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coverage with Python3.8b2 breaks multiprocessing #828

nitzmahone opened this issue Jul 25, 2019 · 3 comments


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commented Jul 25, 2019

Describe the bug
When using coverage with concurrency=multiprocessing under Python3.8b2, forked processes don't fully init. The child worker process starts, and the process init code runs, but the actual Process run target never executes. The minimal repro sample below works fine under other released versions of Python.

To Reproduce
How can we reproduce the problem? Please be specific.

  1. What version of Python are you running? 3.8b2
  2. What versions of what packages do you have installed? coverage==4.5.3
  3. What code are you running?

import multiprocessing

class Worker(multiprocessing.Process):
    def __init__(self, result_queue, input):
        print("in child init")
        super(Worker, self).__init__()
        self._rq = result_queue
        self._input = input
        print("child init done")

    def run(self):
        print("in child run")
        self._rq.put("worker ran with {0}".format(self._input))

rq = multiprocessing.Queue()

w = Worker(rq, "hello")

print("worker pid is {0}, waiting for results...".format(

results = rq.get()
  1. What commands did you run? coverage3 run --concurrency=multiprocessing

Expected behavior
child workloads complete, "done" is printed (along with some debug info)

Additional context
We started hitting this recently in Ansible's nightly coverage runs (which include Python 3.8 prereleases as a canary)...


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commented Jul 25, 2019

an even tinier repro (might make a nice test case ;) ):

import multiprocessing

def do_stuff(result_queue):
    result_queue.put("yay, worker ran")

rq = multiprocessing.Queue()
multiprocessing.Process(target=do_stuff, args=[rq]).start()

print("waiting for results...")

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commented Jul 29, 2019

Thanks, this is fixed in version 4.5.4, and 5.0a5.


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commented Jul 29, 2019

@nedbat Thank you for the quick fix. This has been causing us trouble for a while.

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