476 lines
16 KiB
Python
476 lines
16 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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def yieldable_gather_results(func, iter, *args, **kwargs):
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"""Executes the function with each argument concurrently.
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Args:
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func (func): Function to execute that returns a Deferred
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iter (iter): An iterable that yields items that get passed as the first
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argument to the function
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*args: Arguments to be passed to each call to func
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Returns
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Deferred[list]: Resolved when all functions have been invoked, or errors if
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one of the function calls fails.
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"""
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return logcontext.make_deferred_yieldable(defer.gatherResults([
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run_in_background(func, item, *args, **kwargs)
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for item in iter
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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.debug(
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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.debug("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.debug(
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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.debug("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.debug(
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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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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 defer.TimeoutError(timeout, "Deferred")
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return value
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def timeout_deferred(deferred, timeout, reactor, on_timeout_cancel=None):
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"""The in built twisted `Deferred.addTimeout` fails to time out deferreds
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that have a canceller that throws exceptions. This method creates a new
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deferred that wraps and times out the given deferred, correctly handling
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the case where the given deferred's canceller throws.
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(See https://twistedmatrix.com/trac/ticket/9534)
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NOTE: Unlike `Deferred.addTimeout`, this function returns a new deferred
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Args:
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deferred (Deferred)
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timeout (float): Timeout in seconds
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reactor (twisted.interfaces.IReactorTime): 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 defer.TimeoutError.
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Returns:
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Deferred
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"""
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new_d = defer.Deferred()
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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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try:
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deferred.cancel()
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except: # noqa: E722, if we throw any exception it'll break time outs
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logger.exception("Canceller failed during timeout")
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if not new_d.called:
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new_d.errback(defer.TimeoutError(timeout, "Deferred"))
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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 success_cb(val):
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if not new_d.called:
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new_d.callback(val)
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def failure_cb(val):
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if not new_d.called:
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new_d.errback(val)
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deferred.addCallbacks(success_cb, failure_cb)
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return new_d
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