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

[XuehaiPan]

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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

[b-pass]

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.

[XuehaiPan]

Fixed on latest HEAD.