test_io failed with ASAN failures (heap-use-after-free) on AMD64 Arch Linux Asan 3.13

AMD64 Arch Linux Asan 3.13: https://buildbot.python.org/#/builders/1464/builds/1125

== CPython 3.13.14+ (heads/3.13:303d0acfcdcc, Jul 19 2026, 12:37:17) [GCC 16.1.1 20260625]
== Linux-7.0.14-arch1-1-x86_64-with-glibc2.43 little-endian
== Python build: release ASAN
== cwd: /buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/build/test_python_worker_1123020æ
== CPU count: 2
== encodings: locale=UTF-8 FS=utf-8
== resources: all,-network,-urlfetch
== sanitizers: address
== ASAN_OPTIONS='detect_leaks=0:allocator_may_return_null=1:handle_segv=0'

(...)

FAIL: test_daemon_threads_shutdown_stderr_deadlock (test.test_io.CMiscIOTest.test_daemon_threads_shutdown_stderr_deadlock)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/Lib/test/test_io.py", line 4755, in test_daemon_threads_shutdown_stderr_deadlock
    self.check_daemon_threads_shutdown_deadlock('stderr')
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/buildbot/buildarea/3.13.pablogsal-arch-x86_64.asan/build/Lib/test/test_io.py", line 4745, in check_daemon_threads_shutdown_deadlock
    self.assertFalse(err.strip('.!'))
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
AssertionError: '=================================================================\n
==1136209==ERROR: AddressSanitizer: heap-use-after-free on address 0x7c9453c52a58 at pc 0x555f594278c6 bp 0x7b64519fe080 sp 0x7b64519fe070\n
READ of size 8 at 0x7c9453c52a58 thread T1\n
' is not false

It seems like the test fails randomly since July 14.

Linked PRs

• gh-157013
• gh-157019

Start with Lib/test/test_io.py, especially check_daemon_threads_shutdown_deadlock and test_daemon_threads_shutdown_stderr_deadlock, then inspect the linked buildbot run and ASAN trace. Review gh-157013 and gh-157019 before investigating further; done means the intermittent ASAN heap-use-after-free no longer causes the test to fail.