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Fix linearizer cancellation on twisted < 18.7
Turns out that cancellation of inlineDeferreds didn't really work properly until Twisted 18.7. This commit refactors Linearizer.queue to avoid inlineCallbacks.
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commit
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1 changed files with 68 additions and 43 deletions
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@ -188,62 +188,30 @@ class Linearizer(object):
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# things blocked from executing.
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self.key_to_defer = {}
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@defer.inlineCallbacks
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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 on of the things currently executing finishes it will callback
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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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new_defer = defer.Deferred()
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entry[1][new_defer] = 1
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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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try:
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yield make_deferred_yieldable(new_defer)
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except Exception as 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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raise
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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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yield self._clock.sleep(0)
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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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def _ctx_manager(_):
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try:
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yield
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finally:
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@ -264,7 +232,64 @@ class Linearizer(object):
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# map.
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del self.key_to_defer[key]
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defer.returnValue(_ctx_manager())
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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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