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Re: [PATCH 1/1] worktree refs: fix case sensitivity for 'head'

On Fri, Dec 14, 2018 at 7:50 AM Jacob Keller <jacob.keller@xxxxxxxxx> wrote:
> On Thu, Dec 13, 2018 at 1:16 PM Mike Rappazzo <rappazzo@xxxxxxxxx> wrote:
> >
> > On Thu, Dec 13, 2018 at 3:48 PM Stefan Beller <sbeller@xxxxxxxxxx> wrote:
> > >
> > > > > The current situation is definitely a problem.  If I am in a worktree,
> > > > > using "head" should be the same as "HEAD".
> > >
> > > By any chance, is your file system case insensitive?
> > > That is usually the source of confusion for these discussions.
> >
> > This behavior is the same for MacOS (High Sierra) and Windows 7.  I
> > assume other derivatives of those act the same.
> >
> > On CentOS "head" is an ambiguous ref.  If Windows and Mac resulted in
> > an ambiguous ref, that would also be OK, but as it is now, they return
> > the result of "HEAD" on the primary worktree.
> >
> Because refs are *not* case sensitive, and we know that "HEAD" should
> be per-worktree, it gets checked in the per-worktree refs section. But
> lowercase head is known to not be a per-worktree ref, so we then ask
> the main worktree about head. Since you happen to be on a case
> insensitive file system, it then finds refs/HEAD in the main refs
> worktree, and returns that.
> I don't understand why the CentOS shows it as ambiguous, unless you
> actually happen to have a ref named head. (possibly a branch?)

I think it's just our default answer when we can't decide

$ git rev-parse head
fatal: ambiguous argument 'head': unknown revision or path not in the
working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
$ git rev-parse head --
fatal: bad revision 'head'

> I suspect we could improve things by attempting to figure out if our
> file system is case insensitive and warn users. However, I recall
> patches which tried this, and no suitable method was found. Partly
> because it's not just "case" that is the only problem. There might be
> things like unicode characters which don't get properly encoded, etc.
> The best solution would be to get a non-filesystem backed ref storage
> working that could be used in place of the filesystem.

Even with a new ref storage, I'm pretty sure pseudo refs like HEAD,
FETCH_HEAD... will forever be backed by filesystem. HEAD for example
is part of the repository signature and must exist as a file. We could
also lookup pseudo refs with readdir() instead of lstat(). On
case-preserving-and-insensitive filesystems, we can reject "head" this
way. But that comes with a high cost.