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pty.spawn deadlock #96522

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pty.spawn deadlock#96522
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type-bugAn unexpected behavior, bug, or error

Description

@zhangyoufu
zhangyoufu
opened on Sep 3, 2022

Bug report

% yes | python3 -c "import pty; pty.spawn(['cat'])"
y
...
y^CTraceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python3.10/pty.py", line 181, in spawn
    _copy(master_fd, master_read, stdin_read)
  File "/usr/lib/python3.10/pty.py", line 161, in _copy
    _writen(master_fd, data)
  File "/usr/lib/python3.10/pty.py", line 127, in _writen
    n = os.write(fd, data)
KeyboardInterrupt

You will see 2048 lines of y due to tty echo, then the output stops and hangs forever.
Both python and cat block on write, waiting each other consume something from pty buffer.

Your environment

Tested on Linux/macOS with CPython 3.10.

Linked PRs

Activity

  1. zhangyoufu
    added
    type-bugAn unexpected behavior, bug, or error
    on Sep 3, 2022

  2. zhangyoufu commented on Sep 3, 2022

    @zhangyoufu
    zhangyoufu
    on Sep 3, 2022
    ContributorAuthor

    pty._copy did not check whether the destination fd is writable.

  3. ambv commented on Oct 5, 2022

    @ambv
    ambv
    on Oct 5, 2022
    Contributor

    I can reproduce this with main, and the proposed PR does indeed fix the deadlock.

  4. ambv
    added 3 commits that reference this issue on May 19, 2023

    gh-96522: Fix deadlock in pty.spawn (#96639)

    9c5aa89

    [3.11] pythongh-96522: Fix deadlock in pty.spawn (pythonGH-96639)

    38b5d74

    [3.11] gh-96522: Fix deadlock in pty.spawn (GH-96639) (#104655)

    702ce87

  5. ambv commented on May 19, 2023

    @ambv
    ambv
    on May 19, 2023
    Contributor

    Fixed in 3.11 and 3.12.

  6. ambv
    closed this as completedon May 19, 2023

  7. ambv commented on May 19, 2023

    @ambv
    ambv
    on May 19, 2023
    Contributor

    Thanks, @zhangyoufu! ✨ 🍰 ✨

  8. carljm
    added a commit that references this issue on May 20, 2023

    Merge branch 'main' into gsallam/main

    ea5965a
  9. teamchong
    added a commit that references this issue on Dec 15, 2025

    fix(ci): Remove flaky test_pty from Linux test group

    8a59a4d
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