Support commenting on a PR when closing it about create-pull-request

[peter-evans]

Hi @bgilbert

There is only one case where the action causes a pull request to close. That is when there is no longer a diff with the base branch. The action updates the PR branch and when GitHub sees that it has no diff anymore GitHub automatically closes the PR.

If you want to handle this case and add a comment in the PR, this is how I would recommend achieving it.

permissions:
  pull-requests: write

...

      - name: Create Pull Request
        id: cpr
        uses: peter-evans/create-pull-request@v5

      - if: steps.cpr.outputs.pull-request-operation = 'closed'
        uses: peter-evans/create-or-update-comment@v3
        with:
          issue-number: ${{ steps.cpr.outputs.pull-request-number }}
          body: Pull request was closed because there was no longer a diff with the base branch.

[bgilbert]

That approach sounds reasonable to me, but it currently doesn't work: pull-request-number and pull-request-url aren't set when pull-request-operation is closed.

[peter-evans]

Oh, you're right. 🤦‍♂️

After thinking about this some more, I think this case is not easy to handle in the action because the action only indirectly causes the PR to close. I can't be 100% sure that when the branch is updated it caused the PR to close. A user could have closed it before that and left the branch undeleted. Sorry, but I don't think I have a good solution.

[bgilbert]

In the updated case, we query the API for the PR corresponding to the head branch. Would it make sense to also do that before deleting the branch, and if a PR is found, put its details in pull-request-{number,url}?