synapse-product/synapse/storage/databases/main/lock.py
Erik Johnston 85d237eba7
Add a distributed lock (#10269)
This adds a simple best effort locking mechanism that works cross workers.
2021-06-29 19:15:47 +01:00

335 lines
11 KiB
Python

# 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,
)