Allow tox to use existing environment if no network connectivity

[stephenfin]

What's the problem this feature will solve?

tox - especially v4, with it's vastly improved dependency checking - is heavily reliant on good network connectivity to resolve dependencies. If you have poor/limited internet connectivity - such as when travelling or working from your local cafe ☕ - tox can take an age to get to the point of running tests, assuming it ever gets there. This happens regardless of how recently the environment was built [*].

Describe the solution you'd like

If tox (and by tox I probably mean the underlying pip install invocations) is unable to connect to the internet but a virtualenv already exists for the given target, tox could warn the user and then run commands using the existing virtualenv as a best effort. Alternatively, I could pass a flag to skip the env recreate check (though it should still reinstall the package under test). Something like the opposite of the --notest flag.

Alternative Solutions

My current workaround to this is to just use the virtualenv directly, e.g.:

source .tox/py310/bin/activate
stestr run ...

I can continue to do this, of course, but it would be nice to avoid having to copy/paste commands and resolve any expansions manually.

Additional context

I am happy to work on this myself at some point and consider this a low-priority nice-to-have. I just want to make sure it'll be welcomed if I do the work (and, ideally, get a heads up if this already exists or is simply not possible) 🙏

I did try the --no-provision flag, but that appears to be for preventing creation of the virtualenv: not controlling when stuff gets installed in there. Certainly when I try using it with an old, pre-existing testenv I see things getting recreated:

❯ tox -e py38 --no-provision
py38: recreate env because changed constraint(s) removed ...

[*] Yes, I know this is totally expected and you can't resolve deps without lookup to PyPI or network calls to resolve e.g. remote constraints files. My point is that, as a user, a relatively recent build is probably "good enough", especially if the alternative is to not run anything.