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638d35ef08
Turns out that cancellation of inlineDeferreds didn't really work properly until Twisted 18.7. This commit refactors Linearizer.queue to avoid inlineCallbacks.
441 lines
14 KiB
Python
441 lines
14 KiB
Python
# -*- coding: utf-8 -*-
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# Copyright 2014-2016 OpenMarket Ltd
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# Copyright 2018 New Vector Ltd.
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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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import collections
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import logging
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from contextlib import contextmanager
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from six.moves import range
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from twisted.internet import defer
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from twisted.internet.defer import CancelledError
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from twisted.python import failure
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from synapse.util import Clock, logcontext, unwrapFirstError
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from .logcontext import (
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PreserveLoggingContext,
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make_deferred_yieldable,
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run_in_background,
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)
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logger = logging.getLogger(__name__)
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class ObservableDeferred(object):
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"""Wraps a deferred object so that we can add observer deferreds. These
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observer deferreds do not affect the callback chain of the original
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deferred.
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If consumeErrors is true errors will be captured from the origin deferred.
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Cancelling or otherwise resolving an observer will not affect the original
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ObservableDeferred.
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NB that it does not attempt to do anything with logcontexts; in general
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you should probably make_deferred_yieldable the deferreds
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returned by `observe`, and ensure that the original deferred runs its
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callbacks in the sentinel logcontext.
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"""
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__slots__ = ["_deferred", "_observers", "_result"]
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def __init__(self, deferred, consumeErrors=False):
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object.__setattr__(self, "_deferred", deferred)
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object.__setattr__(self, "_result", None)
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object.__setattr__(self, "_observers", set())
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def callback(r):
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object.__setattr__(self, "_result", (True, r))
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while self._observers:
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try:
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# TODO: Handle errors here.
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self._observers.pop().callback(r)
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except Exception:
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pass
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return r
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def errback(f):
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object.__setattr__(self, "_result", (False, f))
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while self._observers:
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try:
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# TODO: Handle errors here.
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self._observers.pop().errback(f)
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except Exception:
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pass
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if consumeErrors:
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return None
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else:
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return f
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deferred.addCallbacks(callback, errback)
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def observe(self):
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"""Observe the underlying deferred.
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Can return either a deferred if the underlying deferred is still pending
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(or has failed), or the actual value. Callers may need to use maybeDeferred.
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"""
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if not self._result:
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d = defer.Deferred()
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def remove(r):
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self._observers.discard(d)
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return r
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d.addBoth(remove)
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self._observers.add(d)
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return d
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else:
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success, res = self._result
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return res if success else defer.fail(res)
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def observers(self):
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return self._observers
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def has_called(self):
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return self._result is not None
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def has_succeeded(self):
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return self._result is not None and self._result[0] is True
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def get_result(self):
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return self._result[1]
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def __getattr__(self, name):
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return getattr(self._deferred, name)
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def __setattr__(self, name, value):
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setattr(self._deferred, name, value)
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def __repr__(self):
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return "<ObservableDeferred object at %s, result=%r, _deferred=%r>" % (
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id(self), self._result, self._deferred,
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)
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def concurrently_execute(func, args, limit):
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"""Executes the function with each argument conncurrently while limiting
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the number of concurrent executions.
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Args:
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func (func): Function to execute, should return a deferred.
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args (list): List of arguments to pass to func, each invocation of func
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gets a signle argument.
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limit (int): Maximum number of conccurent executions.
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Returns:
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deferred: Resolved when all function invocations have finished.
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"""
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it = iter(args)
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@defer.inlineCallbacks
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def _concurrently_execute_inner():
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try:
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while True:
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yield func(next(it))
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except StopIteration:
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pass
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return logcontext.make_deferred_yieldable(defer.gatherResults([
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run_in_background(_concurrently_execute_inner)
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for _ in range(limit)
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], consumeErrors=True)).addErrback(unwrapFirstError)
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class Linearizer(object):
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"""Limits concurrent access to resources based on a key. Useful to ensure
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only a few things happen at a time on a given resource.
