Bug Report

Today, I managed to create a duplicate branch in a git repository. While this may not be a bug per se, I do think that it is confusing and some way of preventing such issues in the future may be helpful.

I first cloned the repository:

$ git clone https://github.com/CodeForChicago/superclass.git

Then, I created a new branch (or so I thought):

$ git checkout -b lesson_page

However, this branch has already existed for about 4 weeks, without my knowledge. I proceeded to do some work on the files it contained, and when it came time to commit and push, and when I pushed, I got the following message:

To https://github.com/CodeForChicago/superclass.git
 ! [rejected]        lesson_page -> lesson_page (non-fast-forward)
error: failed to push some refs to '
https://github.com/CodeForChicago/superclass.git'
hint: Updates were rejected because the tip of your current branch is behind
hint: its remote counterpart. Integrate the remote changes (e.g.
hint: 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

Given that I had believed that I had created the branch just a few hours prior and was the first to attempt to push to it, this error was consternating.

I may be wrong (I am aware that my understanding of git is limited), but I believe that the git checkout -b command is simply supposed to create a new branch and then switch to it (I'm not aware of any subtle behavior that goes on behind the scenes if the "new" branch that the user is attempting to create already exists). This is why I said it "may not be a bug per se". However, I expect most people who use git to expect this command to create a new branch and then switch to it (this is what most sources online will tell users to do to create a new branch), and as such, it would be extremely beneficial if git were to, at the very least, alert the user to the conflict in some way or another.

Thanks, Ben

Lead Consultant, Northwestern University Information Technology Research Assistant, Center for Interdisciplinary Exploration and Research in Astrophysics at Northwestern University Phsyics, Weinberg College of Arts and Sciences Computer Science, Weinberg College of Arts and Sciences Classics, Weinberg College of Arts and Sciences

On Sat, Apr 2, 2016 at 8:25 PM, Benjamin Sandeen <benjaminsandeen2016@u.northwestern.edu> wrote:

This can be confusing.  I'll hopefully try to help and explain below.

At this point, you created a *local* branch called 'lesson_page' which points to the current HEAD and then switched to that branch . This local branch is independent of the remote branch called 'origin/lesson_page'.

The non-fast-forward push is preventing history being rewritten- this is a good thing :).

The `git checkout -b` command is working as expected. `git checkout -b <name>` is equivalent to `git branch <name> && git checkout <name>`. If you were to execute `git checkout lesson_page`, you would get the desired behavior you were expecting because in the presence of one remote, git will actually execute `git checkout -b lesson_page --track origin/lesson_page` - more details can be found in `git help checkout`. Effectively, it checkouts 'origin/lesson_page' as a new local branch named 'lesson_page'.

However, you indicated that you did not know there was a remote branch already named 'lesson_page'. After cloning the repository, you can use `git branch -a` to see all remotes to determine which form of `git checkout` to use.

Hi,

On Sat, Apr 2, 2016 at 5:25 PM, Benjamin Sandeen <benjaminsandeen2016@u.northwestern.edu> wrote:

The branch existed in the remote repository, but it doesn't exist locally. You never fetched a copy into refs/remotes/origin since you didn't say you were interested, and git will fully allow you to create local branches with the same name as remote branches.

When you tried to push this branch, it will push into refs/heads/lesson_page on the remote, which already exists. Since it cannot perform a fast-forward update, as your local work isn't based directly on the tip of the remote branch, you either need to merge, rebase, or start from scratch.

Let me try to explain. You created your own local branch, which happened to share the same name as an already existing branch. Had you know this you could have fetched and checked out that branch. You can view all branches using "git branch -a" or "git ls-remote".

git checkout -b <branch_name> will create a new branch in your local copy of the repository. Git is distributed. You can do "git checkout --track <branch>" to attempt to create a local branch which tracks the upstream branch, and then git status will provide useful information about the relationship between your local work and the remote work.

It could maybe be improved to notice that a remote has a branch with the same name. However, git can support multiple remotes, so determining which remote to care about is difficult.

In your case you have a couple of options to fix it. I would suggest at least

"git branch --set-upstream-to=origin/<branch_name>" so that git status will give you useful information about the branch relationship. Then you can merge or rebase your work into the branch.

The issue is in understanding how git distributes branches, and how it could handle this. I suspect improvements could be made so that it will attempt to warn you when you create a branch that already exists. However, often you do this *intending* to make it track the specific branch so I am not sure how much good a warning would do. Just a message wouldn't really hurt anything, however.

Thanks, Jake