`sysconfig --generate-posix-vars` can hang on cyclic variable definitions

[brianschubert]

Bug report

Bug description:

This loop can run forever if there's a cycle in the Makefile variable definitions: https://github.com/python/cpython/blob/5334732f9c8a44722e4b339f4bb837b5b0226991/Lib/sysconfig/main.py#L79

To reproduce:

make clean
echo 'foo=$(foo)' > ./Modules/Setup.local
./configure
make  # hangs forever

Or more minimally:

make clean
echo 'foo=$(foo)' > ./Modules/Setup.local
make Makefile
./python -m sysconfig --generate-posix-vars  # hangs forever

Longer cycles are also possible, e.g. v1=$(v2), v2=$(v3), ...., v999=$(v1).

Note that this is less of an issue if the recursive variable definition gets expanded in the Makefile before python -m sysconfig --generate-posix-vars gets invoked. In that case, Make will detect the cycle and exit before the infinite loop occurs.

@serhiy-storchaka you may be interested, this is very similar to #136073

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux