MatrixSynapse/synapse/handlers/worker_lock.py

# 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.

import random
from types import TracebackType
from typing import (
    TYPE_CHECKING,
    AsyncContextManager,
    Collection,
    Dict,
    Optional,
    Tuple,
    Type,
    Union,
)
from weakref import WeakSet

import attr

from twisted.internet import defer
from twisted.internet.interfaces import IReactorTime

from synapse.logging.context import PreserveLoggingContext
from synapse.logging.opentracing import start_active_span
from synapse.metrics.background_process_metrics import wrap_as_background_process
from synapse.storage.databases.main.lock import Lock, LockStore
from synapse.util.async_helpers import timeout_deferred

if TYPE_CHECKING:
    from synapse.logging.opentracing import opentracing
    from synapse.server import HomeServer


# This lock is used to avoid creating an event while we are purging the room.
# We take a read lock when creating an event, and a write one when purging a room.
# This is because it is fine to create several events concurrently, since referenced events
# will not disappear under our feet as long as we don't delete the room.
NEW_EVENT_DURING_PURGE_LOCK_NAME = "new_event_during_purge_lock"


class WorkerLocksHandler:
    """A class for waiting on taking out locks, rather than using the storage
    functions directly (which don't support awaiting).
    """

    def __init__(self, hs: "HomeServer") -> None:
        self._reactor = hs.get_reactor()
        self._store = hs.get_datastores().main
        self._clock = hs.get_clock()
        self._notifier = hs.get_notifier()
        self._instance_name = hs.get_instance_name()

        # Map from lock name/key to set of `WaitingLock` that are active for
        # that lock.
        self._locks: Dict[
            Tuple[str, str], WeakSet[Union[WaitingLock, WaitingMultiLock]]
        ] = {}

        self._clock.looping_call(self._cleanup_locks, 30_000)

        self._notifier.add_lock_released_callback(self._on_lock_released)

    def acquire_lock(self, lock_name: str, lock_key: str) -> "WaitingLock":
        """Acquire a standard lock, returns a context manager that will block
        until the lock is acquired.

        Note: Care must be taken to avoid deadlocks. In particular, this
        function does *not* timeout.

        Usage:
            async with handler.acquire_lock(name, key):
                # Do work while holding the lock...
        """

        lock = WaitingLock(
            reactor=self._reactor,
            store=self._store,
            handler=self,
            lock_name=lock_name,
            lock_key=lock_key,
            write=None,
        )

        self._locks.setdefault((lock_name, lock_key), WeakSet()).add(lock)

        return lock

    def acquire_read_write_lock(
        self,
        lock_name: str,
        lock_key: str,
        *,
        write: bool,
    ) -> "WaitingLock":
        """Acquire a read/write lock, returns a context manager that will block
        until the lock is acquired.

        Note: Care must be taken to avoid deadlocks. In particular, this
        function does *not* timeout.

        Usage:
            async with handler.acquire_read_write_lock(name, key, write=True):
                # Do work while holding the lock...
        """

        lock = WaitingLock(
            reactor=self._reactor,
            store=self._store,
            handler=self,
            lock_name=lock_name,
            lock_key=lock_key,
            write=write,
        )

        self._locks.setdefault((lock_name, lock_key), WeakSet()).add(lock)

        return lock

    def acquire_multi_read_write_lock(
        self,
        lock_names: Collection[Tuple[str, str]],
        *,
        write: bool,
    ) -> "WaitingMultiLock":
        """Acquires multi read/write locks at once, returns a context manager
        that will block until all the locks are acquired.

        This will try and acquire all locks at once, and will never hold on to a
        subset of the locks. (This avoids accidentally creating deadlocks).

        Note: Care must be taken to avoid deadlocks. In particular, this
        function does *not* timeout.
        """

        lock = WaitingMultiLock(
            lock_names=lock_names,
            write=write,
            reactor=self._reactor,
            store=self._store,
            handler=self,
        )

        for lock_name, lock_key in lock_names:
            self._locks.setdefault((lock_name, lock_key), WeakSet()).add(lock)

        return lock

    def notify_lock_released(self, lock_name: str, lock_key: str) -> None:
        """Notify that a lock has been released.

        Pokes both the notifier and replication.
        """

        self._notifier.notify_lock_released(self._instance_name, lock_name, lock_key)

    def _on_lock_released(
        self, instance_name: str, lock_name: str, lock_key: str
    ) -> None:
        """Called when a lock has been released.