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Example:
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with (yield limiter.queue("test_key")):
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# do some work.
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"""
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def __init__(self, name=None, max_count=1, clock=None):
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"""
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Args:
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max_count(int): The maximum number of concurrent accesses
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"""
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if name is None:
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self.name = id(self)
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else:
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self.name = name
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if not clock:
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from twisted.internet import reactor
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clock = Clock(reactor)
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self._clock = clock
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self.max_count = max_count
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# key_to_defer is a map from the key to a 2 element list where
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# the first element is the number of things executing, and
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# the second element is an OrderedDict, where the keys are deferreds for the
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# things blocked from executing.
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self.key_to_defer = {}
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def queue(self, key):
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# we avoid doing defer.inlineCallbacks here, so that cancellation works correctly.
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# (https://twistedmatrix.com/trac/ticket/4632 meant that cancellations were not
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# propagated inside inlineCallbacks until Twisted 18.7)
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entry = self.key_to_defer.setdefault(key, [0, collections.OrderedDict()])
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# If the number of things executing is greater than the maximum
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# then add a deferred to the list of blocked items
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# When one of the things currently executing finishes it will callback
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# this item so that it can continue executing.
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if entry[0] >= self.max_count:
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res = self._await_lock(key)
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else:
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logger.info(
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"Acquired uncontended linearizer lock %r for key %r", self.name, key,
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)
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entry[0] += 1
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res = defer.succeed(None)
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# once we successfully get the lock, we need to return a context manager which
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# will release the lock.
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@contextmanager
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def _ctx_manager(_):
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try:
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yield
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finally:
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logger.info("Releasing linearizer lock %r for key %r", self.name, key)
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# We've finished executing so check if there are any things
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# blocked waiting to execute and start one of them
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entry[0] -= 1
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if entry[1]:
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(next_def, _) = entry[1].popitem(last=False)
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# we need to run the next thing in the sentinel context.
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with PreserveLoggingContext():
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next_def.callback(None)
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elif entry[0] == 0:
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# We were the last thing for this key: remove it from the
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# map.
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del self.key_to_defer[key]
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res.addCallback(_ctx_manager)
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return res
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def _await_lock(self, key):
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"""Helper for queue: adds a deferred to the queue
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Assumes that we've already checked that we've reached the limit of the number
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of lock-holders we allow. Creates a new deferred which is added to the list, and
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adds some management around cancellations.
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Returns the deferred, which will callback once we have secured the lock.
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"""
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entry = self.key_to_defer[key]
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logger.info(
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"Waiting to acquire linearizer lock %r for key %r", self.name, key,
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)
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new_defer = make_deferred_yieldable(defer.Deferred())
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entry[1][new_defer] = 1
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def cb(_r):
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logger.info("Acquired linearizer lock %r for key %r", self.name, key)
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entry[0] += 1
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# if the code holding the lock completes synchronously, then it
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# will recursively run the next claimant on the list. That can
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# relatively rapidly lead to stack exhaustion. This is essentially
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# the same problem as http://twistedmatrix.com/trac/ticket/9304.
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#
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# In order to break the cycle, we add a cheeky sleep(0) here to
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# ensure that we fall back to the reactor between each iteration.
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#
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# (This needs to happen while we hold the lock, and the context manager's exit
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# code must be synchronous, so this is the only sensible place.)
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return self._clock.sleep(0)
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def eb(e):
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logger.info("defer %r got err %r", new_defer, e)
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if isinstance(e, CancelledError):
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logger.info(
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"Cancelling wait for linearizer lock %r for key %r",
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self.name, key,
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)
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else:
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logger.warn(
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"Unexpected exception waiting for linearizer lock %r for key %r",
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self.name, key,
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)
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# we just have to take ourselves back out of the queue.
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del entry[1][new_defer]
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return e
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new_defer.addCallbacks(cb, eb)
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return new_defer
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class ReadWriteLock(object):
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"""A deferred style read write lock.
