Look into caching .tox in GitHub actions

[jonathansick]

If we can cache .tox in GitHub Actions we could speed up the CI a lot. However, in practice, I find that Tox won't run from a cached .tox directory. It could be worth looking into this.

The workflow is:

name: Python CI

on: [push]

jobs:
  build:

    runs-on: ubuntu-latest
    strategy:
      matrix:
        python:
          - 3.7
          - 3.8

    steps:
      - uses: actions/checkout@v2

      - name: Set up Python
        uses: actions/setup-python@v1
        with:
          python-version: ${{ matrix.python }}

      - name: Install tox
        run: pip install tox

      - name: Cache tox environments
        id: cache-tox
        uses: actions/cache@v1
        with:
          path: .tox
          # setup.cfg, pyproject.toml, and .pre-commit-config.yaml have
          # versioning info that would impact the tox environment. hashFiles
          # only takes a single file path or pattern at the moment.
          key: ${{ runner.os }}-${{ matrix.python }}-tox-${{ hashFiles('setup.cfg') }}-${{ hashFiles('pyproject.toml') }}-${{ hashFiles('.pre-commit-config.yaml') }}

      - name: Run tox
        run: tox -e py,lint,typing  # run tox using Python in path

The error is:

 Run tox 1s
15
##[error]Process completed with exit code 2.
1
Run tox -e py,lint,typing
6
ERROR: invocation failed (exit code 1), logfile: /home/runner/work/safir/safir/.tox/.package/log/.package-4.log
7
================================== log start ===================================
8
Traceback (most recent call last):
9
  File "/opt/hostedtoolcache/Python/3.8.1/x64/lib/python3.8/site-packages/tox/helper/build_requires.py", line 7, in <module>
10
    backend = __import__(backend_spec, fromlist=[None])
11
ModuleNotFoundError: No module named 'setuptools'
12

13
=================================== log end ====================================
14
ERROR: FAIL could not package project - v = InvocationError("/home/runner/work/safir/safir/.tox/.package/bin/python /opt/hostedtoolcache/Python/3.8.1/x64/lib/python3.8/site-packages/tox/helper/build_requires.py setuptools.build_meta ''", 1)
15
##[error]Process completed with exit code 2.

[rra]

We are now caching the tox environments and everything seems to work fine, so presumably this is a bug that was subsequently fixed.

[hughesadam87]

@rra you're caching tox like this or some other way?

  - name: Cache tox environments
    id: cache-tox
    uses: actions/cache@v1
    with:
      path: .tox

[rra]

We're using the https://github.com/lsst-sqre/run-tox shared action now, but internally it does the following:

    - name: Cache tox environments
      id: cache-tox
      uses: actions/cache@v3
      if: fromJSON(${{ inputs.use-cache }})
      with:
        path: ${{ inputs.working-directory }}/.tox
        # setup.cfg and pyproject.toml have versioning info that would
        # impact the tox environment.
        key: ${{ inputs.cache-key-prefix}}-${{ inputs.python-version }}-${{ hashFiles('pyproject.toml', 'setup.cfg') }}