bpo-40548: Fix "Check for source changes (pull_request)" GH Action job

[vstinner]

On Git 2.28, "git diff master..." (3 dots) no longer works when
"fetch --depth=1" is used, whereas it works on Git 2.26.

Replace "..." (3 dots) with ".." (2 dots) in the "git diff" command
computing the list of modified files between the base branch and the
PR branch.

https://bugs.python.org/issue40548

[vstinner]

cc @brettcannon @Mariatta @FFY00 @zooba @pablogsal

[vstinner]

The macOS failure is unrelated and can be ignored: https://bugs.python.org/issue40275#msg375122

[vstinner]

This change can be associated to https://bugs.python.org/issue40548 which introduced the "Check for source changes" job.

[shihai1991]

Oh, you are so quick. I try to fix it a minute ago: shihai1991#30 it) Lol~

[shihai1991]

I test it in last weekend, it can work. and removing the git fetch origin $GITHUB_BASE_REF --depth=1 can fix it too.
I guess depth=1 is not enough deep to find the merge base.

[vstinner]

I guess depth=1 is not enough deep to find the merge base.

Correct. But depth=1 is part of https://bugs.python.org/issue40548 design to make the "Check for source changes" job fast. For example, on my latest change, it only took 18 seconds overall!

I don't think that "..." (3 dots) to find the last common commit is needed for this job. ".." (2 dots) should be enough.

[shihai1991]

Thanks, It's can work for me. so LGTM.

[vstinner]

I tested manully: with this change, doc-only PR still skip build jobs.

I created #21812 to test this PR. It's a "documentation-only" PR. Build GitHub Action jobs have been skipped as expected, and "Docs / Docs" is running as expected.

[vstinner]

@shihai1991 @brettcannon: Thanks for the review! I merged my PR. So far, it works as expected. If something goes wrong, we can change the test again.

Python 3.9 is also affected, example: #21809 Check for source changes: "fatal: ... no merge base". I backport the change to 3.8 and 3.9.

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-21813 is a backport of this pull request to the 3.8 branch.

[bedevere-bot]

GH-21814 is a backport of this pull request to the 3.9 branch.

[FFY00]

@vstinner sorry for the delay, I did not have access to my computer over the weekend.

I implemented this in another project and was running into an issue, this was my solution:

https://github.com/FFY00/arch-python-repo/blob/9cf94335c441638a5a7fbed5b58e4852fbdd5792/.github/workflows/build.yml#L39

      - name: Check for source changes
        id: check
        env:
          PREVIOUS: ${{ github.event.before }}
          CURRENT: ${{ github.sha }}
        run: |
          git fetch origin $PREVIOUS
          git diff --name-only $PREVIOUS $CURRENT | grep -q ... || true

Turns out github actions makes the previous object SHA available for us to use. We can use this to explicitly fetch it and to run the diff. It should be more reliable than the current approach. What do you think?

[vstinner]

Turns out github actions makes the previous object SHA available for us to use. We can use this to explicitly fetch it and to run the diff. It should be more reliable than the current approach. What do you think?

I tried git diff origin/$GITHUB_BASE_REF..$GITHUB_SHA: it avoids the "git checkout", but the checkout is part of actions/checkout@v2, we cannot avoid it. With the checkout, origin/$GITHUB_BASE_REF..$GITHUB_SHA is the same than origin/$GITHUB_BASE_REF..HEAD which is the same than origin/$GITHUB_BASE_REF...

From what I understood, in practice, git diff $PREVIOUS $CURRENT is the same than git diff origin/$GITHUB_BASE_REF...

[FFY00]

No, GITHUB_BASE_REF (also github.base_ref) is different from github.event.before.

github.base_ref:

The base_ref or target branch of the pull request in a workflow run. This property is only available when the event that triggers a workflow run is a pull_request.

github.event.before is the SHA of the top commit before the event.

It is documented in the webhooks documentation, see https://docs.github.com/en/actions/reference/events-that-trigger-workflows for context.

github.event.before:

The SHA of the most recent commit on ref before the push.

Github is doing a bad job with the documentation here, it took a lot of digging to uncover this. But it does work and is documented.

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.7.
🐍🍒⛏🤖

[bedevere-bot]

GH-92342 is a backport of this pull request to the 3.7 branch.