Re: git merge commits are non-deterministic? what changed?

[Michael J Gruber <git@drmicha.warpmail.net>]

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 <uqs@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.

Michael