BUG() during criss-cross merge with directory rename and deleted file

From: Emily Shaffer <emilyshaffer@google.com>
Date: Jul 26, 2019, 22:09 UTC
Message-ID: <20190726220928.GG113966@google.com>

Hi all,

As mentioned during the standup in #git-devel today, we encountered a BUG() during certain circumstances of a criss-cross merge. Timing has prevented me from getting the repro case together to send just yet, but it appears that the issue is as follows:

 - During a merge in a repo with fairly complicated, merge-y history,
   the following BUG() is seen:
   "BUG: There are unmerged index entries:
    BUG: 2 baz/bar.txt
    BUG: merge-recursive.c:429: unmerged index entries in
    merge-recursive.c
    Aborted"
 - But when the user then runs `git status` (after clearing
   .git/index.lock) the directory is clean.
 - The repo does not contain a 'baz/bar.txt' (although 'baz/' exists).
 - In the repo's history a 'foo/bar.txt' can be found (that is the only
   'bar.txt' to ever exist in the project).

Digging further shows:

 - The merge had  multiple closest shared ancestors (criss-cross
   merge)
 - Directory 'foo/' was renamed on one side to 'baz/'...
 - ...and 'foo/bar.txt' was deleted on the other side.
 - When the virtual ancestor is generated, the directory rename can't be
   resolved and leaves a conflict.
 - The virtual ancestor being written to disk is in a conflicted state.
 - This causes the top-level merge to fail, printing the BUG() above
   which references a 'baz/bar.txt' that never really lived there.

Furthermore...

 - If merge.directoryrenames is set to any value besides "conflict", the
   merge succeeds (no BUG() generated).

This seems to have been introduced in 8c8e5bd6eb331d055aa7fa6345f6dcdadd658979 although I haven't bisected it yet.

It seems like the solution is to watch for whether we're making a virtual ancestor during a recursive merge, and if so, to treat merge.directoryrenames = "conflict" as "true" or "false" instead.

I plan to modify the tests added in 8c8e5bd6 to highlight this issue (just haven't had time, and probably won't til next week), and send a patch doing the special treatment on merge.directoryrenames.

Happy to hear other ideas folks have.

 - Emily