Re: master^ is not a local branch -- huh?!?

From: Nicolas Pitre <nico@fluxnic.net>
Date: Feb 1, 2010, 23:25 UTC
Message-ID: <alpine.LFD.2.00.1002011809140.1681@xanadu.home>
In-Reply-To: <ron1-6F8B85.14520801022010@news.gmane.org>

On Mon, 1 Feb 2010, Ron Garret wrote:

Show 22 quoted lines

> In article <87aavsu9b3.fsf@osv.gnss.ru>, Sergei Organov <osv@javad.com> 
> wrote:
> 
> > Junio C Hamano <gitster@pobox.com> writes:
> > > Steve Diver <squelch2@googlemail.com> writes:
> > 
> > [...]
> > 
> > > If read carefully (some may argue that it does not need a very careful
> > > reading to get it, though), this hints that "detached HEAD" state is a
> > > substitute for using a temporary branch, but it may not be strong
> > > enough.
> > 
> > For my rather fresh eye it looks more like unnamed (anonymous?) branch
> > than a temporary one. Doesn't detached HEAD behave exactly like a
> > regular HEAD but pointing to the tip of an unnamed branch?
> 
> I strongly concur with this.
> 
> And as long as I'm weighing in, it would also help to prevent confusion 
> if it were made clear that this unnamed branch doesn't actually come 
> into existence unless and until you do a commit.

Nope. Creating a commit doesn't create any branch. A commit creation merely adds a new node in the history graph, and links it to the commit that was the current one before that commit operation. If HEAD is _attached_ to a branch then the branch pointer is also updated to point to that new commit. If HEAD is _detached_ then no branch is updated and HEAD simply carries a direct reference to that new commit.

At a later time you can:

1) Create a new branch pointer which default value is the commit pointed to
   by HEAD.  This is true whether or not HEAD is detached, but in this 
   case this is an interesting property.

2) Move HEAD somewhere else by performing a checkout.  If HEAD was 
   detached then its last position is simply forgotten and those 
   commits that were performed while HEAD was detached, if any, are 
   simply left dangling and eventually garbage collected.  If however a 
   new branch pointer was created in (1) then those commits won't be 
   dangling.

In any case, a detached HEAD is not only a temporary branch, it is also a volatile branch. And in the Git model, it is simply not a branch at all. Hence the 2 states for HEAD: either detached, or attached to a branch pointer.

Nicolas