On Mon, 2012-11-12 at 12:27:31 +0100, Michael J Gruber wrote:
> Ulrich Spörlein venit, vidit, dixit 09.11.2012 19:27:
> > On Fri, 2012-11-09 at 11:16:47 -0500, Jeff King wrote:
> >> On Fri, Nov 09, 2012 at 04:52:48PM +0100, Matthieu Moy wrote:
> >>
> >>> Ulrich Spörlein <u...@spoerlein.net> writes:
> >>>
> >>>>>> 2. Why the hell is the commit hash dependent on the ordering of the
> >>>>>> parent commits? IMHO it should sort the set of parents before
> >>>>>> calculating the hash ...
> >>>>>
> >>>>> What would be the sort key?
> >>>>
> >>>> Trivially, the hash of the parents itself. So you'd always get
> >>>>
> >>>> ...
> >>>> parent 0000
> >>>> parent 1111
> >>>> parent aaaa
> >>>> parent ffff
> >>>
> >>> That would change the behavior of --first-parent. Or you'd need to
> >>> compute the sha1 of the sorted list, but keep the unsorted one in the
> >>> commit. Possible, but weird ;-).
> >>
> >> Right. The reason that merge parents are stored in the order given on
> >> the command line is not random or because it was not considered. It
> >> encodes a valuable piece of information: did the user merge "foo" into
> >> "bar", or did they merge "bar" into "foo"?
> >>
> >> So I think this discussion is going in the wrong direction; git should
> >> never sort the parents, because the order is meaningful. The original
> >> complaint was that a run of svn2git produced different results on two
> >> different git versions. The important question to me is: did svn2git
> >> feed the parents to git in the same order?
> >>
> >> If it did, and git produced different results, then that is a serious
> >> bug.
> >>
> >> If it did not, then the issue needs to be resolved in svn2git (which
> >> _may_ want to sort the parents that it feeds to git, but it would depend
> >> on whether the order it is currently presenting is meaningful).
> > 
> > Yeah, thanks, looks like I have some more work to do. I don't quite get
> > how it could come up with a different order, seeing that it is using svn
> > as the base.
> > 
> > Will run some more experiments, thanks for the info so far.
> There was a change in the order in which "git cherry-pick A B C" applies
> the commits. It's the only odering affecting change in 1.8.0 that I can
> think of right now.

Just to wrap this up, it was of course a "feature" of the converter,
that resulted in this unrepeatable behavior. The SVN API makes use of
apr_hashes, which were traversed in arbitrary order, hence SVN commits
spanning multiple git-branches would be handled in a non-deterministic
order, leading to randomly ordered parent objects for later git merge

It it still debatable, whether a merge commit should have a
list-of-parents or a set-of-parents. Changing it to a set-of-parents
(with a well-defined hash function), would have made this problem go

But this will never be changed, it would break the fundamental git
storage model as it is in place now.

To unsubscribe from this list: send the line "unsubscribe git" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Reply via email to