gh-116646: Add limited C API support to AC fildes converter

[vstinner]

Add tests on the "fildes" converter to _testclinic_limited.

• Issue: C API: Add PyLong_FileDescriptor_Converter(): make private _PyLong_FileDescriptor_Converter() public #116646

[vstinner]

@serhiy-storchaka @erlend-aasland: Ok, this approach should be the least intrusive and the least controversial. It only adds support for the limited C API, but keep the code for the internal C API using converter.

In the worst case, it generates internal C API code for the limited C API, but we can revisit this corner case later.

[serhiy-storchaka]

Why bother with limited_capi? It is okay to use PyObject_AsFileDescriptor unconditionally.

[vstinner]

Why bother with limited_capi? It is okay to use PyObject_AsFileDescriptor unconditionally.

You explained to me that all converters must support the PyArg_ParseTuple() fallback, here using O& and _PyLong_FileDescriptor_Converter(), when parse_arg() cannot be used.

I prefer to move slowly, step by step, when touching Argument Clinic which is fragile and has a small test suite. This change adds a test on the "fildes" converter for the limited C API.

[serhiy-storchaka]

The fallback is still here if you do not remove converter = '_PyLong_FileDescriptor_Converter'.

[serhiy-storchaka]

I prefer to not add an indentation and the limited_capi check. The fallback is still used in a number of modules.

[vstinner]

The fallback is still here if you do not remove converter = '_PyLong_FileDescriptor_Converter'.

Ok, let me elaborate. I tried to move code to converter_init(). The problem is that the logic to decide if an #include must be added or not doesn't work well. It's because converter_init() is always called: calling add_include() means that the include is always added, even if it's not used. I plan to write a follow-up PR to adjust this.

[vstinner]

Merged, thanks for the review @serhiy-storchaka.

[erlend-aasland]

LGTM; thanks.

[erlend-aasland]

[...] when touching Argument Clinic which is fragile and has a small test suite.

You keep repeating this, but the latter part is not true. Please keep to the facts, and don't invent FUD about the Argument Clinic test suite.

[vstinner]

Your right, AC has a large test suite today, and mypy is a great help. Sorry.