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CPython should not assume that pthread_self returns the same value in fork parent and child #126688

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nico opened this issue Nov 11, 2024 · 2 comments
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CPython should not assume that pthread_self returns the same value in fork parent and child #126688

nico opened this issue Nov 11, 2024 · 2 comments
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@colesbury
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3.13 bugs and security fixes 3.14 new features, bugs and security fixes type-bug An unexpected behavior, bug, or error

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@nico
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nico commented Nov 11, 2024 •
edited by bedevere-app bot
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Summarized from SerenityOS/serenity#25263 by request of @colesbury:

It seems that at least Python 3.13 relies on pthread_self returning the same value in both the parent and the child process.
PyThread_get_thread_ident_ex

POSIX doesn't seem to mention if the value returned by pthread_self should be inherited by the child process.
pthread_self
fork

(It is the case on Linux, Hurd, FreeBSD, OpenBSD, Cygwin. It isn't on SerenityOS, and also not on Solaris 9/HP-UX 11 per https://bugs.python.org/issue7242.

It looks like the code add broke things is from this commit e21057b added in #118523 as part of #117657

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@nico nico mentioned this issue Nov 11, 2024
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@colesbury colesbury self-assigned this Nov 11, 2024
@colesbury colesbury added type-bug An unexpected behavior, bug, or error 3.13 bugs and security fixes 3.14 new features, bugs and security fixes labels Nov 11, 2024
colesbury added a commit to colesbury/cpython that referenced this issue Nov 11, 2024
@colesbury
pythongh-126688: Reinit import lock after fork
fc4bdfb 
The PyMutex implementation supports unlocking after fork because we
clear the list of watiers in parking_lot.c. This doesn't work as well
for _PyRecursiveMutex because on some systems, such as SerenityOS, the
thread id is not preserved across fork().
colesbury added a commit to colesbury/cpython that referenced this issue Nov 11, 2024
@colesbury
pythongh-126688: Reinit import lock after fork
5e86cca 
The PyMutex implementation supports unlocking after fork because we
clear the list of watiers in parking_lot.c. This doesn't work as well
for _PyRecursiveMutex because on some systems, such as SerenityOS, the
thread id is not preserved across fork().
@bedevere-app bedevere-app bot mentioned this issue Nov 11, 2024
Merged
colesbury added a commit to colesbury/cpython that referenced this issue Nov 11, 2024
@colesbury
Merge branch 'main' into pythongh-126688-import-lock
4bf74b8
colesbury added a commit that referenced this issue Nov 12, 2024
@colesbury
gh-126688: Reinit import lock after fork (#126692)
5610860 
The PyMutex implementation supports unlocking after fork because we
clear the list of waiters in parking_lot.c. This doesn't work as well
for _PyRecursiveMutex because on some systems, such as SerenityOS, the
thread id is not preserved across fork().
miss-islington pushed a commit to miss-islington/cpython that referenced this issue Nov 12, 2024
@colesbury @miss-islington
pythongh-126688: Reinit import lock after fork (pythonGH-126692)
6f77b81 
The PyMutex implementation supports unlocking after fork because we
clear the list of waiters in parking_lot.c. This doesn't work as well
for _PyRecursiveMutex because on some systems, such as SerenityOS, the
thread id is not preserved across fork().
(cherry picked from commit 5610860)

Co-authored-by: Sam Gross <colesbury@gmail.com>
@bedevere-app bedevere-app bot mentioned this issue Nov 12, 2024
Merged
colesbury added a commit that referenced this issue Nov 12, 2024
@miss-islington @colesbury
[3.13] gh-126688: Reinit import lock after fork (GH-126692) (GH-126765)
13fe8f9 
The PyMutex implementation supports unlocking after fork because we
clear the list of waiters in parking_lot.c. This doesn't work as well
for _PyRecursiveMutex because on some systems, such as SerenityOS, the
thread id is not preserved across fork().
(cherry picked from commit 5610860)

Co-authored-by: Sam Gross <colesbury@gmail.com>
@oskar-skog
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I think this can be closed now.

@ericsnowcurrently
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There may be other code that makes the same assumption about the thread ID. For example, in PyOS_AfterFork_Child():

cpython/Modules/posixmodule.c

Lines 655 to 658 in 09c240f

assert(tstate->thread_id == PyThread_get_thread_ident());
#ifdef PY_HAVE_THREAD_NATIVE_ID
tstate->native_thread_id = PyThread_get_thread_native_id();
#endif

This would break only on debug builds (or at least !defined(NDEBUG)).

I'm guessing there are other places too.

@bedevere-app bedevere-app bot mentioned this issue Nov 21, 2024
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