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

[Jeff King <peff@peff.net>]

On Thu, May 09, 2013 at 10:08:24AM -0700, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > 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.
> 
> I am not sure X and Y mesh well in your "Since X, we would want Y".
> It seems to argue for erring on the side of detached HEAD to me.

Thinking on it more, I don't see that one is actually better than the
other. If you claim a detached HEAD when there isn't one, the user says
"stupid git, that was a branch, and you should tell me its upstream".
But if you claim an undetached HEAD when there isn't one, asking for the
upstream provides wildly inaccurate results (e.g., "git checkout
@{-1}@{u}" taking you somewhere unexpected).

> Checking the "from" name $HEX against old_sha1 is a local and cheap
> measure I added there for the first level of disambiguation.  If
> they do not match, we _know_ we didn't come back from a detached
> HEAD state.
> 
> In order to err on the "favor branch when it could have been one",
> you could additionally look for the reflog .git/logs/refs/heads/$HEX
> when the "from" name $HEX matches old_sha1 (which is likely to be
> detached, but it is possible that we were on the $HEX branch when
> its tip was at $HEX) and making sure the tip of that $HEX branch
> once used to be at $HEX at the time recorded for @{-N} in the HEAD
> reflog in question.

I was thinking in terms of @{-1}@{u}, so that you could say "well, do we
have upstream config for such a branch currently?". Because even though
we are digging into history (and it _may_ have been a branch at the
time, but isn't now), if we are ultimately going to ask about the
upstream config (as it is _now_, not when the entry was made), then it
does not matter if the branch was detached or not: we are still going to
return failure either way.

But there are _other_ uses for @{-1}, and I am probably being too
focused on this one use-case.

So given all of the above, I think I am fine with the direction of the
series.

-Peff