LockStore: fix acquiring a lock via `LockStore.try_acquire_lock` (#12832)

Signed-off-by: Sumner Evans <sumner@beeper.com>
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Sumner Evans 2022-05-30 02:41:13 -06:00 committed by GitHub
parent 28989cb301
commit bda4600399
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3 changed files with 73 additions and 1 deletions

1
changelog.d/12832.bugfix Normal file
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@ -0,0 +1 @@
Fixed a bug which allowed multiple async operations to access database locks concurrently. Contributed by @sumnerevans @ Beeper.

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@ -13,7 +13,7 @@
# limitations under the License.
import logging
from types import TracebackType
from typing import TYPE_CHECKING, Optional, Tuple, Type
from typing import TYPE_CHECKING, Optional, Set, Tuple, Type
from weakref import WeakValueDictionary
from twisted.internet.interfaces import IReactorCore
@ -84,6 +84,8 @@ class LockStore(SQLBaseStore):
self._on_shutdown,
)
self._acquiring_locks: Set[Tuple[str, str]] = set()
@wrap_as_background_process("LockStore._on_shutdown")
async def _on_shutdown(self) -> None:
"""Called when the server is shutting down"""
@ -103,6 +105,21 @@ class LockStore(SQLBaseStore):
context manager if the lock is successfully acquired, which *must* be
used (otherwise the lock will leak).
"""
if (lock_name, lock_key) in self._acquiring_locks:
return None
try:
self._acquiring_locks.add((lock_name, lock_key))
return await self._try_acquire_lock(lock_name, lock_key)
finally:
self._acquiring_locks.discard((lock_name, lock_key))
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).
"""
# Check if this process has taken out a lock and if it's still valid.
lock = self._live_tokens.get((lock_name, lock_key))

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@ -12,6 +12,10 @@
# See the License for the specific language governing permissions and
# limitations under the License.
from twisted.internet import defer, reactor
from twisted.internet.base import ReactorBase
from twisted.internet.defer import Deferred
from synapse.server import HomeServer
from synapse.storage.databases.main.lock import _LOCK_TIMEOUT_MS
@ -22,6 +26,56 @@ class LockTestCase(unittest.HomeserverTestCase):
def prepare(self, reactor, clock, hs: HomeServer):
self.store = hs.get_datastores().main
def test_acquire_contention(self):
# Track the number of tasks holding the lock.
# Should be at most 1.
in_lock = 0
max_in_lock = 0
release_lock: "Deferred[None]" = Deferred()
async def task():
nonlocal in_lock
nonlocal max_in_lock
lock = await self.store.try_acquire_lock("name", "key")
if not lock:
return
async with lock:
in_lock += 1
max_in_lock = max(max_in_lock, in_lock)
# Block to allow other tasks to attempt to take the lock.
await release_lock
in_lock -= 1
# Start 3 tasks.
task1 = defer.ensureDeferred(task())
task2 = defer.ensureDeferred(task())
task3 = defer.ensureDeferred(task())
# Give the reactor a kick so that the database transaction returns.
self.pump()
release_lock.callback(None)
# Run the tasks to completion.
# To work around `Linearizer`s using a different reactor to sleep when
# contended (#12841), we call `runUntilCurrent` on
# `twisted.internet.reactor`, which is a different reactor to that used
# by the homeserver.
assert isinstance(reactor, ReactorBase)
self.get_success(task1)
reactor.runUntilCurrent()
self.get_success(task2)
reactor.runUntilCurrent()
self.get_success(task3)
# At most one task should have held the lock at a time.
self.assertEqual(max_in_lock, 1)
def test_simple_lock(self):
"""Test that we can take out a lock and that while we hold it nobody
else can take it out.