[Bug 2066339] Re: Prefix is not exactly honored when installing with --prefix

[Jeremy Bícha]

** Package changed: python3-setuptools (Ubuntu) => setuptools (Ubuntu)

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https://bugs.launchpad.net/bugs/2066339

Title:
  Prefix is not exactly honored when installing with --prefix

Status in setuptools package in Ubuntu:
  New
Status in setuptools package in Debian:
  New

Bug description:
  In a simple distutils project:

  $ cat hello
  #!/usr/bin/env python3 print("Hello world")

  $ cat setup.py
  from distutils.core import setup
  setup(name='hello', scripts=['hello'])

  Installing it with --prefix is not fully honored, as "local" is always
  ​added (--root is optional, used just to make the reproducer easier):

  $ python3 setup.py install --root=/tmp/py-root --prefix=/opt/py-prefix

  $ find /tmp/py-root/
  /tmp/py-root/
  /tmp/py-root/opt
  /tmp/py-root/opt/py-prefix
  /tmp/py-root/opt/py-prefix/local
  /tmp/py-root/opt/py-prefix/local/bin
  /tmp/py-root/opt/py-prefix/local/bin/hello
  /tmp/py-root/opt/py-prefix/local/lib
  /tmp/py-root/opt/py-prefix/local/lib/python3.12
  /tmp/py-root/opt/py-prefix/local/lib/python3.12/dist-packages
  /tmp/py-root/opt/py-prefix/local/lib/python3.12/dist-packages/hello-0.0.0-py3.12.egg-info
  /tmp/py-root/opt/py-prefix/local/lib/python3.12/dist-packages/hello-0.0.0-py3.12.egg-info/dependency_links.txt
  /tmp/py-root/opt/py-prefix/local/lib/python3.12/dist-packages/hello-0.0.0-py3.12.egg-info/SOURCES.txt
  /tmp/py-root/opt/py-prefix/local/lib/python3.12/dist-packages/hello-0.0.0-py3.12.egg-info/top_level.txt
  /tmp/py-root/opt/py-prefix/local/lib/python3.12/dist-packages/hello-0.0.0-py3.12.egg-info/PKG-INFO

  In fact in such case everything should be installed in
  ​/tmp/py-root/opt/py-prefix prefix and not
  ​/tmp/py-root/opt/py-prefix/local since a prefix is explicitly defined
  ​and so there's no point to add an extra "/local" subpath.

  ---

  In between the others, this breaks jhbuild builds as a test fails for
  ​this specific reason (see bug #2066193 where the missing file is due to
  ​the fact that it gets installed to $PREFIX/local/bin instead of $PREFIX/bin).

  ---

  After some debugging of it, this related to the default scheme change
  ​happened with python3.10 [1] which meant to always use /usr/local as
  ​default and ensure that one should explicitly require for /usr prefix,
  ​but in the case that --prefix= is used, the user request is now ignored because:

   - /usr/lib/python3.*/_distutils_system_mod.py:
     1. Selects the scheme `posix_prefix` since we've requested a prefix explicitly

   - /usr/lib/python3/dist-packages/setuptools/_distutils/command/install.py
     1. Calls sysconfig.get_preferred_scheme("prefix") that now as per the said
        change would return by default `posix_local` unless we're building a
        debian package.
     2. Replaces the variables to use the `posix_local` scheme

  So, in order to fix this we may in theory change python3 packages to
  ​make 1. to select another defined prefix say `posix_explicitprefix`, and
  ​then making `sysconfig.get_preferred_scheme("explicitprefix")` to
  ​instead return `posix_prefix`, but that would not be correct, because
  ​being that a public API it should only support the allowed input
  ​arguments for it ("prefix", "home" or "user").

  As per this, I feel the best way to handle this is instead in the
  ​setuptools module so that we can check once again if --prefix has been
  ​used and react accordingly.

  [1] https://lists.debian.org/debian-python/2022/03/msg00039.html
  [2] https://bugs.launchpad.net/ubuntu/+source/python3.10/+bug/1967920

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