[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:

> "Rather than build the commits that have been pushed to the branch the
pull request is from, we build the merge between the source branch and
the upstream branch."

Okay, great. But this still only happens for Linux. And still has the same problem, the build needs to be re-triggered.

> Both are fine, but...

...

> Bot shouldn't use rebase, but merge master branch.
Rebase in pull request branch break the pull request sometime.
See this thread:
https://discuss.python.org/t/info-rebase-origin-master-push-f-workflow-corrupts-pull-request-rarely/2558/7

Okay, so maybe I was not clear enough here. The PR would be rebased locally in the CI runs before building and running the tests. Nothing would ever be pushed. We don't really want users have to pull their branch before making changes, also the users might not give us write access to their branch.

What I am proposing is to do here the same as Travis apparently does, and then maybe figure out a workflow where we (semi-/)automatically  re-trigger the CI, to make everything works correctly with master.

This is a low risk change. Rethinking the core workflow is the difficult part.

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<https://bugs.python.org/issue40551>
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