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"Task exception was never retrieved" reported for a canceled task #74693

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MiguelGrinberg mannequin opened this issue May 30, 2017 · 7 comments
Closed

"Task exception was never retrieved" reported for a canceled task #74693

MiguelGrinberg mannequin opened this issue May 30, 2017 · 7 comments
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3.7 expert-asyncio type-bug

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@MiguelGrinberg
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@MiguelGrinberg MiguelGrinberg mannequin commented May 30, 2017

BPO 30508
Nosy @ned-deily, @1st1
PRs
  • #2050
  • #2109
  • #2110
  • Files
  • never-retrieved.py
  • 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 2017-06-11.14:12:51.714>
    created_at = <Date 2017-05-30.02:14:06.380>
    labels = ['type-bug', '3.7', 'expert-asyncio']
    title = '"Task exception was never retrieved" reported for a canceled task'
    updated_at = <Date 2017-06-11.15:42:01.895>
    user = 'https://bugs.python.org/MiguelGrinberg'

    bugs.python.org fields:

    activity = <Date 2017-06-11.15:42:01.895>
    actor = 'ned.deily'
    assignee = 'none'
    closed = True
    closed_date = <Date 2017-06-11.14:12:51.714>
    closer = 'yselivanov'
    components = ['asyncio']
    creation = <Date 2017-05-30.02:14:06.380>
    creator = 'Miguel Grinberg'
    dependencies = []
    files = ['46911']
    hgrepos = []
    issue_num = 30508
    keywords = []
    message_count = 7.0
    messages = ['294732', '294733', '295708', '295709', '295711', '295712', '295721']
    nosy_count = 3.0
    nosy_names = ['ned.deily', 'yselivanov', 'Miguel Grinberg']
    pr_nums = ['2050', '2109', '2110']
    priority = 'normal'
    resolution = 'fixed'
    stage = 'resolved'
    status = 'closed'
    superseder = None
    type = 'behavior'
    url = 'https://bugs.python.org/issue30508'
    versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

    @MiguelGrinberg
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    @MiguelGrinberg MiguelGrinberg mannequin commented May 30, 2017

    I am seeing a strange issue that occurs when a task that is awaiting an asyncio.wait_for() is cancelled. I created a simple example that I think demonstrates the issue, even though it isn't exactly how it manifests on my application.

    When I run the attached script never-retrieved.py I get the following error:

    Task exception was never retrieved
    future: <Task finished coro=<crash() done, defined at never-retrieved.py:4> exception=ZeroDivisionError('division by zero',)>
    Traceback (most recent call last):
      File "/usr/lib/python3.5/asyncio/tasks.py", line 239, in _step
        result = coro.send(None)
      File "never-retrieved.py", line 5, in crash
        a = 1/0
    ZeroDivisionError: division by zero

    You can see in the script that the future was cancelled, but the cancel() call was made after the task finished in a zero division error. I think the cancel() call should update the internal state of the future so that the "exception was never retrieved" error does not appear.

    My application has a more complex setup that I have been unable to reproduce with a simple example. I have a task that is waiting on asyncio.wait_for(fut, timeout), with fut subsequently waiting on a websocket server's receive function. When the websocket client closes the connection, a bunch of cancellations happen, but this future inside the wait_for call crashes before wait_for gets to call cancel() on it. Even though I need to investigate this crash, the fact is that wait_for did cancel this future, but because it already ended in an error the "never retried" error is reported anyway.

    @MiguelGrinberg MiguelGrinberg mannequin added expert-asyncio type-bug labels May 30, 2017
    @1st1
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    @1st1 1st1 commented May 30, 2017

    Yes, this is a known problem to me, thank you for creating the issue. Will work on a fix soon.

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    @1st1 1st1 commented Jun 11, 2017

    New changeset 7ce1c6f by Yury Selivanov in branch 'master':
    bpo-30508: Don't log exceptions if Task/Future "cancel()" method called (bpo-2050)
    7ce1c6f

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    @1st1 1st1 commented Jun 11, 2017

    New changeset 176f2eb by Yury Selivanov in branch '3.6':
    bpo-30508: Don't log exceptions if Task/Future "cancel()" method was called. (bpo-2109)
    176f2eb

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    @1st1 1st1 commented Jun 11, 2017

    New changeset d24c828 by Yury Selivanov in branch '3.5':
    bpo-30508: Don't log exceptions if Task/Future "cancel()" method was called. (bpo-2110)
    d24c828

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    @1st1 1st1 commented Jun 11, 2017

    Fixed. Ned, this will be included in 3.6.2, right?

    @1st1 1st1 added the 3.7 label Jun 11, 2017
    @1st1 1st1 closed this as completed Jun 11, 2017
    @ned-deily
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    @ned-deily ned-deily commented Jun 11, 2017

    Yury, yes (3.6.2 cutoff is about 24 hours from now)

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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