gh-151218: Replace sys.flags in PyConfig_Set() by vstinner · Pull Request #151402 · python/cpython

vstinner · 2026-06-12T13:40:53Z

PyConfig_Set() and sys.set_int_max_str_digits() now replace sys.flags, instead of modifying sys.flags in-place.

Modifying sys.flags in-place can lead to data races when multiple threads are reading or writing sys.flags in parallel.

Use _Py_atomic functions to get and set max_str_digits members.

Issue: Data race in sys_set_flag when sys.set_int_max_str_digits() is called concurrently with free-threaded build #151218

PyConfig_Set() and sys.set_int_max_str_digits() now replace sys.flags, instead of modifying sys.flags in-place. Modifying sys.flags in-place can lead to data races when multiple threads are reading or writing sys.flags in parallel. Use _Py_atomic functions to get and set max_str_digits members.

vstinner · 2026-06-12T13:43:01Z

I built Python with ./configure --with-pydebug --disable-gil --with-thread-sanitizer CC=clang LD=clang and I ran the reproducer: TSan doesn't report data races anymore.

If we agree that this change is the right approach, I will add a non-regression test.

skirpichev

I think it's a correct approach.

Add test_sys to "./python -m test --tsan" tests.

vstinner · 2026-06-15T15:13:48Z

I added a thread test to test_free_threading.test_sys which is run by ./python -m test --tsan.

I also added some simple tests on get_int_max_str_digits() / set_int_max_str_digits() (in test_sys).

vstinner · 2026-06-15T15:14:25Z

@picnixz @gpshead: Would you mind to review this fix?

vstinner · 2026-06-15T15:19:12Z

Python 3.13 is not affected: sys.set_int_max_str_digits() only sets tstate->interp->long_state.max_str_digits, it doesn't update sys.flags. And PyConfig_Set() was only added to Python 3.14.

I modified sys.set_int_max_str_digits() in Python 3.14 when I implemented PEP 741 (PyConfig_Set()).

read-the-docs-community · 2026-06-16T11:46:21Z

Documentation build overview

📚 cpython-previews | 🛠️ Build #33163314 | 📁 Comparing 44bc1b2 against main (8646385)

🔍 Preview build

2 files changed

± c-api/init_config.html
± whatsnew/changelog.html

Update also outdated comment.

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

vstinner · 2026-06-16T12:00:27Z

@picnixz: I addressed your review. Please review the updated PR.

picnixz

LGTM. Btw, I tried to delete sys.flags and it made the REPL "crash" (well just exit with an exception). I wonder whether sys.flags should be made non-deletable as well.

picnixz · 2026-06-16T12:15:28Z

-    sys_set_flag(flags, pos, value);
-    Py_DECREF(flags);
-    return 0;
+    new_flags = PyStructSequence_New(&FlagsType);


All this dance makes me wonder whether we need a PyStructSequence_Copy. Would be nicer to just copy the structure and alter one field.

I searched for PyStructSequence_New() and the only other place which creates a copy of an existing structseq is structseq_replace(): structseq.__replace__() method. I'm not sure that it's worth it to write a generic PyStructSequence_Copy() function.

Example using structseq.__replace__() method:

$ python >>> import sys, copy >>> copy.replace(sys.flags, verbose=3) sys.flags(..., verbose=3, ...)

vstinner · 2026-06-16T16:01:26Z

Btw, I tried to delete sys.flags and it made the REPL "crash" (well just exit with an exception). I wonder whether sys.flags should be made non-deletable as well.

In https://peps.python.org/pep-0726/ discussion, I mentioned that protecting sys attributes would be a good motivation to implement module __setattr__() and __delattr__(). Sadly, the SC decided to reject this PEP.

It's not easy to modify how module attributes are set and deleted in a C extension.

miss-islington-app · 2026-06-16T16:17:38Z

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14, 3.15.
🐍🍒⛏🤖

miss-islington-app · 2026-06-16T16:17:47Z

Sorry, @vstinner, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b16d23fc9fe9cb72fa15c8a3036753e5437b5b8c 3.14

bedevere-app · 2026-06-16T16:17:54Z

GH-151552 is a backport of this pull request to the 3.15 branch.

bedevere-app · 2026-06-16T16:33:34Z

GH-151553 is a backport of this pull request to the 3.14 branch.

