[DougWebb]

If you're using CI testing results and tying them to particular commits, you end up with the same problem whether you merge or rebase.

If you test a commit and it passes, and then merge that commit into master, the merge may have changed the code that the commit modified, or something that the commit's code depended on. The green flag you had on the commit is no longer valid because the commit is in a new context now and may not pass.

If you rebase the commit onto master, you're explicitly changing the context of the commit. Yes, you get a different SHA and you're not linked to the original CI result anymore, but that CI result wasn't valid anymore anyway. This is exactly the same situation that the merge put you into, but without the false assurance of the green flag on the original test result.

As many others have noted, rebasing is only recommended on private branches to prepare them for sharing on a public branch. If you're running CI it's probably only on the public branches, so rebasing wouldn't affect that. But if you're running CI on your private branch too, then you're going to want to run it after rebasing onto the public branch and before merging into the public branch. That gives you assurance that your code works on the public branch before you share it. Again, if you're using a merge-based workflow you'd have to do the same testing regardless of your earlier test results.

[notduncansmith]

My intuition of how CI should work is that it tests what the new master would look like were that commit [rebased|merged] into the current master, for that exact reason. Is that not how CI systems tend to work? What you've described above does seem awfully silly.

[jamesmiller5]

This is exactly how I setup our teams workflow. Private commits are pushed into gerrit run against the CI suite.

  1. If reviews + CI tests go well we fast forward merge onto master.
  2. If the commit's parent isn't the latest commit on master, it is automatically rebased and the CI suite is kicked off again.
  3. Upon successful fast forward merge into master, all in-flight reviews are automatically rebased on master's new head and CI's kicked off again.
  4. Any open commit can become the top of master without worry it will break the build.

For our team of ~10 this works exceptionally well with master not being a broken due to our code in the last ~6 months. (edit: formatting)