上下文管理器和多处理池

Question

假设您正在使用multiprocessing.Pool 对象，并且您正在使用构造函数的initializer 设置来传递一个初始化函数，然后该函数会在全局命名空间中创建一个资源。假设资源有一个上下文管理器。如果上下文管理的资源必须在整个进程的生命周期中存在，但在最后被适当地清理，您将如何处理它的生命周期？

到目前为止，我有一些类似的东西：

resource_cm = None
resource = None


def _worker_init(args):
    global resource
    resource_cm = open_resource(args)
    resource = resource_cm.__enter__()

从这里开始，池进程可以使用该资源。到现在为止还挺好。但是处理清理有点棘手，因为multiprocessing.Pool 类不提供destructor 或deinitializer 参数。

我的一个想法是使用atexit 模块，并在初始化程序中注册清理。像这样的：

def _worker_init(args):
    global resource
    resource_cm = open_resource(args)
    resource = resource_cm.__enter__()

    def _clean_up():
        resource_cm.__exit__()

    import atexit
    atexit.register(_clean_up)

这是一个好方法吗？有没有更简单的方法？

编辑：atexit 似乎不起作用。至少不是我上面使用它的方式，所以到目前为止我还没有解决这个问题的方法。

Answer 1

首先，这是一个非常好的问题！在multiprocessing 代码中挖掘了一下之后，我想我找到了一种方法：

当您启动multiprocessing.Pool 时，Pool 对象在内部为池的每个成员创建一个multiprocessing.Process 对象。当这些子进程启动时，它们会调用_bootstrap 函数，如下所示：

def _bootstrap(self):
    from . import util
    global _current_process
    try:
        # ... (stuff we don't care about)
        util._finalizer_registry.clear()
        util._run_after_forkers()
        util.info('child process calling self.run()')
        try:
            self.run()
            exitcode = 0 
        finally:
            util._exit_function()
        # ... (more stuff we don't care about)

run 方法是实际运行 target 您给 Process 对象的方法。对于Pool 进程，这是一种具有长时间运行的while 循环的方法，该循环等待工作项通过内部队列进入。我们真正感兴趣的是在self.run: util._exit_function() 被调用之后发生了什么。

事实证明，该函数做了一些清理工作，听起来很像您正在寻找的内容：

def _exit_function(info=info, debug=debug, _run_finalizers=_run_finalizers,
                   active_children=active_children,
                   current_process=current_process):
    # NB: we hold on to references to functions in the arglist due to the
    # situation described below, where this function is called after this
    # module's globals are destroyed.

    global _exiting

    info('process shutting down')
    debug('running all "atexit" finalizers with priority >= 0')  # Very interesting!
    _run_finalizers(0)

这是_run_finalizers的文档字符串：

def _run_finalizers(minpriority=None):
    '''
    Run all finalizers whose exit priority is not None and at least minpriority

    Finalizers with highest priority are called first; finalizers with
    the same priority will be called in reverse order of creation.
    '''

该方法实际上会遍历终结器回调列表并执行它们：

items = [x for x in _finalizer_registry.items() if f(x)]
items.sort(reverse=True)

for key, finalizer in items:
    sub_debug('calling %s', finalizer)
    try:
        finalizer()
    except Exception:
        import traceback
        traceback.print_exc()

完美。那么我们如何进入_finalizer_registry呢？ multiprocessing.util 中有一个名为 Finalize 的未记录对象，它负责向注册表添加回调：

class Finalize(object):
    '''
    Class which supports object finalization using weakrefs
    '''
    def __init__(self, obj, callback, args=(), kwargs=None, exitpriority=None):
        assert exitpriority is None or type(exitpriority) is int

        if obj is not None:
            self._weakref = weakref.ref(obj, self)
        else:
            assert exitpriority is not None

        self._callback = callback
        self._args = args
        self._kwargs = kwargs or {}
        self._key = (exitpriority, _finalizer_counter.next())
        self._pid = os.getpid()

        _finalizer_registry[self._key] = self  # That's what we're looking for!

好的，所以把它们放在一个例子中：

import multiprocessing
from multiprocessing.util import Finalize

resource_cm = None
resource = None

class Resource(object):
    def __init__(self, args):
        self.args = args

    def __enter__(self):
        print("in __enter__ of %s" % multiprocessing.current_process())
        return self

    def __exit__(self, *args, **kwargs):
        print("in __exit__ of %s" % multiprocessing.current_process())

def open_resource(args):
    return Resource(args)

def _worker_init(args):
    global resource
    print("calling init")
    resource_cm = open_resource(args)
    resource = resource_cm.__enter__()
    # Register a finalizer
    Finalize(resource, resource.__exit__, exitpriority=16)

def hi(*args):
    print("we're in the worker")

if __name__ == "__main__":
    pool = multiprocessing.Pool(initializer=_worker_init, initargs=("abc",))
    pool.map(hi, range(pool._processes))
    pool.close()
    pool.join()

输出：

calling init
in __enter__ of <Process(PoolWorker-1, started daemon)>
calling init
calling init
in __enter__ of <Process(PoolWorker-2, started daemon)>
in __enter__ of <Process(PoolWorker-3, started daemon)>
calling init
in __enter__ of <Process(PoolWorker-4, started daemon)>
we're in the worker
we're in the worker
we're in the worker
we're in the worker
in __exit__ of <Process(PoolWorker-1, started daemon)>
in __exit__ of <Process(PoolWorker-2, started daemon)>
in __exit__ of <Process(PoolWorker-3, started daemon)>
in __exit__ of <Process(PoolWorker-4, started daemon)>

如您所见，当我们 join() 池时，__exit__ 会在我们所有的工作人员中被调用。

Answer 2

您可以继承Process 并覆盖其run() 方法，以便它在退出前执行清理。然后你应该继承 Pool 以便它使用你的子类进程：

from multiprocessing import Process
from multiprocessing.pool import Pool

class SafeProcess(Process):
    """ Process that will cleanup before exit """
    def run(self, *args, **kw):
        result = super().run(*args, **kw)
        # cleanup however you want here
        return result


class SafePool(Pool):
    Process = SafeProcess


pool = SafePool(4)  # use it as standard Pool

Answer 3

这是我想出的解决方案。它使用billiard，这是 Python 的多处理包的一个分支。此解决方案需要使用私有 API Worker._ensure_messages_consumed，因此我不建议在生产中使用此解决方案。我只是需要这个来做一个附带项目，所以这对我来说已经足够了。使用此功能需您自担风险。

from billiard import pool
from billiard.pool import Pool, Worker

class SafeWorker(Worker):
    # this function is called just before a worker process exits
    def _ensure_messages_consumed(self, *args, **kwargs):
        # Not necessary, but you can move `Pool.initializer` logic here if you want.
        out = super()._ensure_messages_consumed(*args, **kwargs)
        # Do clean up work here
        return out

class SafePool(Pool):
    Worker = SafeWorker

我尝试的另一个解决方案是将我的清理逻辑实现为信号处理程序，但这不起作用，因为multiprocessing 和billiard 都使用exit() 来杀死他们的工作进程。我不确定atexit 是如何工作的，但这可能是该方法也不起作用的原因。