Re: unexpected git-merge result

From: Junio C Hamano <gitster@pobox.com>
Date: Feb 19, 2010, 22:27 UTC
Message-ID: <7vaav4amjj.fsf@alter.siamese.dyndns.org>
In-Reply-To: <loom.20100219T202452-35@post.gmane.org>

Dale Rowley <ddrowley3@juno.com> writes:

Show 15 quoted lines

> Our branch history looked something like the following:
>
>   B---C
>  /     \
> A---D---E---H
>      \     /
>       F---G
>
> In commit D a few lines were added to a file, and then in commit G the same
> lines were removed. When branches E and G were merged, those lines unexpectedly
> re-appeared in the merge commit (H), even though no other commits had touched
> that file.
>
> I think I understand why this happened: git-merge-base reports that the
> common ancestor for E and G is A.

Do you?  I don't.

D is the merge base between E and G. A and D are both common ancestor between E and G, but because D is a descendant of A, A is not a merge base (the definition of merge-base is "common ancestors none of whose children is common ancestor"). A is not.

Are you sure you really have ancestry relationship between D and G? If the history were like this:

   B---C
  /     \
 A---D---E---H
  \         /
   F-------G

and both D and F add the same set of "few lines", and G removes it, then I can understand how you would get these "removed by G" lines appear in H, as the side branch F--G didn't have no net effect on these lines and D is the only one that added them.

It can happen if F is a cherry-pick of D and G is a revert of F, for example.