Re: [PATCH 1/2] sha1_name.c: signal if @{-N} was a true branch nameor a detached head

[Jeff King <peff@peff.net>]

On Wed, May 08, 2013 at 02:12:29PM -0700, Junio C Hamano wrote:

> The original API read "checkout: moving from (.*) to ..." from the
> reflog of the HEAD, and returned the substring between "from" and
> "to", but there was no way, if the substring was a 40-hex string, to
> tell if we were on a detached HEAD at that commit object, or on a
> branch whose name happened to be the 40-hex string.
> 
> At this point, we cannot afford to change the format recorded in the
> reflog, so introduce a heuristics to see if the 40-hex matches the
> object name of the commit we are switching out of.  This will
> unfortunately mishandle this case:
> 
> 	HEX=$(git rev-parse master)
> 	git checkout -b $HEX master
> 	git checkout master

I do not think I've ever seen a 40-hex branch name in practice, but I
would think a branch named after the commit tip would be a reasonably
common reason to have one, and would trigger this case.

Since the point of marking the detached HEAD is to turn off things like
"@{-1}@{u}", we would want to be generous and err on the side of
assuming it is a branch if it _might_ be one. IOW, shouldn't we treat
the above sequence as a branch, and therefore mishandle:

  git checkout $HEX^0 master
  git checkout master

by erroneously assuming that we moved to the branch $HEX?

-Peff