Re: [PATCH] Detached HEAD (experimental)

From: Junio C Hamano <junkio@cox.net>
Date: Jan 9, 2007, 22:37 UTC
Message-ID: <7vy7obj07k.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <20070109213117.GB25012@fieldses.org>

"J. Bruce Fields" <bfields@fieldses.org> writes:

Show 25 quoted lines

> On Tue, Jan 09, 2007 at 01:20:27PM -0800, Junio C Hamano wrote:
>> Jeff King <peff@peff.net> writes:
>> 
>> > For example, with what's in next now, I can do this:
>> >
>> >   git checkout v1.4.0
>> >   hack hack hack
>> >   git commit -m -a 'some changes which will never be seen again'
>> >   git checkout v1.2.0
>> >
>> > I thought the _point_ of the safety valve was not to lose those changes.
>> 
>> Fair enough.
>> 
>> We could always do the check upon "git checkout" from a detached
>> HEAD state, whether it takes you back on some existing branch or
>> leaves your HEAD still detached.
>
> Stupid question: why can't checkout do something like this?
>
> 	if we're currently not on a branch, fail if .git/PREV
> 		doesn't point to the same commit as .git/HEAD.
>
> 	if we're checking out a non-branch, store its SHA1 into
> 		.git/PREV.

I do not want to think about the consequences of adding more cruft under .git/ directory. For example, should PREV be noticed by fsck and prune? What should various forms of 'git-reset' do with it? How does it interact with 'git-bisect'?

Being able to test merge or even make commits without being on a branch is vastly useful. It might or might not lead to anywhere even after you make a handful commits -- and I would imagine that it would be very handy to be able to be lazy and not having to decide if it is worth a new branch.

But that may be just my imagination; I generally prefer any feature that allows me to defer decision over something that makes me decide early. If Carl wants to do a patch to teach 'git-commit' (and all other things that can create commits) not to do things from working in a detached HEAD, I would probably not opposed to it too much, but I am fairly certain that I won't be coding it myself.

It's tempting to forget about this whole "safety" business. Because we allow "reset --hard" and other forms of operations that can lose history if they were done while on a branch, only giving the safety to "git checkout" feels somewhat silly. And the primary motive for detached HEAD as I understand it is for sightseeing, and not allowing "reset --hard" to jump around is just plain silly.

That is, after:

	git checkout v1.4.0

you are not on any branch, and we would still allow

	git reset --hard v1.2.0

which is exactly the same as:

	git checkout v1.2.0

You can still say:

	git checkout master

and we do not even check.

Which makes the "merge-base --check-ancestry" stuff I did last night pretty much unnecessary, but that's Ok. It will find other uses.