ci: enable automatic merging of pull requests

[jamacku]

PR that meets the following criteria will be automatically merged:

• Passing CI
• Approved by at least one reviewer
• No Draft or WIP title
• No conflicts

The merging workflow runs on schedule every 45 minutes. It can be executed manually on demand from the Actions tab. It internally uses several GitHub Actions to perform the merge.

• https://github.com/redhat-plumbers-in-action/gather-pull-request-metadata to gather PR metadata
• https://github.com/redhat-plumbers-in-action/pull-request-validator to check reviews and CI status
• https://github.com/redhat-plumbers-in-action/auto-merge to perform the merge
• https://github.com/redhat-plumbers-in-action/issue-commentator to comment on PRs about the merge status

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I have tested the workflow on jamacku#1

A working example can be found at redhat-plumbers/systemd-rhel10#45 (comment). It contains a few more pieces, but merging logic is also present.

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There are a few limitations/requirements:

• When merging PR this way, it doesn't trigger on: push workflows - https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow
• Maintainers who approve PRs need to have member visibility set to public; otherwise, GitHub Action will not see that they are members eligible to review PRs.

/cc @xsuchy

[mfocko]

Just out of interest, what's the reason for using GH Action instead of the GitHub's built-in functionality?

At the same time, from the reviewer's PoV, if I review the PR and click on the big green button denoting merge when ready, it has basically the same effect, cause in the branch protection rules I can require specific CI checks to pass, and also there cannot be any merge conflicts.

[jamacku]

Just out of interest, what's the reason for using GH Action instead of the GitHub's built-in functionality?

This is a valid question. We (rhel systemd) had this automation before GitHub introduced all the Auto-merge stuff. We have other automation pieces related to Jira and Upstream that work together with auto-merge functionality. It allows us to have more customization and flexibility regarding the behavior. For example, we can waive the CI, all CI checks block us, and we pick non-blocking checks; when PR is merged, we transition Jira into a new state, etc.

For the copr project, it might be sufficient to use rules and auto-merge. But I know the copr project has a policy about merging approved PRs only after 24 hours from submission. This is currently not possible by using only rules. It is currently not possible by our action either, but it can be easily extended to support this requirement.