[austinmatherne-wk]

Fixes actions/setup-python#1376

Python 3.15 no longer falls back to the bundled libmpdec when configure can't find a system copy (what's new). The Ubuntu builds of 3.15.0 ship without the C _decimal module as a result. decimal then falls back to _pydecimal, which is slower and raises ValueError for values over 4300 digits. The RHEL builder has the same problem.

Installing libmpdec-dev doesn't fix it. Ubuntu 24.04 has no mpdecimal package, and the tarballs would then need libmpdec installed on the runner. --without-system-libmpdec works for 3.15, but 3.16 removes the bundled copy and the flag (what's new).

For 3.15+, the Ubuntu and RHEL builders now download mpdecimal 4.0.1, check its SHA-256, build it as a static library, and pass it to configure through LIBMPDEC_CFLAGS and LIBMPDEC_LIBS. It's the same tarball and hash that CPython's macOS installer uses. _decimal has no runtime dependency on libmpdec, and builds before 3.15 are unchanged.

python-modules.py now checks for _decimal as well. Importing decimal succeeds through _pydecimal, so the existing check didn't catch this.

Testing:

• 3.15.0 on all six Ubuntu platforms, default and free-threaded, in my fork: https://github.com/austinmatherne-wk/python-versions/actions/runs/40336601178. All 24 build and test jobs pass, and CPython's test_decimal passes during the PGO build.
• 3.16.0a1 on Ubuntu 24.04 in Docker, using the builder's Configure(). _decimal imports and doesn't link to libmpdec.
• I couldn't test RHEL because those runners aren't available to forks.

Notes:

• The published 3.15.0 Linux tarballs need a rebuild after this merges to pick up _decimal.
• Updating mpdecimal later means changing the version and hash together in BuildLibmpdec().

[pylaterreur]

I checked this locally with the builder's configure flags on 3.15.0 (Ubuntu, gcc 15.2): without libmpdec _decimal is missing, as in the 24.04 build log; with this PR's static libmpdec 4.0.1 it is built, needs only libc and libm, and test_decimal -u decimal,cpu passes (736 run, 9 skipped). The free-threaded configure behaves the same. Thanks for this, it also fixes the 22.04 and arm64 builds.

Two optional notes, neither blocking:

• Building mpdecimal with CFLAGS='-fPIC -O3 -fvisibility=hidden' matches how CPython compiled its bundled libmpdec in the 3.14 builds. With -fPIC alone, _decimal exports 239 libmpdec symbols (3.14.8 exports none) and big-number arithmetic is slower: 200k-digit division and sqrt took 17.00G instructions against 14.12G with the flags above, and factorial(100000) 2.47G against 2.34G. Small precisions don't change. Diff: main...pylaterreur:python-versions:pr418-libmpdec-cflags (one line in nix-python-builder.psm1).
• Execute-Command doesn't stop on failure, so if the mpdecimal build ever failed, _decimal would silently disappear again; the new _decimal import test is what would catch it.

The published 3.15.0 Linux tarballs will need a rebuild after this merges.

[austinmatherne-wk]

Thanks! I cherry picked your commit. I also added added a commit which stops the build if libmpdec.a doesn't get built.

[pylaterreur]

@v-HarithaVattikuti could you review this when you have a moment? Until it ships, the Ubuntu 3.15.0 builds that setup-python serves have no _decimal (CPython 3.15 removed the bundled libmpdec fallback), so import decimal fails on hosted runners. Verified earlier in this thread that the PR fixes it.

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