Automatically provisioned Tox hangs on pyproject_api if `--installpkg` is used

[vytas7]

Issue

Automatically provisioned Tox hangs on pyproject_api backend if --installpkg is used.

At first glance, this seems to be very similar to #3512, except that instead of multiple environments the trigger now is recursively provisioning tox in a separate venv. A single environment is enough to reproduce.

I'll take a quick debugging session to see if the reason could be any similar to #3512.
In any case, it seems that my fix from #3530 does not help in this scenario.

A quick workaround exists, though -- it is enough to install the missing dependencies into the current environment (normally just setuptools was enough in the example at hand).

Environment

CPython 3.12 on Ubuntu/Debian Linux.

Minimal example

Clone https://github.com/ansible/pylibssh that @webknjaz is maintaining.

Build an sdist:

python -m build --sdist

Run tox:

tox --installpkg dist/ansible_pylibssh-*.tar.gz --notest

In the case of recursively provisioned environment, tox hangs. Otherwise it terminates cleanly.

[webknjaz]

Woah! I couldn't have thought it was this twisted! I imagine that it's failing on Python 3.12+ only because this CPython version no longer bundles setuptools within its ensurepip module. So removing setuptools in older Python versions would probably trigger the same behavior. Or having any non-bundled dep in there, for that matter.

[webknjaz]

normally just setuptools was enough in the example at hand

I wonder if having a newer version of tox itself would trigger this... That'd be fun!