Subject: [PATCH v2] merge-recursive: fix the diff3 common ancestor label for virtual commits

From: Elijah Newren <[email-masked]>
Date: Mon, 30 Sep 2019 23:58:49 -0700

In commit 743474cbfa8b ("merge-recursive: provide a better label for
diff3 common ancestor", 2019-08-17), the label for the common ancestor
was changed from always being

         "merged common ancestors"

to instead be based on the number of merge bases:

    >=2: "merged common ancestors"
      1: <abbreviated commit hash>
      0: "<empty tree>"

Unfortunately, this did not take into account that when we have a single
merge base, that merge base could be fake or constructed.  In such
cases, this resulted in a label of "00000000".  Of course, the previous
label of "merged common ancestors" was also misleading for this case.
Since we have an API that is explicitly about creating fake merge base
commits in merge_recursive_generic(), we should provide a better label
when using that API with one merge base.  So, when
merge_recursive_generic() is called with one merge base, set the label
to:

         "constructed merge base"

Note that callers of merge_recursive_generic() include the builtin
commands git-am (in combination with git apply --build-fake-ancestor),
git-merge-recursive, and git-stash.

Reported-by: Jeff King <[email-masked]>
Signed-off-by: Elijah Newren <[email-masked]>
---
Applies to the top of en/merge-recursive-cleanup, which is in next.

Changes since v1:
  - We only had a problem if the number of fake merge bases was exactly
    one; update the patch to check for that and update the commit message
    accordingly.

Range-diff:
1:  e3b5015985 ! 1:  3f4444bfd7 merge-recursive: fix the diff3 common ancestor label for virtual commits
    @@ Commit message
         Unfortunately, this did not take into account that when we have a single
         merge base, that merge base could be fake or constructed.  In such
         cases, this resulted in a label of "00000000".  Of course, the previous
    -    label of "merged common ancestors" was also misleading for these cases.
    -    Since we have an API that is explicitly about creating fake commits in
    -    merge_recursive_generic(), we should provide a better label when using
    -    that API.  So, when merge_recursive_generic() is called, set the label
    +    label of "merged common ancestors" was also misleading for this case.
    +    Since we have an API that is explicitly about creating fake merge base
    +    commits in merge_recursive_generic(), we should provide a better label
    +    when using that API with one merge base.  So, when
    +    merge_recursive_generic() is called with one merge base, set the label
         to:
     
                  "constructed merge base"
     
    -    Note that users of merge_recursive_generic include the builtin commands
    -    git-am (in combination with git apply --build-fake-ancestor),
    +    Note that callers of merge_recursive_generic() include the builtin
    +    commands git-am (in combination with git apply --build-fake-ancestor),
         git-merge-recursive, and git-stash.
     
         Reported-by: Jeff King <[email-masked]>
    @@ merge-recursive.c: int merge_recursive_generic(struct merge_options *opt,
      					   oid_to_hex(merge_bases[i]));
      			commit_list_insert(base, &ca);
      		}
    -+		opt->ancestor = "constructed merge base";
    ++		if (num_merge_bases == 1)
    ++			opt->ancestor = "constructed merge base";
      	}
      
      	repo_hold_locked_index(opt->repo, &lock, LOCK_DIE_ON_ERROR);

 merge-recursive.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/merge-recursive.c b/merge-recursive.c
index b058741f00..e12d91f48a 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -3550,6 +3550,8 @@ static int merge_recursive_internal(struct merge_options *opt,
 		merged_merge_bases = make_virtual_commit(opt->repo, tree,
 							 "ancestor");
 		ancestor_name = "empty tree";
+	} else if (opt->ancestor) {
+		ancestor_name = opt->ancestor;
 	} else if (merge_bases) {
 		ancestor_name = "merged common ancestors";
 	} else {
@@ -3689,7 +3691,8 @@ int merge_recursive(struct merge_options *opt,
 {
 	int clean;
 
-	assert(opt->ancestor == NULL);
+	assert(opt->ancestor == NULL ||
+	       !strcmp(opt->ancestor, "constructed merge base"));
 
 	if (merge_start(opt, repo_get_commit_tree(opt->repo, h1)))
 		return -1;
@@ -3741,6 +3744,8 @@ int merge_recursive_generic(struct merge_options *opt,
 					   oid_to_hex(merge_bases[i]));
 			commit_list_insert(base, &ca);
 		}
+		if (num_merge_bases == 1)
+			opt->ancestor = "constructed merge base";
 	}
 
 	repo_hold_locked_index(opt->repo, &lock, LOCK_DIE_ON_ERROR);
-- 
2.23.0.25.ge3b5015985.dirty