kart merge doesn't work with shallow clones

[olsen232]

Trying to merge two commits that share an ancestor, and that ancestor has a parent, and that parent is missing due to being a shallow clone, fails with the following error:

  ...
  File "kart/merge.py", line 313, in merge
  File "kart/merge.py", line 57, in do_merge
_pygit2.GitError: object not found - no match for id (e9cdf8481b46172e8cb21b0696747f9672eef66e)

It's not clear why this wouldn't work - I'll do some digging. It fails within libgit2 function git_merge_trees somewhere.

There is no issue with merging in a shallow clone as long as both the ancestor and the ancestor's parent are both present. So, this can be avoided by doing a clone one deeper than you think you need - if you think you need a clone with depth=1, clone it with depth=2 so that you have one more parent and everything will work fine.

There may also be an issue where merges fail if the ancestor itself is missing due to shallow-clone - this is more understandable / unavoidable however, since, without the merge-ancestor, we don't know how each branch has changed, so we don't really know what we are merging. In this case, the fix would be to deepen the clone at least as far as the merge ancestor before performing the merge.

This was found as part of spatial filtering testing but is not actually related to spatial filtering and is not a regression - all versions of Kart have this issue.

[olsen232]

Correction - this fails within libgit2 function git_merge_base

[olsen232]

Related - libgit2 shallow-clones tracking issue libgit2/libgit2#3058