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unittest: on failure, TestCase.run() keeps a reference to the exception #64079

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vstinner opened this issue Dec 3, 2013 · 7 comments
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unittest: on failure, TestCase.run() keeps a reference to the exception #64079

vstinner opened this issue Dec 3, 2013 · 7 comments

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@vstinner
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vstinner commented Dec 3, 2013

BPO 19880
Nosy @pitrou, @vstinner, @ezio-melotti
Files
  • unittest_leak.py
  • unittest_workaround.patch
  • contextmanager_leak.py
  • contextmanager_leak2.py
  • generator_workaround.patch
  • frame_ref_cycle.py
  • unittest_leak.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 2013-12-09.00:56:40.951>
created_at = <Date 2013-12-03.22:46:40.957>
labels = []
title = 'unittest: on failure, TestCase.run() keeps a reference to the exception'
updated_at = <Date 2013-12-09.00:56:40.949>
user = 'https://github.com/vstinner'

    bugs.python.org fields:

    activity = <Date 2013-12-09.00:56:40.949>
actor = 'python-dev'
assignee = 'none'
closed = True
closed_date = <Date 2013-12-09.00:56:40.951>
closer = 'python-dev'
components = []
creation = <Date 2013-12-03.22:46:40.957>
creator = 'vstinner'
dependencies = []
files = ['32951', '32952', '32953', '32954', '32955', '32956', '32957']
hgrepos = []
issue_num = 19880
keywords = ['patch']
message_count = 7.0
messages = ['205167', '205168', '205173', '205174', '205177', '205179', '205630']
nosy_count = 4.0
nosy_names = ['pitrou', 'vstinner', 'ezio.melotti', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue19880'
versions = ['Python 3.4']
    @vstinner
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    vstinner commented Dec 3, 2013

    Test attached unittest_leak.py script: you will see MyException.ninstance counter increased up to 10, whereas I expect that MyException is destroyed at TestCase.run() exit.

    It looks like a tricky reference cycle between:

    • frames
    • exc_info local variable of _Outcome.testPartExecutor() context manager
    • _Outcome.errors list
    • _Outclass instance

    Attached unittest_workaround.patch patch works around the issue.

    @vstinner
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    vstinner commented Dec 3, 2013

    contextmanager_leak.py: shorter script to demonstrate the issue.

    Replacing "exc_info = sys.exc_info()" with "sys.exc_info()" works around the issue.

    @vstinner
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    vstinner commented Dec 3, 2013

    contextmanager_leak2.py: even simpler example, storing a current frame in a local variable of the frame is enough.

    generator_workaround.patch is another workaround: call frame.clear() when at generator exit to explicitly break the reference cycle.

    @vstinner
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    vstinner commented Dec 3, 2013

    frame_ref_cycle.py: if I understood correctly, unittest_leak.py can be simplified to this script. A frame contains a local variable which contains the frame: reference cycle.

    @vstinner
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    vstinner commented Dec 3, 2013

    I found this issue while working the memory limit feature of tracemalloc module (issue bpo-19817). I opened bpo-19835 to workaround an unlimited loop on PyErr_NoMemory() when Python is out of memory.

    See also the issue bpo-17807 and the PEP-442 for a similar reference cycle with generators.

    @vstinner
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    vstinner commented Dec 4, 2013

    unittest_leak.patch: Fix the the initial bug, unittest_leak.py.

    I don't think that it's possible to write a generic fix for frame_ref_cycle.py. Storing sys.exc_info() to format it as a traceback later is a common pattern. Clearing a frame at function exit breaks this use case.

    @python-dev
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    python-dev mannequin commented Dec 9, 2013

    New changeset 09658ea0b93d by Victor Stinner in branch 'default':
    Close bpo-19880: Fix a reference leak in unittest.TestCase. Explicitly break
    http://hg.python.org/cpython/rev/09658ea0b93d

    @python-dev python-dev mannequin closed this as completed Dec 9, 2013
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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