python-peps/pep-0656.rst

PEP: 656
Title: Platform Tag for Linux Distributions Using Musl
Author: Tzu-ping Chung <uranusjr@gmail.com>
Sponsor: Brett Cannon <brett@python.org>
PEP-Delegate: TBD
Discussions-To: https://discuss.python.org/t/7165
Status: Draft
Type: Informational
Content-Type: text/x-rst
Created: 2021-03-17


Abstract
========

This PEP proposes a new platfrom tag series ``musllinux`` for
binary Python package distributions for a Python installation linked
against musl on a Linux distribution. The tag works similarly to the
"perennial manylinux" platform tags specified in :pep:`600`, but
targeting platforms based on musl instead.


Motivation
==========

With the wide use of containers, distributions such as Alpine Linux
[alpine]_, have been gaining more popularity than ever. Many of them
based on musl [musl]_, a different libc implementation from glibc, and
therefore cannot use the existing ``manylinux`` platform tags. This
means that Python package projects cannot deploy binary distributions
on PyPI for them. Users of such projects demand build constraints from
those projects, putting unnecessary burden on project maintainers.


Rationale
=========

According to the documentation, musl has a stable ABI, and maintains
backwards compatibility [musl-compatibility]_ [compare-libcs]_, so a
binary compiled against an earlier version of musl is guaranteed to
run against a newer musl runtime [musl-compat-ml]_. Therefore, we use
a scheme similar to the glibc-version-based manylinux tags, but
against musl versions instead of glibc.

Logic behind the new platform tag largely follows :pep:`600`, and
require wheels using this tag make similar promises. Please refer to
the PEP for more details on rationale and reasoning behind the design.


Specification
=============

Tags using the new scheme will take the form::

    musllinux_${MUSLMAJOR}_${MUSLMINOR}_${ARCH}

Distributions using the tag make similar promises to those described
in :pep:`600`, including:

1. The distribution works on any mainstream Linux distributions with
   musl version ``${MUSLMAJOR}.${MUSLMINOR}`` or later.
2. The distribution's ``${ARCH}`` matches the return value of
   ``sysconfig.get_platform()`` on the host system.

Example values::

    musllinux_1_1_x86_64   # musl 1.1 running on x86-64.
    musllinux_1_2_aarch64  # musl 1.2 running on ARM 64-bit.

The value can be formatted with the following Python code::

    import sysconfig

    def format_musllinux(musl_version: tuple[int, int]) -> str:
        os_name, sep, arch = sysconfig.get_platform().partition("-")
        assert os_name == "linux" and sep, "Not a Linux"
        arch = arch.replace("-", "_")
        return f"musllinux_{musl_version[0]}_{musl_version[1]}_{arch}"

It is recommended for Python package repositories, including PyPI, to
accept platform tags matching the following regular expression::

    musllinux_([0-9]+)_([0-9]+)_([^-]+)

Python package repositories may impose additional requirements to
reject Wheels with known issues, including but not limited to:

* A ``musllinux_1_1`` wheel containing symbols only available in musl 1.2.
* Wheel that depends on external libraries not considered generally
  available to the intended audience of the package index.
* A platform tag claiming compatibility to a non-existent musl version
  (like ``musllinux_9000_0``).

Such policies are ultimately up to individual package repositories.
It is not the author's intention to impose restrictions to the
maintainers.


Backwards Compatibility
=======================

There are no backwards compatibility concerns in this PEP.


Rejected Ideas
==============

Create a platform tag based specifically for Alpine Linux
---------------------------------------------------------

Past experience on the ``manylinux`` tag series shows this approach
would be too costly time-wise. The author feels the "works well with
others" rule both is more inclusive and works well enough in practice.


References
==========

.. [alpine] https://alpinelinux.org/

.. [musl] https://musl.libc.org

.. [musl-compatibility] https://wiki.musl-libc.org/compatibility.html

.. [compare-libcs] https://www.etalabs.net/compare_libcs.html

.. [musl-compat-ml] https://mail.python.org/archives/list/distutils-sig@python.org/message/VRXSTNXWHPAVUW253ZCWWMP7WDTBAQDL/


Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.


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