[issue40551] PRs should be rebased on top of master before running the build/tests

[Filipe Laíns]

Filipe Laíns <filipe.lains at gmail.com> added the comment:

> Just like Travis, Github actions also automatically rebases pull requests onto the latest master. As you can see from a recent workflow run, it uses the special ref: "refs/remote/pull/20047/merge" https://github.com/python/cpython/pull/20047/checks?check_run_id=665377507#step:2:898

Thanks for letting me know. I did search through the documentation but I hadn't found anything regarding this. Well, I guess [1] hinted it but I did not pay enough attention :D

> Unless there's real world examples of ongoing problems with PRs being merged with outdated test runs, I don't think this is worth complicating our CIs for.

Although I understand your point, I think this is one of those cases where things can easily get out of hand. And when that happens, it could be very difficult to revert unless you catch it right away. There's too much noise for me to find examples, sorry.

> As Inada-san mentioned, the occasional slip through will eventually be caught by the buildbots and promptly reverted. Re-triggering the CI for very stale pull requests might be worth doing. I'm kinda opposed to an automated system because this creates an undue burden on our CI providers, Travis graciously provides us with extra time for our coverage builds to finish already. Re-running pull requests that might never be merged is a lot of added work for them.

Well, I was not really proposing that. I suggested to retrigger on approvals (and by this I mean approvals from core devs) or have a bot retriggering and merging automatically.

> Maybe we can get one of the bots to tag old pull requests and then have some guidance in the core-dev guide to re-run the CIs manually when merging those PRs.

Yes, this seems reasonable, and could be expanded in the future. I think we should document this and maybe add a bot command to allow devs to retrigger more easily (I was thinking they could just comment with /test).

[1] https://github.com/actions/checkout#Checkout-pull-request-HEAD-commit-instead-of-merge-commit

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue40551>
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