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test_no_refcycle_through_target sometimes fails in test_threading #46748

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pitrou opened this issue Mar 27, 2008 · 8 comments
Closed

test_no_refcycle_through_target sometimes fails in test_threading #46748

pitrou opened this issue Mar 27, 2008 · 8 comments
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stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@pitrou
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pitrou commented Mar 27, 2008

BPO 2496
Nosy @pitrou
Files
  • test_threading.patch
  • test_threading2.patch
    		•

    Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.

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    GitHub fields:

    assignee = None
closed_at = <Date 2008-03-28.04:12:52.791>
created_at = <Date 2008-03-27.12:04:29.607>
labels = ['type-bug', 'library']
title = 'test_no_refcycle_through_target sometimes fails in test_threading'
updated_at = <Date 2008-03-28.04:12:52.789>
user = 'https://github.com/pitrou'

    bugs.python.org fields:

    activity = <Date 2008-03-28.04:12:52.789>
actor = 'jyasskin'
assignee = 'jyasskin'
closed = True
closed_date = <Date 2008-03-28.04:12:52.791>
closer = 'jyasskin'
components = ['Library (Lib)']
creation = <Date 2008-03-27.12:04:29.607>
creator = 'pitrou'
dependencies = []
files = ['9874', '9878']
hgrepos = []
issue_num = 2496
keywords = ['patch']
message_count = 8.0
messages = ['64584', '64586', '64596', '64600', '64602', '64608', '64612', '64614']
nosy_count = 3.0
nosy_names = ['nnorwitz', 'pitrou', 'jyasskin']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue2496'
versions = ['Python 2.6']
    @pitrou
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    pitrou commented Mar 27, 2008

    This is a reminder for the failing test which is affecting some buildbots.
    I can't reproduce it right now (under Linux), even by surrounding the
    test code with a pair of gc.disable() / gc.enable().

    @pitrou pitrou added stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Mar 27, 2008
    @pitrou
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    pitrou commented Mar 27, 2008

    This is a tentative patch. I can't verify it fixes anything but at least
    it shouldn't do any harm ;)
    If it doesn't fix it I see two possible explanations:

    • the buildbots are running some kind of debug build which keeps
      references to local variables, preventing them to be deallocated
    • the C thread implementation needs fixing on some platforms

    @pitrou
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    pitrou commented Mar 27, 2008

    Hmm, even with a Py_DEBUG build I can't reproduce the bug.

    @pitrou
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    pitrou commented Mar 27, 2008

    Hmm, I think I know what happens. t_bootstrap() in threadmodule.c calls
    the self.__bootstrap() method in the Thread object, and it is this
    method which sets the __stopped flag at its end, which in turns wakes up
    the join() method.

    The problem is that at this point, t_bootstrap() still (rightly) holds a
    reference to the Thread object, since it has a reference to its
    __bootstrap() method which is still running. Depending on how the
    operating system switches threads, this reference may or may not be
    released when the join() method returns.

    So I think it's the test that is flaky. Instead of calling the join()
    method, it should wait for the OS-level thread to finish. Or it should
    find another way of testing for the reference cycle.

    @pitrou
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    pitrou commented Mar 27, 2008

    I'm attaching a patch which tries to make the test a bit less flaky
    (well, it still is, since I introduce a time.sleep() :-)).

    @jyasskin
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    jyasskin mannequin commented Mar 27, 2008

    I'll look at this tonight.

    @jyasskin jyasskin mannequin self-assigned this Mar 27, 2008
    @jyasskin
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    jyasskin mannequin commented Mar 28, 2008

    I think I've confirmed your diagnosis. If I add a _sleep(.01) to
    Thread.__bootstrap_inner() just after the call to self.__stop(), the
    test fails reliably. Very good catch! Given that, I think just adding a
    short sleep to the test before counting references will fix it nearly
    every time, but I'd like to kill the race dead if we can.

    @jyasskin
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    jyasskin mannequin commented Mar 28, 2008

    Fixed in r61984. I believe the exception info was actually keeping the
    object alive. The thread itself didn't have any references to it, but
    the traceback did.

    @jyasskin jyasskin mannequin closed this as completed Mar 28, 2008
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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