Pass real paths when running subprocesses

[dechamps]

Summary of changes

Fixes #1164 by making sure real paths are passed to executables that are not subject to path redirection.

Test plan

winget install python --source=msstore
pip install .
python -m pipx ensurepath
pipx install pycowsay
pycowsay

The above fails without this PR with the error shown in #1164, but succeeds with this PR.

I tried to add a GitHub Actions job that does the above so that any future regressions will be caught, but unfortunately installing Microsoft Store apps in a GitHub Actions workflow does not appear to be easy (see e.g. Cyberboss/install-winget#1).

[uranusjr]

Seems harmless enough.

[dechamps]

I looked at the test failure and it turns out that my PR in its current state breaks every OS except Windows 😅 One thing I did not realize is that, on every OS except Windows, the python binary in the venv is actually a symlink, and Python expects argv[0] to contain the path to the symlink (not the real path) so that it can resolve the venv. My PR results in pipx resolving the venv symlink manually and calling python with the real path in argv[0]. This results in Python being called without any venv, which is obviously wrong.

This is a bit of a catch-22: on Windows using MS Store Python, argv[0] cannot be the redirected path (that doesn't work), but on *nix, it has to be the redirected path (i.e. the symlink).

I'll see if I can come up with a fix that doesn't assume too much about the particular implementation details of venvs on each platform.

[dechamps]

Okay I managed to make it work on every OS by only using the real path if the executable is not a symlink. The basic idea is that if the target is a symlink then the target might care about argv[0] being the symlink, so we pass the symlink untouched. If, however, it is not a symlink, then using the real path is a no-op, except on MS Store Python, where it will fix argv[0] so that the venv's Scripts/python.exe receives its real path and is able to locate its own venv. (Phew!)

[gaborbernat]

Test please and changelog entry.

[dechamps]

@gaborbernat

Test please

These code paths are already covered, and both branches are taken in GitHub Actions workflows (the non-symlink branch is taken on Windows, and the symlink branch is taken on every other OS).

If you want me to add a regression test for #1164, well, I tried to do that, but sadly it's not as easy as it seems because I was unable to get MS Store Python to install in a GitHub Actions Windows runner (see Cyberboss/install-winget#1 for one approach I tried).

changelog entry

Done.

[gaborbernat]