On Mon, Jan 16, 2023 at 12:30 PM Carlo Marcelo Arenas Belón
[off-list ref] wrote:

builtin/checkout: check the branch used in -B with worktrees

Thanks for working on this and coming up with a fix. As mentioned
earlier, I had started looking into it[1], but lacked the time to
disentangle the logic, so I'm glad to see a patch arrive so quickly.

When multiple worktrees are being used, checkout/switch check
that the target branch is not already checked out with code that
evolved from 8d9fdd7087 (worktree.c: check whether branch is rebased
in another worktree, 2016-04-22), but that logic wasn't valid for
-B/-C

Avoid reusing the same `branch_info` structure for the checks and
assumming that it will be rejected later if is a new branch that
already exists as that doesn't apply to -B/-C.

Even though I'm familiar with the bug report[2] which sparked this
patch, I find the above description somewhat hard to digest; the
high-level problem it is addressing doesn't jump off the page at me.
Perhaps it can be rewritten something like this:

    checkout/switch: disallow checking out same branch in multiple worktrees

    Commands `git switch -C` and `git checkout -B` neglect to check
    whether the branch being forcibly created is already checked out
    in some other worktree, which can result in the undesirable
    situation of the same branch being checked out in multiple
    worktrees. For instance:

        $ git worktree list
        .../foo    beefb00f [main]
        $ git worktree add ../other
        Preparing worktree (new branch 'other')
        HEAD is now at beefb00f first
        $ cd ../other
        $ git switch -C main
        Switched to and reset branch 'main'
        $ git worktree list
        .../foo    beefb00f [main]
        .../other  beefb00f [main]
        $

    Fix this problem by teaching `git switch -C` and `git checkout -B`
    to check whether the branch in question is already checked out
    elsewhere.

after which you might include some details which you wrote about initially.

This variable name change (`new_branch_info` => `check_branch_info`)
helps make the code clearer. Good. (I had found it more than a little
confusing to have similar named variables `new_branch_info` and
`opts->new_branch` even though they are unrelated and have very
different purposes.)

Although `git switch` and `git checkout` currently share
implementation, that might not always be the case going forward. As
such, this test could be made a bit more robust by testing both
commands rather than just `git-checkout`. So, perhaps:

    test_expect_success 'die the same branch is already checked out' '
        (
            cd here &&
            test_must_fail git checkout -B newmain &&
            test_must_fail git switch -C newmain
        )
    '

+test_expect_success 'not die on re-checking out current branch (checkout -B)' '
+       (
+               cd there &&
+               git checkout -B newmain
+       )
+'

Good to see you considered this case too. (I had tested it myself
manually when trying out your patch.)

[1]: https://lore.kernel.org/git/CAPig+cQc1+D9gH7BAC-r03bGKWx3a9jpPyLuP-ehH-X2P+fV6Q@mail.gmail.com/ (local)
[2]: https://lore.kernel.org/git/CAA3Q-aaO=vcZd9VLFr8UP-g06be80eUWN_GjygfyGkYmrLx9yQ@mail.gmail.com/ (local)