Description

zhangyoufu

opened on Sep 3, 2022

Issue body actions

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

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

Activity

1. zhangyoufu

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Sep 3, 2022
2. zhangyoufu commented on Sep 3, 2022

   

   zhangyoufu

   on Sep 3, 2022

   ContributorAuthor

   More actions

   pty._copy did not check whether the destination fd is writable.
3. ambv commented on Oct 5, 2022

   

   ambv

   on Oct 5, 2022

   Contributor

   More actions

   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

   on May 19, 2023

   Contributor

   More actions

   Fixed in 3.11 and 3.12.
6. ambv

   closed this as completedon May 19, 2023
7. ambv commented on May 19, 2023

   

   ambv

   on May 19, 2023

   Contributor

   More actions

   Thanks, @zhangyoufu! ✨ 🍰 ✨

   Reacted by Youfu Zhang
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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