Factor out pybind11/compat/pybind11_platform_abi_id.h

[rwgk]

Description

Follow-on to PR #5296:

Objective: Maximize reusability of PYBIND11_PLATFORM_ABI_ID (for other Python/C++ binding systems).

Pure refactoring. No functional changes.

This PR also factors out pybind11/compat/pybind11_conduit_v1.h (from tests/exo_planet_c_api.cpp), but this is mainly to provide a reference implementation. See the long comment at the top of pybind11_conduit_v1.h

Birds-eye view of the refactoring steps:

• Parts of pybind11/detail/common.h → pybind11/compat/wrap_include_python_h.h
• Parts of include/pybind11/detail/internals.h → pybind11/compat/pybind11_platform_abi_id.h
• Parts of tests/exo_planet_c_api.cpp → pybind11/compat/pybind11_conduit_v1.h

Suggested changelog entry:

pybind11/compat/pybind11_platform_abi_id.h was factored out, to maximize reusability of ``PYBIND11_PLATFORM_ABI_ID`` (for other Python/C++ binding systems).

[rwgk]

@henryiii I want to have a README.txt in include/pybind11/compat, but I'm getting the error (see below for full context):

Documentation build test

Only in ./pybind11/compat: README.txt

What's the best way to handle this situation?

I believe having a README.txt right there (vs a new section in docs/) will be valuable, because this is for people who might not (want to) use pybind11.

---

Run python3 -m pip install --user -U ../dist/*.tar.gz
  python3 -m pip install --user -U ../dist/*.tar.gz
  installed=$(python3 -c "import pybind11; print(pybind11.get_include() + '/pybind11')")
  diff -rq $installed ./pybind11
  shell: /usr/bin/bash -e {0}
  env:
    PIP_BREAK_SYSTEM_PACKAGES: 1
    PIP_ONLY_BINARY: numpy
    FORCE_COLOR: 3
    PYTEST_TIMEOUT: 300
    VERBOSE: 1
    pythonLocation: /opt/hostedtoolcache/Python/3.1[2](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:2).5/x64
    PKG_CONFIG_PATH: /opt/hostedtoolcache/Python/[3](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:3).12.5/x64/lib/pkgconfig
    Python_ROOT_DIR: /opt/hostedtoolcache/Python/3.12.5/x64
    Python2_ROOT_DIR: /opt/hostedtoolcache/Python/3.12.5/x6[4](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:4)
    Python3_ROOT_DIR: /opt/hostedtoolcache/Python/3.12.[5](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:5)/x64
    LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.12.5/x[6](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:6)4/lib
Processing /home/runner/work/pybind11/pybind11/dist/pybind11-2.14.0.dev1.tar.gz
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
  Preparing metadata (pyproject.toml): started
  Preparing metadata (pyproject.toml): finished with status 'done'
Processing /home/runner/work/pybind11/pybind11/dist/pybind11_global-2.14.0.dev1.tar.gz
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'done'
  Preparing metadata (pyproject.toml): started
  Preparing metadata (pyproject.toml): finished with status 'done'
Building wheels for collected packages: pybind11, pybind11_global
  Building wheel for pybind11 (pyproject.toml): started
  Building wheel for pybind11 (pyproject.toml): finished with status 'done'
  Created wheel for pybind11: filename=pybind11-2.14.0.dev1-py3-none-any.whl size=24[7](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:7)323 sha256=6c45c16d00dffe226c663b7697a99caf943c6e[8](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:8)6d7a5f40072905fc242a3d1cb
  Stored in directory: /home/runner/.cache/pip/wheels/33/71/4b/48a8068c064[9](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:9)936366f43f4c61326da95fee8e122155830acd
  Building wheel for pybind[11](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:11)_global (pyproject.toml): started
  Building wheel for pybind11_global (pyproject.toml): finished with status 'done'
  Created wheel for pybind11_global: filename=pybind11_global-2.14.0.dev1-py3-none-any.whl size=448987 sha256=2f4c576fc669328b4c0df69dcaa0795c02a0f8c4a60c37f40[12](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:12)f2035d4e8a7f3
  Stored in directory: /home/runner/.cache/pip/wheels/d9/50/de/12393b3b6dd623bc8e25dde2f7665815fbd256d158981672fe
Successfully built pybind11 pybind11_global
Installing collected packages: pybind11_global, pybind11
Successfully installed pybind11-2.[14](https://github.com/pybind/pybind11/actions/runs/10869192453/job/30160061106?pr=5375#step:9:14).0.dev1 pybind11_global-2.14.0.dev1
Only in ./pybind11/compat: README.txt
Error: Process completed with exit code 1.

[rwgk]

@henryiii I decided it's best to add pybind11/compat/README.txt to the wheels. (commit a0eb9f3).

Currently waiting to see if that resolves the "Documentation build test" issue.

[rwgk]

@henryiii I just marked this as ready for review.

A nice to have, if and only if easy:

Is there an easy way to build tests/exo_planet_c_api.so with -fno-exceptions?

Just one build would be sufficient. E.g. there is no need to get fancy under Windows. Whatever is within easy reach.

[henryiii]

You can add it only to the sdist with MANIFEST.in, if you'd rather. Though having it in the wheel seems fine. I can try the test you suggest tomorrow or so (remind me if I forget).

[rwgk]

You can add it only to the sdist with MANIFEST.in, if you'd rather. Though having it in the wheel seems fine.

I think having the README also in the wheels might be useful for people looking around there.

I can try the test you suggest tomorrow or so (remind me if I forget).

I figured out something simple that passes local testing at least: 75871ef

I just see some GHA jobs aren't happy. I'll put this PR back in draft mode until I have that resolved.

[rwgk]

@henryiii This is as complete now as I'd want to make it.