PEP 656 – 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:
- Discourse thread
- Status:
- Final
- Type:
- Standards Track
- Topic:
- Packaging
- Created:
- 17-Mar-2021
- Post-History:
- 17-Mar-2021, 18-Apr-2021
- Resolution:
- Discourse message
Abstract
This PEP proposes a new platform 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:
- The distribution works on any mainstream Linux distributions with
musl version
${MUSLMAJOR}.${MUSLMINOR}
or later. - The distribution’s
${ARCH}
matches the return value ofsysconfig.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_9500_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
Copyright
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Source: https://github.com/python/peps/blob/main/peps/pep-0656.rst
Last modified: 2023-09-09 17:39:29 GMT