`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:

cpython/Lib/sysconfig/__main__.py

Line 79 in 5334732

while len(variables) > 0:

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

Linked PRs

• gh-136097: Fix sysconfig._parse_makefile() #136166

[serhiy-storchaka]

Thank you for report. This is much complex code, and it is not always clear what behavior is correct.

Recursive solution fixes the problem of infinite recursion and other bugs, but it introduces other problem -- stack consumption. If you create several hundred forward referencing variables, you can now get RecursionError. It would be possible to solve this using topological sorting, but I am not sure that it is worth to do this.