Subject: Re: Able to checkout same branch in multiple worktrees when using symbolic-refs

From: Junio C Hamano <[email-masked]>
Date: Fri, 29 Apr 2022 13:19:08 -0700

Austin Morton <[email-masked]> writes:

> When using a symbolic-ref I am able to inadvertently checkout the same
> branch in multiple worktrees when using the symbolic-ref name, despite
> being prevented from doing so if I use the target branch name.

Don't do that if it hurts ;-)

If you checked out 'main' in the secondary worktree and made a
commit there, the symbolic-ref makes 'master' to be updated,
confusing the primary worktree whose index and the working tree
files record work relative to the old commit before the secondary
worktree updated it via the symbolic-ref, which is exactly the same
situation that the "do not check out the same branch in two
different worktrees" protection tries to save the user from, but I
do not think we currently do so.

Because this "do not check a branch out twice and let it be futzed
with from the two places" is primarily a mechanism to help
inexperienced users from getting confused (and there is an escape
hatch mechanism to allow it to those who know what they are doing),
and use of symbolic-ref to make 'main' and 'master' synonyms with
each other is not something inexperienced users who have no clue on
what they are doing would be doing anyway, it may not be a huge
deal.

I also suspect there is no way, other than scanning _all_ the local
branches, to see if some other branch aliases the branch we are
about to check out.  It may potentially be somewhat expensive.

But it would be nice if it can get fixed in inexpensively.

Thanks for a report.

> Below is a minimal reproducer:
>
> $ git --version
> git version 2.36.0
> $ git init .
> $ git status
> $ git commit --allow-empty -m "Initial commit"
> $ git symbolic-ref refs/heads/main refs/heads/master
> $ git worktree add ../worktree2
> $ git worktree list
> /home/amorton/test/worktree1 cd8312d [master]
> /home/amorton/test/worktree2 cd8312d [worktree2]
> $ cd ../worktree2
> $ git checkout master
> fatal: 'master' is already checked out at '/home/amorton/test/worktree1'
> $ git checkout main
> Switched to branch 'main'
> $ git worktree list
> /home/amorton/test/worktree1 cd8312d [master]
> /home/amorton/test/worktree2 cd8312d [master]