gh-116417: Add _testlimitedcapi C extension

[vstinner]

Add a new C extension "_testlimitedcapi" which is only built with the limited C API.

Move heaptype_relative.c and vectorcall_limited.c from Modules/_testcapi/ to Modules/_testlimitedcapi/.

• Issue: C API: Move limited C API tests from _testcapi to a new _testlimitedcapi extension #116417

[vstinner]

@erlend-aasland @encukou @sobolevn @colesbury: What do you think of this change? (See the issue for the rationale.)

[vstinner]

If we land this change, should we move tests which don't use the non-limited C API (tests only using the limited C API) to _testlimitedapi?

This change should be backported to 3.11 and 3.12 branches to ease backport of future changes.

[colesbury]

This seems like a good idea to me.

• The #include "pyconfig.h" can be removed from vectorcall_limited.c and heaptype_relative.c now that it's part of parts.h
• heaptype_relative.c can also remove the Py_LIMITED_API define now that it's defined in parts.h.

[zooba]

Can we remove xxlimited.c then? I'm pretty sure that's only in there to test the limited API, so if we have another module doing it, there isn't a need for that one.

[vstinner]

@colesbury:

This seems like a good idea to me. (...)

Fixed: nicely spotted for remaining code in heaptype_relative.c.

I had a lot of build issues to build the new C extension properly on Linux and Windows, and then I forgot these lines.

@zooba:

Can we remove xxlimited.c then? I'm pretty sure that's only in there to test the limited API, so if we have another module doing it, there isn't a need for that one.

I don't think that it's a test, but more an example to write code using the limited C API. I started with the comment:

/* Use this file as a template to start implementing a module that
   also declares object types. All occurrences of 'Xxo' should be changed

cc @encukou

[vstinner]

@colesbury: I addressed your remarks and tests now pass. You can review the updated PR ;-)

[colesbury]

LGTM

[sobolevn]

This looks like a good idea, thank you!

[erlend-aasland]

I'd like it if you could create an entry for this module in configure.ac; that way it is easier to see if this module is enabled or not. Relevant output from sample ./configure and make invocations:

The following modules are *disabled* in configure script:
_ctypes_test              _testbuffer               _testcapi              
_testclinic               _testclinic_limited       _testexternalinspection
_testimportmultiple       _testinternalcapi         _testmultiphase        
_testsinglephase          _xxtestfuzz               xxsubtype              

checking for stdlib extension module _testcapi... disabled
checking for stdlib extension module _testclinic... disabled
checking for stdlib extension module _testclinic_limited... disabled
checking for stdlib extension module _testinternalcapi... disabled
checking for stdlib extension module _testbuffer... disabled
checking for stdlib extension module _testimportmultiple... disabled
checking for stdlib extension module _testmultiphase... disabled
checking for stdlib extension module _testsinglephase... disabled
checking for stdlib extension module _testexternalinspection... disabled

If you "hide" this new extension module behind the _testcapi module, it will not show up in these outputs.

[vstinner]

I'd like it if you could create an entry for this module in configure.ac; that way it is easier to see if this module is enabled or not.

Sure! It's done.

Example:

$ ./configure --disable-test-modules|grep testlimited
checking for stdlib extension module _testlimitedcapi... disabled

The test extension is now disabled as expected.

[erlend-aasland]

Thanks! On more nit :)

[erlend-aasland]

It's a nice change; it makes sense to have this as a separate test module IMO.

[vstinner]

Thanks for your helpful reviews! I merged my PR.

[vstinner]

Backporting this change to Python 3.12 looks non trivial, I get many conflicts:

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   Lib/test/support/__init__.py
	both modified:   Lib/test/test_capi/test_misc.py
	both modified:   Modules/Setup.stdlib.in
	both modified:   Modules/_testcapi/parts.h
	both modified:   Modules/_testcapimodule.c
	both modified:   Modules/_testlimitedcapi/heaptype_relative.c
	both modified:   Modules/_testlimitedcapi/vectorcall_limited.c
	both modified:   PCbuild/pcbuild.proj
	both modified:   PCbuild/pcbuild.sln
	both modified:   PCbuild/readme.txt
	both modified:   Tools/c-analyzer/c_parser/preprocessor/gcc.py
	both modified:   Tools/msi/test/test_files.wxs
	both modified:   configure
	both modified:   configure.ac

The split of Modules/_testinternalcapi.c was not backported. Maybe we should not backport these changes to 3.11 and 3.12.