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PEP 656 -- Platform Tag for Linux Distributions Using Musl

PEP:656
Title:Platform Tag for Linux Distributions Using Musl
Author:Tzu-ping Chung <uranusjr at gmail.com>
Sponsor:Brett Cannon <brett at python.org>
PEP-Delegate:Paul Moore <p.f.moore at gmail.com>
Discussions-To:https://discuss.python.org/t/7165
Status:Accepted
Type:Informational
Created:17-Mar-2021
Post-History:17-Mar-2021 18-Apr-2021
Resolution:https://discuss.python.org/t/7165/32

Abstract

This PEP proposes a new platfrom tag series musllinux for binary Python package distributions for a Python installation that depends on 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 ("perennial manylinux"), and requires wheels using this tag make similar promises. Please refer to PEP 600 for more details on rationale and reasoning behind the design.

The musllinux platform tags only apply to Python interpreters dynamically linked against the musl libc and executed on the runtime shared library, on a Linux operating system. Statically linked interpreters or mixed builds with other libc implementations (such as glibc) are out of scope and not supported by platform tags defined in this document. Such interpreters should not claim compatibility with musllinux platform tags.

Specification

Tags using the new scheme will take the form:

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

This tag promises the wheel works on any mainstream Linux distribution that uses musl version ${MUSLMAJOR}.${MUSLMINOR}, following the perennial design. All other system-level dependency requirements rely on the community's definition to the intentionally vague "mainstream" description introduced in PEP 600. A wheel may make use of newer system dependencies when all mainstream distributions using the specified musl version provide the dependency by default; once all mainstream distributions on the musl version ship a certain dependency version by default, users relying on older versions are automatically removed from the coverage of that musllinux tag.

Reading the musl version

The musl version values can be obtained by executing the musl libc shared library the Python interpreter is currently running on, and parsing the output:

import re
import subprocess

def get_musl_major_minor(so: str) -> tuple[int, int] | None:
    """Detect musl runtime version.

    Returns a two-tuple ``(major, minor)`` that indicates musl
    library's version, or ``None`` if the given libc .so does not
    output expected information.

    The libc library should output something like this to stderr::

        musl libc (x86_64)
        Version 1.2.2
        Dynamic Program Loader
    """
    proc = subprocess.run([so], stderr=subprocess.PIPE, text=True)
    lines = (line.strip() for line in proc.stderr.splitlines())
    lines = [line for line in lines if line]
    if len(lines) < 2 or lines[0][:4] != "musl":
        return None
    match = re.match(r"Version (\d+)\.(\d+)", lines[1])
    if match:
        return (int(match.group(1)), int(match.group(2)))
    return None

There are currently two possible ways to find the musl library's location that a Python interpreter is running on, either with the system ldd command [ldd], or by parsing the PT_INTERP section's value from the executable's ELF header [elf].

Formatting the tag

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, replacing period (.) and hyphen (-) characters with underscores (_), as outlined in PEP 425 and PEP 427.

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(".", "_").replace("-", "_")
    return f"musllinux_{musl_version[0]}_{musl_version[1]}_{arch}"

Recommendations to package indexes

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 or later.
  • 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.

Source: https://github.com/python/peps/blob/master/pep-0656.rst