock_name, NEW.lock_key, NEW.write_lock, NEW.token)
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ON CONFLICT (lock_name, lock_key)
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DO NOTHING;
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END;
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-- Ensure that we keep `worker_read_write_locks_mode` up to date whenever a lock
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-- is released (i.e. a row deleted from `worker_read_write_locks`). Either we
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-- update the `worker_read_write_locks_mode.token` to match another instance
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-- that has currently acquired the lock, or we delete the row if nobody has
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-- currently acquired a lock.
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CREATE TRIGGER IF NOT EXISTS delete_read_write_lock_parent_trigger
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AFTER DELETE ON worker_read_write_locks
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FOR EACH ROW
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BEGIN
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DELETE FROM worker_read_write_locks_mode
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WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key
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AND NOT EXISTS (
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SELECT 1 FROM worker_read_write_locks
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WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key
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);
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UPDATE worker_read_write_locks_mode
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SET token = (
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SELECT token FROM worker_read_write_locks
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WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key
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)
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WHERE lock_name = OLD.lock_name AND lock_key = OLD.lock_key;
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END;
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