Coverage fails with os.fork and os._exit

Issue #310 closed
Marc Schlaich created an issue

It is a common pattern to exit a child process with os._exit after a fork (ref). However, in this case coverage for the child process fails.


import os

def child():
    a = 1 + 1
    assert a == 2

def main():
    pid = os.fork()
    if pid:
        _, systemstatus = os.waitpid(pid, 0)
        assert not systemstatus

if __name__ == '__main__':

Test run:

$ coverage run -p
$ coverage combine && coverage report -m
Name    Stmts   Miss  Cover   Missing
cov        13      4    69%   6-7, 16-17

Comments (24)

  1. Ned Batchelder repo owner

    Your test run data is incorrect, it actually is:

    Name     Stmts   Miss  Cover   Missing
    bug310      13      4    69%   4-5, 14-15

    I'm not sure what I can do to fix this. os._exit skips any further work in the process, including the work needs to do to write out its measured data.

  2. Marc Schlaich reporter

    Your test run data is incorrect, it actually is:

    Bitbucket has stripped some new lines ;)

    I'm not sure what I can do to fix this. os._exit skips any further work in the process

    Yes, I feared this cannot be handled automatically. Anyway, it would be enough if I can do the clean up manually before os._exit but AFAIS there is no useful entry point in the API. Something like coverage.shutdown would be nice.

  3. Ryan Stuart

    Here is a dirty hack to work around this issue:

        def _coverage_hack(cls):
            """God awful hack to make coverage and py.test work together."""
            class Cov(object):
                stop = lambda self: None
                save = lambda self: None
            cov = Cov()
            if "coverage" in sys.modules:
                import coverage
                    raise ZeroDivisionError
                except ZeroDivisionError:
                    f = sys.exc_info()[2].tb_frame
                tb = []
                while f:
                    f = f.f_back
                t = tb[-3]
                if 'self' in t.f_locals:
                    slf = t.f_locals['self']
                    if hasattr(slf, "coverage"):
                        if isinstance(slf.coverage, coverage.coverage):
                            cov = slf.coverage
            return cov
        _coverage_hack = classmethod(_coverage_hack)

    It can be used as follows:

    self.__coverage = self._coverage_hack()
    child_pid = os.fork()
    if child_pid == 0:
        # Do work
        ((self.__coverage.stop(),) and (,) and os._exit(0))
  4. Ned Batchelder repo owner

    @rstuart85 If I read this code right, it's a way to get access to the coverage instance (if any), and then calling it explicitly from the product code. If the product code can be changed to accommodate this problem, then there's lots of possibilities, but people generally don't want to do that.

  5. Ned Batchelder repo owner

    @schlamar The method you want is currently called coverage._atexit. I could rename that to coverage.shutdown. Try using coverage._atexit() now and tell me if it works well. Or you could use coverage.stop();

  6. Ryan Stuart

    I don't actually want to use that code, its just the only way I can get coverage and py.test with os._exit to work properly.


  7. Ned Batchelder repo owner
    • changed status to open

    Hmm, we have a few ideas here, let's not close this just yet.

    And somehow, no one has suggested monkey-patching os._exit yet.

  8. Ryan Stuart

    @schlamar It isn't fixed for me.

    coverage run --source my_dir -p -m py.test my_dir returns 100%

    py.test --cov my_dir my_dir returns 95%

    Where is the appropriate place to take this discussion?

  9. Ryan Stuart

    @schlamar Just to be clear, the problem of it not capturing lines seems to be fixed but it certainly doesn't capture the lines in the subprocesses no matter if I exit with os._exit(), sys.exit() or my hack above.

  10. Marc Schlaich reporter

    @rstuart85 As already said, I have no idea how to implicitly support os._exit. pytest-cov supports py.process.ForkedFunc (which is a wrapper around fork/os._exit) by using explicit before and after process hooks.

    However, in theory it should work with sys.exit. If that's not working, please report it at with a test case. Thanks!

  11. Loic Dachary

    @spaceone when running the proposed reproducer I get the following ouptut:

    Name     Stmts   Miss  Cover
    ----------------------------       6      1    83%

    The code it contains is more complicated than the reproducer provided in the description of this issue. It would be great if you could explain why and how it demonstrates something different.

  12. space one
  13. Loic Dachary

    @spaceone the result I get (83% coverage) is with 4.3.1 and I confirm that I get exactly the same result as you with 4.2 (67% coverage), using python 2.7.12.

  14. Loic Dachary

    @spaceone would you be so kind as to try to reproduce the issue with coverage 4.3.1 ? If you're still experiencing problems with that version I'll try to figure out why.

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