[BUG]: Segmentation fault on Python finalization with PYBIND11_HAS_SUBINTERPRETER_SUPPORT on Python 3.12d

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What version (or hash if on master) of pybind11 are you using?

master @ 98bd78f

Problem description

Building with Python 3.12+ will automatically enable subinterpreter support after #5564.

feat: support for sub-interpreters #5564

While testing with Python 3.12 with Py_DEBUG enabled, a segmentation fault will be raised on Python finalization:

Fatal Python error: _PyInterpreterState_GET: the function must be called with the GIL held, after Python initialization and before Python finalization, but the GIL is released (the current Python thread state is NULL)

Python 3.12d on Linux: https://github.com/metaopt/optree/actions/runs/15216857124/job/42804275481?pr=216#step:15:62
- Coredump backtrace: https://github.com/metaopt/optree/actions/runs/15216857124/job/42804275481?pr=216#step:17:237
Python 3.12d on macOS: https://github.com/metaopt/optree/actions/runs/15216857124/job/42804275470?pr=216#step:15:56
- Coredump backtrace: https://github.com/metaopt/optree/actions/runs/15216857124/job/42804275470?pr=216#step:17:120
Python 3.12d on Windows: https://github.com/metaopt/optree/actions/runs/15216857124/job/42804275480?pr=216#step:15:191

Reproducible example code

Is this a regression? Put the last known working version here if it is.

Not a regression

Ah, I think this is caused by the multiphase init changes and not (directly) by subinterpreter support. Are you able to try this one-line change and see if it fixes the issue?

diff --git a/include/pybind11/detail/common.h b/include/pybind11/detail/common.h
index 2e81150f..6a89d332 100644
--- a/include/pybind11/detail/common.h
+++ b/include/pybind11/detail/common.h
@@ -386,6 +386,7 @@ PYBIND11_WARNING_DISABLE_CLANG("-Wgnu-zero-variadic-macro-arguments")
                 slots,                                                                            \
                 ##__VA_ARGS__);                                                                   \
         }();                                                                                      \
+        Py_SET_REFCNT(result.ptr(), _Py_IMMORTAL_REFCNT);                                         \
         return result.ptr();                                                                      \
     }

If this works for you I will make a PR for it.

Fixed on latest HEAD.

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[BUG]: Segmentation fault on Python finalization with `PYBIND11_HAS_SUBINTERPRETER_SUPPORT` on Python 3.12d #5687

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What version (or hash if on master) of pybind11 are you using?

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Is this a regression? Put the last known working version here if it is.

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[BUG]: Segmentation fault on Python finalization with PYBIND11_HAS_SUBINTERPRETER_SUPPORT on Python 3.12d #5687

Description

Required prerequisites

What version (or hash if on master) of pybind11 are you using?

Problem description

Reproducible example code

Is this a regression? Put the last known working version here if it is.

Activity

b-pass commented on May 23, 2025

XuehaiPan commented on May 26, 2025

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[BUG]: Segmentation fault on Python finalization with `PYBIND11_HAS_SUBINTERPRETER_SUPPORT` on Python 3.12d #5687