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Example:
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with (yield read_write_lock.read("test_key")):
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# do some work
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"""
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# IMPLEMENTATION NOTES
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#
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# We track the most recent queued reader and writer deferreds (which get
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# resolved when they release the lock).
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#
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# Read: We know its safe to acquire a read lock when the latest writer has
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# been resolved. The new reader is appeneded to the list of latest readers.
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#
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# Write: We know its safe to acquire the write lock when both the latest
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# writers and readers have been resolved. The new writer replaces the latest
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# writer.
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def __init__(self):
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# Latest readers queued
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self.key_to_current_readers = {}
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# Latest writer queued
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self.key_to_current_writer = {}
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@defer.inlineCallbacks
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def read(self, key):
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new_defer = defer.Deferred()
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curr_readers = self.key_to_current_readers.setdefault(key, set())
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curr_writer = self.key_to_current_writer.get(key, None)
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curr_readers.add(new_defer)
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# We wait for the latest writer to finish writing. We can safely ignore
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# any existing readers... as they're readers.
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yield make_deferred_yieldable(curr_writer)
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@contextmanager
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def _ctx_manager():
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try:
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yield
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finally:
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new_defer.callback(None)
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self.key_to_current_readers.get(key, set()).discard(new_defer)
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defer.returnValue(_ctx_manager())
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@defer.inlineCallbacks
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def write(self, key):
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new_defer = defer.Deferred()
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curr_readers = self.key_to_current_readers.get(key, set())
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curr_writer = self.key_to_current_writer.get(key, None)
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# We wait on all latest readers and writer.
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to_wait_on = list(curr_readers)
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if curr_writer:
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to_wait_on.append(curr_writer)
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# We can clear the list of current readers since the new writer waits
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# for them to finish.
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curr_readers.clear()
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self.key_to_current_writer[key] = new_defer
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yield make_deferred_yieldable(defer.gatherResults(to_wait_on))
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@contextmanager
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def _ctx_manager():
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try:
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yield
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finally:
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new_defer.callback(None)
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if self.key_to_current_writer[key] == new_defer:
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self.key_to_current_writer.pop(key)
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defer.returnValue(_ctx_manager())
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class DeferredTimeoutError(Exception):
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"""
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This error is raised by default when a L{Deferred} times out.
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"""
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def add_timeout_to_deferred(deferred, timeout, reactor, on_timeout_cancel=None):
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"""
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Add a timeout to a deferred by scheduling it to be cancelled after
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timeout seconds.
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This is essentially a backport of deferred.addTimeout, which was introduced
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in twisted 16.5.
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If the deferred gets timed out, it errbacks with a DeferredTimeoutError,
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unless a cancelable function was passed to its initialization or unless
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a different on_timeout_cancel callable is provided.
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Args:
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deferred (defer.Deferred): deferred to be timed out
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timeout (Number): seconds to time out after
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reactor (twisted.internet.reactor): the Twisted reactor to use
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on_timeout_cancel (callable): A callable which is called immediately
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after the deferred times out, and not if this deferred is
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otherwise cancelled before the timeout.
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It takes an arbitrary value, which is the value of the deferred at
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that exact point in time (probably a CancelledError Failure), and
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the timeout.
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The default callable (if none is provided) will translate a
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CancelledError Failure into a DeferredTimeoutError.
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"""
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timed_out = [False]
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def time_it_out():
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timed_out[0] = True
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deferred.cancel()
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delayed_call = reactor.callLater(timeout, time_it_out)
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def convert_cancelled(value):
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if timed_out[0]:
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to_call = on_timeout_cancel or _cancelled_to_timed_out_error
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return to_call(value, timeout)
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return value
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deferred.addBoth(convert_cancelled)
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def cancel_timeout(result):
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# stop the pending call to cancel the deferred if it's been fired
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if delayed_call.active():
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delayed_call.cancel()
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return result
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deferred.addBoth(cancel_timeout)
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def _cancelled_to_timed_out_error(value, timeout):
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if isinstance(value, failure.Failure):
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value.trap(CancelledError)
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raise DeferredTimeoutError(timeout, "Deferred")
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return value
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