gh-75723: Fix double evaluation of .pth and .site files in venvs

[warsaw]

This is a more comprehensive fix for Python 3.15 for #75723 reported by @asottile (thanks!). This should not be backported since it's specific to the additions made for PEP 829. I'll evaluate and update !147951 for 3.13 and 3.14 separately.

Huge thanks to @asottile for finding, reporting, and fixing the original bug.

• Issue: pth files in site-packages of venvs are executed twice #75723

[ncoghlan]

Looks good to me!

[warsaw]

Thanks for the review @ncoghlan !

[scoder]

We are seeing CI test failures in Cython with the latest CPython main branch since yesterday, and this PR seems the most likely culprit. Some of our tests start Python commands in subprocesses (on Linux/fork) that build code using setuptools, which injects a fake distutils import via a .pth file. The error we see is that the distutils package can no longer be found and imported from the subprocesses, whereas the main process has no problem using it.

From a quick look through this change, it feels like a status leak into the child process, but I couldn't immediately pin point one. Any idea what might go wrong here?

[warsaw]

We are seeing CI test failures in Cython with the latest CPython main branch since yesterday, and this PR seems the most likely culprit. Some of our tests start Python commands in subprocesses (on Linux/fork) that build code using setuptools, which injects a fake distutils import via a .pth file. The error we see is that the distutils package can no longer be found and imported from the subprocesses, whereas the main process has no problem using it.

From a quick look through this change, it feels like a status leak into the child process, but I couldn't immediately pin point one. Any idea what might go wrong here?

If you're able, please try with #149659

[scoder]

If you're able, please try with #149659

I patched the site.py changes of that PR into my local CPython (and created a fresh venv from it) but couldn't find a difference in behaviour. Still failing.