Re: worktree add already exists

[Eric Sunshine <sunshine@sunshineco.com>]

On Mon, May 27, 2019 at 11:32 AM Ingo Wolf <ingo.wolf@gmx.de> wrote:
> $ ls -a barework
> ./  ../  test.txt
> $ git -C bare worktree add --no-checkout ../barework
> Preparing worktree (new branch 'barework')
> fatal: '../barework' already exists
> $ git -C bare branch
>    barework
> * master
> Why this doesn't just work and if not why is barework branch made then,
> why at all ?

It is by design that "git worktree add" (in general) fails if the
target directory already exists and is non-empty. This is consistent
with how "git clone" behaves. As for why your particular use-case
isn't directly supported, it's likely that nobody has yet asked for
it, and nobody thought about this particular case when --no-checkout
was added (which came some time after basic "git worktree add" itself
was implemented).

It is an accident of implementation that the new branch gets created
before "git worktree add" errors out due to the existing non-empty
directory (and, likely, nobody complained about it, so it went
unnoticed). This particular issue probably can be easily fixed now
that the logic for checking if the target directory can be a valid
worktree has been factored out of the code which actually creates the
new directory[1].

[1]: 45059e6468 (worktree: prepare for more checks of whether path can
become worktree, 2018-08-28)

> I would like to attach an existing dir to git (make it a workdir) and
> then update the index with git reset and checkin the differences.

I haven't thought through the possible ramifications, but the actual
implementation might be as simple as changing this code in
builtin/worktree.c:validate_worktree_add():

    if (file_exists(path) && !is_empty_dir(path))
        die(_("'%s' already exists"), path);

to:

    if (opts->checkout && file_exists(path) && !is_empty_dir(path))
        die(_("'%s' already exists"), path);

or something.