gh-115119: removed implicit fallback to the bundled libmpdec

[skirpichev]

• Issue: Unvendor libmpdec sources #115119

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📚 Documentation preview 📚: https://cpython-previews--134078.org.readthedocs.build/

[skirpichev]

On top of configure changes in #133997.

N/B: all linux jobs, except for changed (not sure if it worth) - have no system libmpdec. For MacOS we run tests with system libmpdec.

[AA-Turner]

AC_MSG_ERROR? I think opting in to the pure Python version should be explicit.

[skirpichev]

AC_MSG_ERROR ?

Well, that could be an option.

Though, more complex wrt implementation: all linux jobs will fail, unless we either provide system libmpdec or change ./configure invocations to use a new option.

I think opting in to the pure Python version should be explicit.

I'm not sure it's useful. After all, the pure-Python version is always available as the _pydecimal.py.

Did you suggest a new option like --with-purepython-decimal?

[picnixz]

A warning is actually ok, but users may not see it and then, they would have a slow version of decimal. OTOH, an error might be too abrupt but at least the tansition would be more explicit and would force people to upgrade if they want an efficient way to do it.

I'm +0.25 for AC_MSG_ERROR just to force people to update. We can add an option --allow-fallback-to-pydecimal to handle CI possible failures, though this means that devs should be aware that their C code might not be tested in the CI due to that.

[vstinner]

LGTM

[vstinner]

@AA-Turner @picnixz @hugovk: Do you think that this change is ready to be merged?

cc @erlend-aasland @corona10 who modify configure tools sometimes.

[picnixz]

I see a double review but I'm on mobile. Sorry if it got double posted. I'm ok with this change, modulo some wording nits.

[vstinner]

I think that I now prefer PR gh-135568: don't add --with-libmpdec option. I don't think that ./configure should fail if mpdecimal library is missing, but a warning should be written at the end of the ./configure script.

[skirpichev]

warning should be written at the end of the ./configure script.

Done. I moved this after _decimal module check at the end.

JFR:

1. No system libmpdec, default:

$ ./configure --with-pydebug 2>&1|grep -i mpd
checking for --with-system-libmpdec... yes
checking for libmpdec >= 2.5.0... no
checking for rl_compdisp_func_t... yes
configure: WARNING: no system libmpdecimal found; falling back to pure-Python version for the decimal module
$ make -s
Written build/lib.linux-x86_64-3.15/_sysconfigdata_d_linux_x86_64-linux-gnu.py
Written build/lib.linux-x86_64-3.15/_sysconfig_vars_d_linux_x86_64-linux-gnu.json
The necessary bits to build these optional modules were not found:
_decimal
To find the necessary bits, look in configure.ac and config.log.

Checked 114 modules (35 built-in, 77 shared, 1 n/a on linux-x86_64, 0 disabled, 1 missing, 0 failed on import)

2. No system libmpdec, with bundled:

$ ./configure --with-pydebug --without-system-libmpdec 2>&1|grep -i mpd
checking for --with-system-libmpdec... no
configure: WARNING: the bundled copy of libmpdecimal is scheduled for removal in Python 3.16; consider using a system installed mpdecimal library.
checking for decimal libmpdec machine... uint128
checking for rl_compdisp_func_t... yes

3. System libmpdec, with bundled:

$ ./configure --with-pydebug --without-system-libmpdec 2>&1|grep -i mpd
checking for --with-system-libmpdec... no
configure: WARNING: the bundled copy of libmpdecimal is scheduled for removal in Python 3.16; consider using a system installed mpdecimal library.
checking for decimal libmpdec machine... uint128
checking for rl_compdisp_func_t... yes

4. System libmpdec, default:

$ ./configure --with-pydebug 2>&1|grep -i mpd
checking for --with-system-libmpdec... yes
checking for libmpdec >= 2.5.0... yes
checking for rl_compdisp_func_t... yes

[ned-deily]

Thanks for making the changes. I think this solution makes for a better user experience.