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contextvars.Context.run hangs forever in ProccessPoolExecutor #83438

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contextvars.Context.run hangs forever in ProccessPoolExecutor#83438
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3.8 (EOL)end of lifetopic-multiprocessingtype-bugAn unexpected behavior, bug, or error

Description

@Ronserruya
Ronserruya
mannequin
opened on Jan 8, 2020
BPO 39257
Nosy @1st1, @Ronserruya

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 = None
created_at = <Date 2020-01-08.10:34:34.026>
labels = ['type-bug', '3.8']
title = 'contextvars.Context.run hangs forever in ProccessPoolExecutor'
updated_at = <Date 2020-01-10.19:23:50.063>
user = 'https://github.com/Ronserruya'

bugs.python.org fields:

activity = <Date 2020-01-10.19:23:50.063>
actor = 'yselivanov'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2020-01-08.10:34:34.026>
creator = 'ronserruya'
dependencies = []
files = []
hgrepos = []
issue_num = 39257
keywords = []
message_count = 2.0
messages = ['359575', '359749']
nosy_count = 2.0
nosy_names = ['yselivanov', 'ronserruya']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39257'
versions = ['Python 3.8']

Activity

  1. Ronserruya commented on Jan 8, 2020

    @Ronserruya
    Ronserruyamannequin
    on Jan 8, 2020
    MannequinAuthor

    Sending Context.run to another process via ProccessPoolExecutor hangs forever:

    from contextvars import ContextVar, copy_context
from concurrent.futures import ProcessPoolExecutor
from multiprocessing import Process

var: ContextVar[int] = ContextVar('var',default=None)

if __name__ == '__main__':
    
    # ***** This hangs forever *****
    with ProcessPoolExecutor(max_workers=1) as pp:
        ctx = copy_context()
        pp.submit(ctx.run, list)

    # ****** This throws 'cannot pickle Context' # ***** This hangs forever *****
    ctx = copy_context()
    p = Process(target=ctx.run, args=(list,))
    p.start()
    p.join()

    python version is 3.8.0
    running on Mac OSX 10.15.1

  2. Ronserruya
    added
    3.8 (EOL)end of life
    type-bugAn unexpected behavior, bug, or error
    on Jan 8, 2020

  3. 1st1 commented on Jan 10, 2020

    @1st1
    1st1
    on Jan 10, 2020
    Member

    This throws 'cannot pickle Context'

    Yes, this is expected. contextvars are not compatible with multiprocessing.

    **** This hangs forever *****

    This hanging part is weird, and most likely hints at a bug in multiprocessing.

  4. ezio-melotti
    transferred this issue fromon Apr 10, 2022
  5. serhiy-storchaka
    added this to Multiprocessing issueson May 20, 2022
  6. vstinner
    added
    topic-multiprocessing
    on Jun 6, 2022
  7. seathatflowsinourveins
    added 2 commits that reference this issue on Oct 3, 2026

    Complete the critique's missed sources and matrix sharding; qualify h…

    2ac89db

    Record the failed hosted trial of unittest-parallel on the two requir…

    4cfac6c
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