passenv should fail on environment variables containing spaces

[ericzolf]

Issue

Since tox 4.0, passenv is a comma separated list of environment variables, where it used to be a space separated list:
https://tox.wiki/en/4.0.3/faq.html#tox-4-changed-ini-rules

I'm fine with this, BUT tox silently accepts a space separated list of environment variables, even though the environment variables are silently not passed, making root cause analysis more difficult than necessary for people (like me) migrating from tox 3 to 4. Since no shell (AFAIK) accepts environment variables with spaces in it, once the list has been split, none of its elements should contain spaces, or a meaningful error should be output.

Environment

Provide at least:

• OS: Fedora 37 or Ubuntu 20.02
• pip list of the host Python where tox is installed:

$ pip list
Package       Version
------------- -------
cachetools    5.2.0
chardet       5.1.0
colorama      0.4.6
distlib       0.3.6
filelock      3.8.2
packaging     22.0
pip           22.3.1
platformdirs  2.6.0
pluggy        1.0.0
py            1.11.0
pyproject_api 1.2.1
setuptools    62.6.0
six           1.16.0
tox           4.0.3
virtualenv    20.17.1
wheel         0.38.4

Output of running tox

MYENV None

I'd expect an error message of some kind that the passenv format isn't correct.

Minimal example

[testenv]
passenv = MYENV YOURENV
commands =
    python -c 'import os; print("MYENV", os.getenv("MYENV"))'

[gaborbernat]

PR welcome 😁

[ericzolf]

Taking the challenge but I might need a bit of help:

• pass_env_post_process in src/tox/tox_env/api.py seems to be a good place to detect the error
• I guess logging.error is to be used to report errors
• but how to properly handle the error, exit the program? Raise a Fail exception?

[gaborbernat]

I think raising the failure error.

raise HandledError(msg)

[ericzolf]

Raising a HandledError made tox fail with a useless message (not mine at least!), so I raised "Fail", which looks fine to me. I'll complete the draft PR #2671 once I've got a sign that my changes look OK.