Jeff King [off-list ref] 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.
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.
That is far more expensive, though; I doubt it is worth doing.
On Thu, May 09, 2013 at 10:08:24AM -0700, Junio C Hamano wrote:
Jeff King [off-list ref] writes:
quoted
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