Re: [PATCH 1/5] t1508 (at-combinations): more tests; document failures

> On Thu, May 02, 2013 at 02:34:01AM +0530, Ramkumar Ramachandra wrote:
>
>> Junio C Hamano wrote:
>> > Just making sure.  HEAD@{$n} and @{$n} for non-negative $n mean
>> > totally different things.  @{0} and HEAD@{0} are almost always the
>> > same, and @{1} and HEAD@{1} may often happen to be the same, but as
>> > a blanket statement, I find "Since HEAD is implicit in @{}" very
>> > misleading.
>> 
>> When will they be different?  I'm looking at this from the parser's
>> point of view: when the part before @{} is missing, we dwim a "HEAD".
>
> The difference is that HEAD@{} refers to HEAD's reflog, but @{} refers
> to the reflog of the branch pointed to by HEAD. For example, try:
>
>   git init repo && cd repo
>   git commit --allow-empty -m one &&
>   git commit --allow-empty -m two &&
>   git checkout HEAD^ &&
>   git commit --allow-empty -m three &&
>   git checkout master &&
>   for i in 0 1 2; do
>     echo "HEAD@{$i}: $(git log -1 --oneline HEAD@{$i} 2>&1)"
>     echo "    @{$i}: $(git log -1 --oneline     @{$i} 2>&1)"
>   done
>
> which produces:
>
>   HEAD@{0}: 1fbb097 two
>       @{0}: 1fbb097 two
>   HEAD@{1}: 42f3f4d three
>       @{1}: 1089d0e one
>   HEAD@{2}: 1089d0e one
>       @{2}: fatal: Log for '' only has 2 entries.
>
> Unless your reflogs are screwed up, the 0th reflog should always point
> to the same commit (since you just moved HEAD there), but beyond that
> there is not necessarily any relation. And even for the 0th reflog
> entry, the reflog information is not the same. Try this on the repo
> above:
>
>   echo "HEAD@{0}: $(git log -g -1 --format='%gd %gs' HEAD@{0})"
>   echo "    @{0}: $(git log -g -1 --format='%gd %gs'     @{0})"
>
> which yields:
>
>   HEAD@{0}: HEAD@{0} checkout: moving from ... to master
>       @{0}: master@{0} commit: two
>
> -Peff