The current implementation of the PEP 587 is not ready for future evolutions of the PyPreConfig and PyConfig structure. Any change will break the ABI compatibility. Their _config_version field is useless.

The first versions of the PEP 587 used a macro to statically initialize the structure. It was decided to get ride of macros to use functions instead.

Example:

   PyConfig config;
   PyStatus status = PyConfig_InitPythonConfig(&config);

PyConfig_InitPythonConfig() gets unitialized memory and doesn't know the size of the config variable.

I propose to require to store the size of the structure in the structure directly:

* Add PyPreConfig.struct_size and PyConfig.struct_size
* Require to initialize these fields to sizeof(PyPreConfig) and sizeof(PyConfig) respectively

Example:
     
   PyConfig config;
   config.struct_size = sizeof(PyConfig);
   PyStatus status = PyConfig_InitPythonConfig(&config);

Thanks to the struct_size field, PyConfig_InitPythonConfig() can support old PyConfig structures without breaking the ABI.

If an old PyConfig is passed to PyConfig_Read(): newer fields will be ignored.

I updated the PEP 587:
https://github.com/python/peps/commit/afa38c0bef7738b8fcc3173daee8488e0420833d

New changeset 441b10cf2855955c86565f8d59e72c2efc0f0a57 by Victor Stinner in branch 'master':
bpo-38304: Add PyConfig.struct_size (GH-16451)
https://github.com/python/cpython/commit/441b10cf2855955c86565f8d59e72c2efc0f0a57

New changeset 6e128382b3e7f83d91d5e73d83a051ff5fb6469c by Victor Stinner in branch '3.8':
bpo-38304: Add PyConfig.struct_size (GH-16451) (GH-16453)
https://github.com/python/cpython/commit/6e128382b3e7f83d91d5e73d83a051ff5fb6469c

Ok, the API should now be ABI future proof thanks to this change.

I experimented a change locally: it's possible to support two sizes of the PyConfig structure (need to modify a few functions in initconfig.c).

I sent an email to python-dev:
https://mail.python.org/archives/list/python-dev@python.org/thread/C7Z2NA2DTM3DLOZCFQAK5A2WFYO3PHHX/

The hidden _config_version field wasn't in the accepted PEP.

Both it and struct_size should be removed, as we don't support updating an embedded Python runtime to a new X.Y.0 release without rebuilding the embedding application.

Nick, do you consider this a 3.8.0 release blocker?  If so, you should reopen it and mark it as such, since the 3.8.0rc1 cutoff is imminent.

> The hidden _config_version field wasn't in the accepted PEP.

It was in the accepted PEP and I replaced it with struct_size:
https://github.com/python/peps/commit/afa38c0bef7738b8fcc3173daee8488e0420833d

I suggest to discuss the issue on the thread that I started on python-dev.

> Both it and struct_size should be removed, as we don't support updating an embedded Python runtime to a new X.Y.0 release without rebuilding the embedding application.

See my reply on python-dev:
https://mail.python.org/archives/list/python-dev@python.org/message/ZE5K7MEKN32YGHNA5457AROWUNI7JOWC/

New changeset 89f8177dcfdbcf17c85bb6998c946c9f42bf6e27 by Victor Stinner in branch 'master':
bpo-38304: Fix PyConfig usage in python_uwp.cpp (GH-16487)
https://github.com/python/cpython/commit/89f8177dcfdbcf17c85bb6998c946c9f42bf6e27

New changeset 81f6b031c46721478372d77fe2e55aa1f8300ae1 by Victor Stinner in branch '3.8':
bpo-38304: Fix PyConfig usage in python_uwp.cpp (GH-16488)
https://github.com/python/cpython/commit/81f6b031c46721478372d77fe2e55aa1f8300ae1

I reopen the issue since there seems to be disagreement about the whole feature (provide a stable ABI for embedded Python): see bpo-38326.

Will you forward port your change to master, Victor?

> Will you forward port your change to master, Victor?

Sure, it was my plan: PR 16508. I chose to fix 3.8 first to unblock the 3.8.0rc1 release.

New changeset 3c30a76f3d3c0dcc1fb4de097fa4a3a4c92c0b0b by Victor Stinner in branch 'master':
bpo-38304: Remove PyConfig.struct_size (GH-16500) (GH-16508)
https://github.com/python/cpython/commit/3c30a76f3d3c0dcc1fb4de097fa4a3a4c92c0b0b

New changeset 8462a4936b3a551dc546a6adea04a70b0a07ca67 by Victor Stinner in branch 'master':
bpo-38304: PyConfig_InitPythonConfig() cannot fail anymore (GH-16509)
https://github.com/python/cpython/commit/8462a4936b3a551dc546a6adea04a70b0a07ca67

New changeset d49f096cc41f57155efe71cd089c29b38c218488 by Miss Islington (bot) in branch '3.8':
bpo-38304: PyConfig_InitPythonConfig() cannot fail anymore (GH-16509)
https://github.com/python/cpython/commit/d49f096cc41f57155efe71cd089c29b38c218488

It was decided to abandon the idea of stable ABI for PyConfig: see bpo-38326. I now close this issue.