如何从Twisted的deferToThread API中延迟添加延迟？

Posted 2021-03-27

from twisted.internet import reactor
from twisted.internet import threads
from twisted.internet import defer
import time

def worker(arg):
    print 'Hello world'
     time.sleep(10)
    return 1

def run():
    print 'Starting workers'
    l = []
    for x in range(2):
        l.append(threads.deferToThread(worker, x))
    return defer.DeferredList(l)

def res(results):
    print results
    reactor.stop()

d = run()
d.addCallback(res)
reactor.run()

如何通过超时停止工人？

答案

除非与您合作，否则不能中断线程。 time.sleep(10)不会合作，所以我认为你不能打断这个工人。如果您有另一种具有多个离散阶段的工作者，或者在某些任务的循环中操作，那么您可以执行以下操作：

def worker(stop, jobs):
    for j in jobs:
        if stop:
            break
        j.do()

stop = []
d = deferToThread(worker)

# This will make the list eval to true and break out of the loop.
stop.append(None)

这也不是Twisted特定的。这就是线程在Python中的工作方式。

另一答案

虽然可能无法中断线程，但可以通过cancel函数停止延迟，我认为这在Twisted 10.1.0及更高版本中可用。

我已经使用以下类来使Deferreds回调特定函数，如果Deferred在一段时间后没有触发。对于与OP主题具有相同问题的人来说，这可能是有用的。

编辑：正如下面的评论所建议的，最好不要继承defer.Deferred。因此，我已经更改了代码以使用实现相同效果的包装器。

class DeferredWrapperWithTimeout(object):
    '''
    Holds a deferred that allows a specified function to be called-back
    if the deferred does not fire before some specified timeout.
    '''
    def __init__(self, canceller=None):
        self._def = defer.Deferred(canceller)

    def _finish(self, r, t):
        '''
        Function to be called (internally) after the Deferred
        has fired, in order to cancel the timeout.
        '''
        if ( (t!=None) and (t.active()) ):
            t.cancel()
        return r

    def getDeferred(self):
        return self._def

    def addTimeoutCallback(self, reactr, timeout,
                           callUponTimeout, *args, **kw):
        '''
        The function 'callUponTimeout' (with optional args or keywords)
        will be called after 'timeout' seconds, unless the Deferred fires.
        '''

        def timeoutCallback():
            self._def.cancel()
            callUponTimeout(*args, **kw)
        toc = reactr.callLater(timeout, timeoutCallback)
        return self._def.addCallback(self._finish, toc)

超时前的示例回调：

from twisted.internet import reactor

from DeferredWithTimeout import *

dw = DeferredWrapperWithTimeout()
d  = dw.getDeferred()

def testCallback(x=None):
    print "called"

def testTimeout(x=None):
    print "timedout"

d.addCallback(testCallback)
dw.addTimeoutCallback(reactor, 20, testTimeout, "to")
reactor.callLater(2, d.callback, "cb")
reactor.run()

打印“被叫”，没有别的。

回调前的示例超时：

from twisted.internet import reactor

from DeferredWithTimeout import *

dw = DeferredWrapperWithTimeout()
d  = dw.getDeferred()

def testCallback(x=None):
    print "called"

def testTimeout(x=None):
    print "timedout"

d.addCallback(testCallback)
dw.addTimeoutCallback(reactor, 20, testTimeout, "to")
reactor.run()

20秒后打印“timedout”，没有别的。

另一答案

好吧，我的答案不是关于线程，但正如所说，你可以实现超时功能作为一个单独的帮助：

from twisted.internet import defer

def add_watchdog(deferred, timeout=0.05):

    def callback(value):
        if not watchdog.called:
            watchdog.cancel()
        return value

    deferred.addBoth(callback)

    from twisted.internet import reactor
    watchdog = reactor.callLater(timeout, defer.timeout, deferred)

d = defer.Deferred()
add_watchdog(d)

然后，如果需要，你可以在延迟的错误中捕获defer.TimeoutError。

另一答案

我们这样做是使用装饰器。此方法的优点是在达到超时时取消延迟。这应该以某种方式成为Twisted库imho的一部分

from twisted.internet import defer, reactor

def timeout(secs):
    """Decorator to add timeout to Deferred calls"""
    def wrap(func):
        @defer.inlineCallbacks
        def _timeout(*args, **kwargs):
            raw_d = func(*args, **kwargs)
            if not isinstance(raw_d, defer.Deferred):
                defer.returnValue(raw_d)

            timeout_d = defer.Deferred()
            times_up = reactor.callLater(secs, timeout_d.callback, None)

            try:
                raw_result, timeout_result = yield defer.DeferredList(
                    [raw_d, timeout_d], fireOnOneCallback=True, fireOnOneErrback=True,
                    consumeErrors=True)
            except defer.FirstError as e:  # Only raw_d should raise an exception
                assert e.index == 0
                times_up.cancel()
                e.subFailure.raiseException()
            else:  # timeout
                if timeout_d.called:
                    raw_d.cancel()
                    raise Exception("%s secs have expired" % secs)

            # no timeout
            times_up.cancel()
            defer.returnValue(raw_result)
        return _timeout
return wrap