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I think GitHub is pretty good here: If you make PR #1 from branch foo with target main, and PR #2 from branch bar (a descendant of foo) with target foo (that is, a "sub-PR" of #1), then if #1 gets merged first (into main), #2's target branch will auto-update to main, which is exactly what you want.

If #2 gets merged first (into foo), then branch foo of course updates to include the commits in bar, which is again what you want.

What behaviour were you hoping to see?

Wow, I didn't realize github did this!

I was wrong about when GitHub performs the auto-update. It happens not when the PR is merged, but rather when its underlying branch is deleted. From https://docs.github.com/en/pull-requests/collaborating-with-...:

>Note: If you delete a head branch after its pull request has been merged, GitHub checks for any open pull requests in the same repository that specify the deleted branch as their base branch. GitHub automatically updates any such pull requests, changing their base branch to the merged pull request's base branch.

(This makes more sense, since if you keep the original branch around, it must be because you want to do more with it.)

Ah, I didn't know that, thanks. Does it work branch "foo" is not in the same repository as "main" (eg. because you don't have write access to the repository)?

I haven't tried that specific case, but there's no reason for it not to work -- if #1 (you/foo) gets merged first, the only automatic modification that needs to happen is to change #2's target branch from you/foo to them/main, which is no problem since that target branch is just metadata that lives in your repo. (Merging #2 afterwards would again need to be done by someone with write access to them/main, as you'd expect.)

I mean, if both branches "foo" and "bar" are in your fork, you can't open a PR in the upstream repo, can you? As soon as I try, GitHub redirects me to my fork to create the PR there

bar is a descendant of foo, correct? That's the situation I'm talking about.

Assuming that's the case, you want to make #2 a local PR anyway -- that is, its source is you/bar and its target is you/foo. Once #1 gets merged to them/main (by someone with the access to the upstream repo, "them"), #2 will (I predict, based on reasons I've already given) automatically become a PR "in" the repo them. That is, its target branch will automatically change to them/main.

I just tested, and it does not: after merging https://github.com/ZN3val/testrepo/pull/1, https://github.com/progval/testrepo/pull/2 is not updated.

Thanks for checking, it seems I was wrong about when GitHub performs the auto-update. It happens not when the PR is merged, but rather when its underlying branch is deleted. From https://docs.github.com/en/pull-requests/collaborating-with-...:

>Note: If you delete a head branch after its pull request has been merged, GitHub checks for any open pull requests in the same repository that specify the deleted branch as their base branch. GitHub automatically updates any such pull requests, changing their base branch to the merged pull request's base branch.

(This makes more sense, since if you keep the original branch around, it must be because you want to do more with it.)

So try deleting the branch underlying your PR #1. I think #2's target should then auto-update.

It auto-closed PR #2 and did not open a new one. Also, interestingly, there was no button on PR #1 to delete the branch from there as usual, I had to go to https://github.com/progval/testrepo/branches

Too late to edit my post unfortunately so replying instead:

I was wrong about when GitHub performs the auto-update. It happens not when the PR is merged, but rather when its underlying branch is deleted. From https://docs.github.com/en/pull-requests/collaborating-with-...:

>Note: If you delete a head branch after its pull request has been merged, GitHub checks for any open pull requests in the same repository that specify the deleted branch as their base branch. GitHub automatically updates any such pull requests, changing their base branch to the merged pull request's base branch.

(This makes more sense, since if you keep the original branch around, it must be because you want to do more with it.)

If your PR is that big and has that many interconnected pieces, then it should probably be broken up and serially merged anyway. Otherwise it becomes a pain for the reviewers to read, and a pain to make any large changes to.

Huh, Gitlab does have "depends on MR" merging which will block until all the dependent MRs have been merged. Including on other projects.

Either the second one includes the first one's commits so the second one's diff is unreadable, or it doesn't and the CI fails because it's missing the features introduced by the first one (or merge-conflicts).

You can have merge trains where it will build the MR based on the MRs that are merging to master.