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Diffstat (limited to 'synapse/storage/databases/main/lock.py')
| -rw-r--r-- | synapse/storage/databases/main/lock.py | 334 |
1 files changed, 334 insertions, 0 deletions
diff --git a/synapse/storage/databases/main/lock.py b/synapse/storage/databases/main/lock.py new file mode 100644 index 0000000000..e76188328c --- /dev/null +++ b/synapse/storage/databases/main/lock.py @@ -0,0 +1,334 @@ +# Copyright 2021 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. +import logging +from types import TracebackType +from typing import TYPE_CHECKING, Dict, Optional, Tuple, Type + +from twisted.internet.interfaces import IReactorCore + +from synapse.metrics.background_process_metrics import wrap_as_background_process +from synapse.storage._base import SQLBaseStore +from synapse.storage.database import DatabasePool, LoggingTransaction +from synapse.storage.types import Connection +from synapse.util import Clock +from synapse.util.stringutils import random_string + +if TYPE_CHECKING: + from synapse.server import HomeServer + + +logger = logging.getLogger(__name__) + + +# How often to renew an acquired lock by updating the `last_renewed_ts` time in +# the lock table. +_RENEWAL_INTERVAL_MS = 30 * 1000 + +# How long before an acquired lock times out. +_LOCK_TIMEOUT_MS = 2 * 60 * 1000 + + +class LockStore(SQLBaseStore): + """Provides a best effort distributed lock between worker instances. + + Locks are identified by a name and key. A lock is acquired by inserting into + the `worker_locks` table if a) there is no existing row for the name/key or + b) the existing row has a `last_renewed_ts` older than `_LOCK_TIMEOUT_MS`. + + When a lock is taken out the instance inserts a random `token`, the instance + that holds that token holds the lock until it drops (or times out). + + The instance that holds the lock should regularly update the + `last_renewed_ts` column with the current time. + """ + + def __init__(self, database: DatabasePool, db_conn: Connection, hs: "HomeServer"): + super().__init__(database, db_conn, hs) + + self._reactor = hs.get_reactor() + self._instance_name = hs.get_instance_id() + + # A map from `(lock_name, lock_key)` to the token of any locks that we + # think we currently hold. + self._live_tokens: Dict[Tuple[str, str], str] = {} + + # When we shut down we want to remove the locks. Technically this can + # lead to a race, as we may drop the lock while we are still processing. + # However, a) it should be a small window, b) the lock is best effort + # anyway and c) we want to really avoid leaking locks when we restart. + hs.get_reactor().addSystemEventTrigger( + "before", + "shutdown", + self._on_shutdown, + ) + + @wrap_as_background_process("LockStore._on_shutdown") + async def _on_shutdown(self) -> None: + """Called when the server is shutting down""" + logger.info("Dropping held locks due to shutdown") + + for (lock_name, lock_key), token in self._live_tokens.items(): + await self._drop_lock(lock_name, lock_key, token) + + logger.info("Dropped locks due to shutdown") + + async def try_acquire_lock(self, lock_name: str, lock_key: str) -> Optional["Lock"]: + """Try to acquire a lock for the given name/key. Will return an async + context manager if the lock is successfully acquired, which *must* be + used (otherwise the lock will leak). + """ + + now = self._clock.time_msec() + token = random_string(6) + + if self.db_pool.engine.can_native_upsert: + + def _try_acquire_lock_txn(txn: LoggingTransaction) -> bool: + # We take out the lock if either a) there is no row for the lock + # already or b) the existing row has timed out. + sql = """ + INSERT INTO worker_locks (lock_name, lock_key, instance_name, token, last_renewed_ts) + VALUES (?, ?, ?, ?, ?) + ON CONFLICT (lock_name, lock_key) + DO UPDATE + SET + token = EXCLUDED.token, + instance_name = EXCLUDED.instance_name, + last_renewed_ts = EXCLUDED.last_renewed_ts + WHERE + worker_locks.last_renewed_ts < ? + """ + txn.execute( + sql, + ( + lock_name, + lock_key, + self._instance_name, + token, + now, + now - _LOCK_TIMEOUT_MS, + ), + ) + + # We only acquired the lock if we inserted or updated the table. + return bool(txn.rowcount) + + did_lock = await self.db_pool.runInteraction( + "try_acquire_lock", + _try_acquire_lock_txn, + # We can autocommit here as we're executing a single query, this + # will avoid serialization errors. + db_autocommit=True, + ) + if not did_lock: + return None + + else: + # If we're on an old SQLite we emulate the above logic by first + # clearing out any existing stale locks and then upserting. + + def _try_acquire_lock_emulated_txn(txn: LoggingTransaction) -> bool: + sql = """ + DELETE FROM worker_locks + WHERE + lock_name = ? + AND lock_key = ? + AND last_renewed_ts < ? + """ + txn.execute( + sql, + (lock_name, lock_key, now - _LOCK_TIMEOUT_MS), + ) + + inserted = self.db_pool.simple_upsert_txn_emulated( + txn, + table="worker_locks", + keyvalues={ + "lock_name": lock_name, + "lock_key": lock_key, + }, + values={}, + insertion_values={ + "token": token, + "last_renewed_ts": self._clock.time_msec(), + "instance_name": self._instance_name, + }, + ) + + return inserted + + did_lock = await self.db_pool.runInteraction( + "try_acquire_lock_emulated", _try_acquire_lock_emulated_txn + ) + + if not did_lock: + return None + + self._live_tokens[(lock_name, lock_key)] = token + + return Lock( + self._reactor, + self._clock, + self, + lock_name=lock_name, + lock_key=lock_key, + token=token, + ) + + async def _is_lock_still_valid( + self, lock_name: str, lock_key: str, token: str + ) -> bool: + """Checks whether this instance still holds the lock.""" + last_renewed_ts = await self.db_pool.simple_select_one_onecol( + table="worker_locks", + keyvalues={ + "lock_name": lock_name, + "lock_key": lock_key, + "token": token, + }, + retcol="last_renewed_ts", + allow_none=True, + desc="is_lock_still_valid", + ) + return ( + last_renewed_ts is not None + and self._clock.time_msec() - _LOCK_TIMEOUT_MS < last_renewed_ts + ) + + async def _renew_lock(self, lock_name: str, lock_key: str, token: str) -> None: + """Attempt to renew the lock if we still hold it.""" + await self.db_pool.simple_update( + table="worker_locks", + keyvalues={ + "lock_name": lock_name, + "lock_key": lock_key, + "token": token, + }, + updatevalues={"last_renewed_ts": self._clock.time_msec()}, + desc="renew_lock", + ) + + async def _drop_lock(self, lock_name: str, lock_key: str, token: str) -> None: + """Attempt to drop the lock, if we still hold it""" + await self.db_pool.simple_delete( + table="worker_locks", + keyvalues={ + "lock_name": lock_name, + "lock_key": lock_key, + "token": token, + }, + desc="drop_lock", + ) + + self._live_tokens.pop((lock_name, lock_key), None) + + +class Lock: + """An async context manager that manages an acquired lock, ensuring it is + regularly renewed and dropping it when the context manager exits. + + The lock object has an `is_still_valid` method which can be used to + double-check the lock is still valid, if e.g. processing work in a loop. + + For example: + + lock = await self.store.try_acquire_lock(...) + if not lock: + return + + async with lock: + for item in work: + await process(item) + + if not await lock.is_still_valid(): + break + """ + + def __init__( + self, + reactor: IReactorCore, + clock: Clock, + store: LockStore, + lock_name: str, + lock_key: str, + token: str, + ) -> None: + self._reactor = reactor + self._clock = clock + self._store = store + self._lock_name = lock_name + self._lock_key = lock_key + + self._token = token + + self._looping_call = clock.looping_call( + self._renew, _RENEWAL_INTERVAL_MS, store, lock_name, lock_key, token + ) + + self._dropped = False + + @staticmethod + @wrap_as_background_process("Lock._renew") + async def _renew( + store: LockStore, + lock_name: str, + lock_key: str, + token: str, + ) -> None: + """Renew the lock. + + Note: this is a static method, rather than using self.*, so that we + don't end up with a reference to `self` in the reactor, which would stop + this from being cleaned up if we dropped the context manager. + """ + await store._renew_lock(lock_name, lock_key, token) + + async def is_still_valid(self) -> bool: + """Check if the lock is still held by us""" + return await self._store._is_lock_still_valid( + self._lock_name, self._lock_key, self._token + ) + + async def __aenter__(self) -> None: + if self._dropped: + raise Exception("Cannot reuse a Lock object") + + async def __aexit__( + self, + _exctype: Optional[Type[BaseException]], + _excinst: Optional[BaseException], + _exctb: Optional[TracebackType], + ) -> bool: + if self._looping_call.running: + self._looping_call.stop() + + await self._store._drop_lock(self._lock_name, self._lock_key, self._token) + self._dropped = True + + return False + + def __del__(self) -> None: + if not self._dropped: + # We should not be dropped without the lock being released (unless + # we're shutting down), but if we are then let's at least stop + # renewing the lock. + if self._looping_call.running: + self._looping_call.stop() + + if self._reactor.running: + logger.error( + "Lock for (%s, %s) dropped without being released", + self._lock_name, + self._lock_key, + ) |