[build] Multiarch: DESTSHARED placement makes stdlib extensions non-importable without downstream patching

[nanonyme]

Feature or enhancement

Proposal:

Summary

The current CPython build system does not produce a working multiarch layout without downstream hacks.
When --libdir=/usr/lib/<triplet> is used, extension modules are installed into a directory that is not on sys.path, unless --with-platlibdir is also set.
But setting --with-platlibdir collapses the purelib/platlib split. This leaves downstreams with two bad choices.

Observed behavior

• With --libdir=/usr/lib/<triplet> and no --with-platlibdir:

  • Pure‑Python stdlib → /usr/lib/pythonX.Y
  • Extension modules → /usr/lib/<triplet>/pythonX.Y/lib-dynload
  • Problem: extensions are not importable, because that path is not on sys.path.

• With --with-platlibdir=<triplet>:

  • Both pure‑Python and extension modules collapse under /usr/lib/<triplet>/pythonX.Y
  • Problem: the traditional split is lost.

Downstream workarounds

• Debian: patches DESTSHARED to follow LIBDEST (/usr/lib/pythonX.Y/lib-dynload).
• Flatpak freedesktop‑sdk: overrides DESTSHARED at make install, but this leaks the Python version into build tooling.
• Both approaches have long‑term maintenance downsides.

Why this is a problem

• Upstream CPython does not provide a working, policy‑neutral way to support multiarch layouts.
• Every downstream that cares about multiarch must carry a patch or override.
• This increases divergence and makes it harder to reason about where stdlib files live.

Proposed direction

Introduce a supported configure‑time option or default logic that:

• Keeps purelib under ${prefix}/lib/pythonX.Y
• Places extension modules under ${libdir}/pythonX.Y/lib-dynload
• Ensures both are added to sys.path at runtime
• Updates sysconfig schemes accordingly

This would eliminate the need for downstream patches and make multiarch a first‑class citizen in CPython’s build system.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

[nanonyme]

Actually I will abandon this. Looks like it is possible to have Debian style split without patching or setting environment variable. This only means that the split does not exist in certain downstreams but that probably doesn't matter much. For history, the right mechanism to have Debian-style split without patches or messing with internal variables is:

--libdir=/usr/lib/arch
--with-platlibdir=lib