        Wakes up any locks that might be waiting on this.
        """
        locks = self._locks.get((lock_name, lock_key))
        if not locks:
            return

        def _wake_deferred(deferred: defer.Deferred) -> None:
            if not deferred.called:
                deferred.callback(None)

        for lock in locks:
            self._clock.call_later(0, _wake_deferred, lock.deferred)

    @wrap_as_background_process("_cleanup_locks")
    async def _cleanup_locks(self) -> None:
        """Periodically cleans out stale entries in the locks map"""
        self._locks = {key: value for key, value in self._locks.items() if value}


@attr.s(auto_attribs=True, eq=False)
class WaitingLock:
    reactor: IReactorTime
    store: LockStore
    handler: WorkerLocksHandler
    lock_name: str
    lock_key: str
    write: Optional[bool]
    deferred: "defer.Deferred[None]" = attr.Factory(defer.Deferred)
    _inner_lock: Optional[Lock] = None
    _retry_interval: float = 0.1
    _lock_span: "opentracing.Scope" = attr.Factory(
        lambda: start_active_span("WaitingLock.lock")
    )

    async def __aenter__(self) -> None:
        self._lock_span.__enter__()

        with start_active_span("WaitingLock.waiting_for_lock"):
            while self._inner_lock is None:
                self.deferred = defer.Deferred()

                if self.write is not None:
                    lock = await self.store.try_acquire_read_write_lock(
                        self.lock_name, self.lock_key, write=self.write
                    )
                else:
                    lock = await self.store.try_acquire_lock(
                        self.lock_name, self.lock_key
                    )

                if lock:
                    self._inner_lock = lock
                    break

                try:
                    # Wait until the we get notified the lock might have been
                    # released (by the deferred being resolved). We also
                    # periodically wake up in case the lock was released but we
                    # weren't notified.
                    with PreserveLoggingContext():
                        await timeout_deferred(
                            deferred=self.deferred,
                            timeout=self._get_next_retry_interval(),
                            reactor=self.reactor,
                        )
                except Exception:
                    pass

        return await self._inner_lock.__aenter__()

    async def __aexit__(
        self,
        exc_type: Optional[Type[BaseException]],
        exc: Optional[BaseException],
        tb: Optional[TracebackType],
    ) -> Optional[bool]:
        assert self._inner_lock

        self.handler.notify_lock_released(self.lock_name, self.lock_key)

        try:
            r = await self._inner_lock.__aexit__(exc_type, exc, tb)
        finally:
            self._lock_span.__exit__(exc_type, exc, tb)

        return r

    def _get_next_retry_interval(self) -> float:
        next = self._retry_interval
        self._retry_interval = max(5, next * 2)
        return next * random.uniform(0.9, 1.1)


@attr.s(auto_attribs=True, eq=False)
class WaitingMultiLock:
    lock_names: Collection[Tuple[str, str]]

    write: bool

    reactor: IReactorTime
    store: LockStore
    handler: WorkerLocksHandler

    deferred: "defer.Deferred[None]" = attr.Factory(defer.Deferred)

    _inner_lock_cm: Optional[AsyncContextManager] = None
    _retry_interval: float = 0.1
    _lock_span: "opentracing.Scope" = attr.Factory(
        lambda: start_active_span("WaitingLock.lock")
    )

    async def __aenter__(self) -> None:
        self._lock_span.__enter__()

        with start_active_span("WaitingLock.waiting_for_lock"):
            while self._inner_lock_cm is None:
                self.deferred = defer.Deferred()

                lock_cm = await self.store.try_acquire_multi_read_write_lock(
                    self.lock_names, write=self.write
                )

                if lock_cm:
                    self._inner_lock_cm = lock_cm
                    break

                try:
                    # Wait until the we get notified the lock might have been
                    # released (by the deferred being resolved). We also
                    # periodically wake up in case the lock was released but we
                    # weren't notified.
                    with PreserveLoggingContext():
                        await timeout_deferred(
                            deferred=self.deferred,
                            timeout=self._get_next_retry_interval(),
                            reactor=self.reactor,
                        )
                except Exception:
                    pass

        assert self._inner_lock_cm
        await self._inner_lock_cm.__aenter__()
        return

    async def __aexit__(
        self,
        exc_type: Optional[Type[BaseException]],
        exc: Optional[BaseException],
        tb: Optional[TracebackType],
    ) -> Optional[bool]:
        assert self._inner_lock_cm

        for lock_name, lock_key in self.lock_names:
            self.handler.notify_lock_released(lock_name, lock_key)

        try:
            r = await self._inner_lock_cm.__aexit__(exc_type, exc, tb)
        finally:
            self._lock_span.__exit__(exc_type, exc, tb)

        return r

    def _get_next_retry_interval(self) -> float:
        next = self._retry_interval
        self._retry_interval = max(5, next * 2)
        return next * random.uniform(0.9, 1.1)