…51552) gh-151218: Replace sys.flags in PyConfig_Set() (GH-151402) PyConfig_Set() and sys.set_int_max_str_digits() now replace sys.flags (create a new object), instead of modifying sys.flags in-place. Modifying sys.flags in-place can lead to data races when multiple threads are reading or writing sys.flags in parallel. Use _Py_atomic functions to get and set max_str_digits members. (cherry picked from commit b16d23f) Co-authored-by: Victor Stinner <vstinner@python.org> Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

) gh-151218: Replace sys.flags in PyConfig_Set() (#151402) PyConfig_Set() and sys.set_int_max_str_digits() now replace sys.flags (create a new object), instead of modifying sys.flags in-place. Modifying sys.flags in-place can lead to data races when multiple threads are reading or writing sys.flags in parallel. Use _Py_atomic functions to get and set max_str_digits members. (cherry picked from commit b16d23f) Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

vstinner requested a review from ericsnowcurrently as a code owner June 12, 2026 13:40

bedevere-app Bot added the awaiting core review label Jun 12, 2026

bedevere-app Bot mentioned this pull request Jun 12, 2026

Data race in sys_set_flag when sys.set_int_max_str_digits() is called concurrently with free-threaded build #151218

Closed

vstinner mentioned this pull request Jun 12, 2026

gh-151218: Fix data race in sys_set_flag for free-threading #151220

Closed

skirpichev self-requested a review June 12, 2026 13:44

skirpichev reviewed Jun 12, 2026

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vstinner added 3 commits June 15, 2026 17:04

Add unit tests

19e863c

Add test_sys to "./python -m test --tsan" tests.

Move test to test_free_threading.test_sys

a66a331

Complete Changelog entry

4ca409d

vstinner added needs backport to 3.15 pre-release feature fixes, bugs and security fixes needs backport to 3.14 bugs and security fixes labels Jun 15, 2026

picnixz reviewed Jun 15, 2026

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Comment thread Misc/NEWS.d/next/Core_and_Builtins/2026-06-12-15-30-25.gh-issue-151218.5M_nv8.rst Outdated

Comment thread Python/sysmodule.c Outdated

Comment thread Python/sysmodule.c

Comment thread Python/sysmodule.c

vstinner added 2 commits June 16, 2026 13:35

Address picnixz's review

f114294

Add get_int_max_str_digits() to the test

e5280f1

vstinner requested review from FFY00 and ZeroIntensity as code owners June 16, 2026 11:42

picnixz reviewed Jun 16, 2026

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Comment thread Doc/c-api/init_config.rst

Move versionadded

770d385

picnixz reviewed Jun 16, 2026

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Comment thread Python/sysmodule.c Outdated

vstinner and others added 2 commits June 16, 2026 13:56

Use also _Py_ID() in _PySys_UpdateConfig()

df57cf2

Update also outdated comment.

Update Python/sysmodule.c

44bc1b2

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

picnixz approved these changes Jun 16, 2026

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bedevere-app Bot removed the awaiting core review label Jun 16, 2026

bedevere-app Bot added the awaiting merge label Jun 16, 2026

PyStructSequence_GET_ITEM() cannot fail

1843290

vstinner enabled auto-merge (squash) June 16, 2026 16:03

vstinner merged commit b16d23f into python:main Jun 16, 2026
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vstinner deleted the replace_sys_flags branch June 16, 2026 16:17

bedevere-app Bot removed the awaiting merge label Jun 16, 2026

miss-islington-app Bot assigned vstinner Jun 16, 2026

bedevere-app Bot removed the needs backport to 3.15 pre-release feature fixes, bugs and security fixes label Jun 16, 2026

bedevere-app Bot removed the needs backport to 3.14 bugs and security fixes label Jun 16, 2026

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