gh repo sync should be able to sync a local branch with an upstream remote

[benebsiny]

Fixes #8100

Suppose there are 3 remotes

$ git remote -v
origin     git@github.com:me/project.git (fetch)
origin     git@github.com:me/project.git (push)
upstream   git@github.com:owner/project.git (fetch)
upstream   git@github.com:owner/project.git (push)
stranger   git@github.com:strangers/strangers.git (fetch)
stranger   git@github.com:strangers/strangers.git (push)

The orgin is upstream's fork, and local repo is cloned from origin. Besides, the stranger has nothing to do with the origin, upstream, and local ones.

If you are trying gh repo sync --source owner/project, the program will enter sync.go#236 block. Next, check if this remote has the same ancestor as local repo, and this is done by func HasCommonAncestor (sync.go#237), which runs git merge-base branch_name FETCH_HEAD (Git Doc). Since the local repo is a fork of upstream, func HasCommonAncestor should return true. Then the program continues.

If you are trying gh repo sync --source strangers/strangers, func HasCommonAncestor should return false. Therefore, an error message is shown: can't sync because master is not tracking strangers/strangers

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Assume that the upstream is removed by git remote remove upstream, then run gh repo sync --source owner/project, an error message is shown: can't sync because master is not tracking owner/project.
So, should this PR also handle (try to sync) the remotes which are not listed in git remote? Or just show the aforementioned error message?

If there's something unclarity, please let me know, thank you!

[samcoe]

@benebsiny Appreciate you getting started on this work and apologies for the delayed review. I think this implementation is a bit more complicated than it needs to be unless I have missed something. Perhaps you can explain why git.HasCommonAncestorAncestor is necessary?

My thoughts are that we can simply remove the mismatch remotes check to resolve #8100. The git.IsAncestor check should continue to work properly and make sure the user only gets a fast forward merge when not using the --force flag.

[benebsiny]

@samcoe Thanks for your feedback!

The reason why I add git.HasCommonAncestorAncestor is to distinguish the mismatchRemotesError and divergingError. The
HasCommonAncestor is to check the mismatchRemotesError, and git.IsAncestor is to check the divergingError.

In my opinion, mismatchRemotesError should be raised when the --source has nothing to do with the local repo, and the divergingError should be raised when the --source and the local are diverged.

---

Consider this case:
If the local is 1 commit behind the remote, and then I create a new commit at local. Next, I try the sync command with source flag, the git.HasCommonAncestor will return true. Thus mismatchRemotesError will not be raised. And git.IsAncestor will return false, so the divergingError will be raised.

---

Another case:
The --source has nothing to do with the local, the git.HasCommonAncestor should return false, so the mismatchRemotesError is thrown.

[samcoe]

@benebsiny Thanks for the explanation, I understand better now.

In my opinion, mismatchRemotesError should be raised when the --source has nothing to do with the local repo, and the divergingError should be raised when the --source and the local are diverged.

I think this is where we are diverging a bit. I think this is overly defensive behavior from gh. The user configured their remotes, if they have them set up in such a way that they do not align then it is not for gh to complain about. Having mismatching remotes should not prohibit the user from being able to do gh repo sync, they will likely have to use the --force flag but it is their choice to do so.

[benebsiny]

@samcoe Thanks for your feedback! I've removed the checking for the mismatchRemotesError. Since there's no usage of mismatchRemotesError, I also remove the mismatchRemotesError variable. Have a look at the latest update.

[samcoe]

@benebsiny Thanks for the effort on this PR! I pushed a small polish 💅 commit and I think this is ready to 🚢