Re: bug? in checkout with ambiguous refnames

On Fri, 7 Jan 2011, Jeff King wrote:

On Fri, Jan 07, 2011 at 11:46:50AM +0100, Uwe Kleine-K?nig wrote:

quoted

	ukl@octopus:~/gsrc/linux-2.6$ git diff; git diff --cached

	ukl@octopus:~/gsrc/linux-2.6$ git checkout sgu/mxs-amba-uart
	warning: refname 'sgu/mxs-amba-uart' is ambiguous.
	Previous HEAD position was c13fb17... Merge commit '65e29a85a419f9a161ab0f09f9d69924e36d940e' into HEAD
	Switched to branch 'sgu/mxs-amba-uart'

OK, it might be a bad idea to have this ambiguity, still ...
[...]
So working copy and cache are at refs/tags/sgu/mxs-amba-uart, HEAD
points to refs/heads/sgu/mxs-amba-uart

Yeah, we generally resolve ambiguities in favor of the tag (and that
warning comes from deep within get_sha1_basic). So the real bug here is
that it still said "Switched to branch", which is totally wrong.

That being said, it probably would make more sense for "git checkout" to
prefer branches to tags.

What was the rationale for generally favoring tags? Why does that
reasoning not apply to 'git checkout' too? At least without knowing
the answer to that question, I think I would prefer to have checkout
behave the same as the other commands do. Maybe a bit academic, but it
seems like it would be nice if e.g. 'git checkout $anything && git
show $anything' would show the HEAD commit and if 'git checkout
$anything && git diff HEAD $anything' was always empty.

Btw, what exactly does "generally" mean, i.e. which other commands
don't favor tags? I know rebase is one example of a command that does
not favor tags.

Slightly off topic, but why does 'git rev-parse --symbolic-full-name'
not output anything when the input is ambiguous? 'git rev-parse'
without any flags favors tags, so I would have expected to get
something like refs/tags/$name back.

The reason I'm asking is because I just happened to see in the rebase
code the other day that it will rebase a detached head if the <branch>
parameter is not "completely unqualified". For example 'git rebase
master heads/topic' or 'git rebase master refs/heads/topic' will not
update refs/heads/topic. I was trying to fix that by using 'git
rev-parse --symbolic-full-name' to parse <branch>. That seemed to work
fine until I saw this thread :-).