Re: git subtree bug produces divergent descendants

[David Ware <davidw@realtimegenomics.com>]

Sorry for the double post, I received a mail blocking notification
message (due to the attached .sh file) and erroneously thought this
message had been blocked from the entire list. My later one includes
the test case as part of the attached patch.

Cheers,
Dave Ware

On Mon, Dec 7, 2015 at 9:41 AM, David Ware <davidw@realtimegenomics.com> wrote:
> My group has run into a bug with "git-subtree split". Under some
> circumstances a split created from a descendant of another earlier
> split is not a descendant of that earlier split (thus blocking
> pushes). We originally noticed this on v1.9.1 but have also checked it
> on v2.6.3
>
> When scanning the commits to produce the subtree it seems to skip
> creating a new commit if any of the parent commits have the same tree
> and instead uses that tree in its place. This is fine when the cause
> is a branch that did not cause any changes to the subtree.  However it
> creates an issue when the cause is both branches ending up with the
> same tree through identical alterations (or more likely, one of the
> branches has just a subset of the alterations on the other, such as a
> branch just containing cherry-picks).
>
> The attached bash script (makerepo.sh) reproduces the problem. To use
> create an empty directory and run the script in it. The resulting
> 'master' branch has had the latest commits on the 'branch' branch
> merged into it, so it follows that a subtree on 'folder/' at 'master'
> should contain all the commits of a subtree on 'folder/' at 'branch'.
> (These subtrees have been produced at 'subtree_tip' and
> 'subtree_branch' respectively.)
>
> The attached patch (against v2.6.3) fixes the issue for the cases
> we've encountered, however since we're not particularly familiar with
> git internals we may not have approached this optimally. We suspect it
> could be improved to also handle the cases where there are more than 2
> parents.
>
> Cheers,
> Dave Ware