Postmortem: degradation involving python `language_version` fixed

[pre.commit.ci]

This is a post-mortem followup for this issue: https://github.com/pre-commit-ci/issues/issues/26

Between 2020-11-20 14:00 PST and 2020-11-23 09:00 PST `language: python` hooks with `language_version` set to anything but `python` / `python3` / `python3.8` failed to build with a message similar to:

[INFO] Installing environment for https://github.com/psf/black.

[INFO] Once installed this environment will be reused.

[INFO] This may take a few minutes...

An unexpected error has occurred: CalledProcessError: command: ('/venv/bin/python3', '-mvirtualenv', '/pc/clone/HxuGbCv5Sg-bj1aDQ52Uiw/py_env-python3.7', '-p', 'python3.7')

return code: 1

expected return code: 0

stdout:

    RuntimeError: read-only app data python info cannot be updated

stderr: (none)

Check the log at /pc/pre-commit.log

If you had this issue on one of your pull request runs, you can close + reopen to re-trigger a run.

An optimization was rolled out on Friday which reduces disk usage significantly (up to 97%) for python hooks but an oversight in testing allowed this issue to slip through.  It has been fixed and a test has been added to the runner-image repository to handle this case: https://github.com/pre-commit-ci/runner-image/pull/25

A follow-up item is to improve the observability for failed builds such that time to detection for an issue like this can be improved.

Thanks,

Anthony