Why does git branch -t fail with "Not tracking: ambiguous information"?

Question

When I try to create a new branch tracking a remote branch, I get this:

che@nok ~/prj/git-ipc $ git branch -t test main/some_remote_branch
error: Not tracking: ambiguous information for ref refs/remotes/main/some_remote_branch

The source seems to somehow search for branches to track and throws me out because it finds less more than one, but I don't exactly get what it's looking for since I already told it what to track on the command line.

Can anybody tell me what's going on and how to fix it?

Answer 1

because it finds less than one

Nope: because it finds more than one matching remote branch, which means the function remote_find_tracking() returns more than one tracking branch for a given local branch ref.

Is some_remote_branch not already tracked by one of your local branches?
(a git config -l would allow you to check what you have currently have set up).
(a git branch -r can also help to list your current remote-tracking branches. )

---

remote branches, which I thought are something different that remote-tracking branches.

Wrong, as illustrated by this thread:

remote-branches are the "real" remote-tracking-branches. You don't commit to them locally, they are essentially read-only copies of exactly what is happening in a remote repository.
If you try to 'git-checkout' a remote-tracking branch, you will get a detached HEAD.

Local branch:
A branch to which you may commit changes. Optionally, the branch can be configured to "follow" one of your remote-tracking branches. This means that a 'git-pull' without arguments (when your local branch is checked out), will automatically 'git-fetch' and then 'git-merge' the remote-tracking branch.

Now:

it's the job of git-fetch to update remote-tracking branches with any changes found in the remote repository.
Git-pull runs git-fetch and then runs a git-merge to update the currently-checked-out branch.

The problem is, for git-merge:

When this happens, git-merge must decide which remote-tracking-branch to merge into the currently checked out local branch.
You can set which remote-tracking-branch will be selected in this situation with the --track option.

--track sets up a local following branch to refer to a remote's branch, not to the tracking branch

Consider that remote_find_tracking() takes a single remote and a refspec with src filled, and returns the given refspec after filling its dst, if an appropriate tracking was configured for the remote, meaning git.

/*
 * For the given remote, reads the refspec's src and sets the other fields.
 */
int remote_find_tracking(struct remote *remote, struct refspec *refspec);

May be it considers it already has a local following branch matching some_remote_branch. Do you have any local branch with that exact same name?
Or, the other way around: your current branch has a remote branch with a similar name, which makes it a natural candidate for any git-merge: trying to make it track another remote branch would make the git-merge unable to choose which local branch to update/merge with changes of a remote.

Answer 2

Got it! The problem was that I have previously set up a remote with --mirror, for the purpose of having a backup / public copy of my repository.

If you run

git remote add --mirror <name> <url>

it does not only flag the remote as mirror (which is what I wanted for pushes), but also configures remote.<mirror>.fetch option for the mirror to +refs/*:refs/*, which means that all of your branches suddenly "track" your mirror repository and any attempt to create a tracking branch is going to fail.

(As an added bonus, running git fetch <mirror> is going to overwrite all your refs with old ones from your backup repo.)

The solution that seems to fix this issue is setting remote.<mirror>.fetch to : (which, I hope, means "never fetch anything"). This apparently fixes the tracking issue and eliminates the deadly fetch.

Answer 3

I saw this also when I had two remote repo's with the same (default) fetch pattern (fetch = +refs/heads/*:refs/remotes/origin/*) and the same branches.

I still haven't worked out how to fix it properly, since I want to continue to push/pull to both repo's, but manually adding the info to the project's .git/config works, e.g.:

[branch "mybranch"]
  remote = origin
  merge = refs/heads/mybranch