[PATCH v2 1/2] revision: ensure MERGE_HEAD is a ref in prepare_show_merge

[Michael Lohmann <mi.al.lohmann@gmail.com>]

This is done to
(1) ensure MERGE_HEAD is a ref,
(2) obtain the oid without any prefixing by refs.c:repo_dwim_ref()

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
---
Hi!

On 12. Jan 2024, at 01:15, Junio C Hamano <gitster@pobox.com> wrote:
> > This extends the functionality of `git log --merge` to also work with
> > conflicts for rebase, cherry pick and revert.
> > 
> > Signed-off-by: Michael Lohmann <mi.al.lohmann@gmail.com>
> > ---
> > ... It is basically the counterpart to
> > `git show ${ACTION}_HEAD` for understanding the source of the conflict.
> 
> I do not know about the validity of that approach to use *_HEAD,

What I wanted to convey is that e.g. "git show CHERRY_PICK_HEAD" will
tell you about the conflict from the perspective of the commit that is
currently to be applied while "git log --merge" tells the story from the
perspective of HEAD. So they are by no means the same, but can
complement each other in getting an understanding about the conflict.

> but we may want to tighten the original's use of repo_get
> > -	if (repo_get_oid(the_repository, "MERGE_HEAD", &oid))
> > -		die("--merge without MERGE_HEAD?");
> > -	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
> 
> ... so that we won't be confused in a repository that has a tag
> whose name happens to be MERGE_HEAD.  We probably should be using
> refs.c:refs_resolve_ref_unsafe() instead to _oid() here ...

Here I am really at the limit of my understanding of the core functions.
Is this roughly what you had in mind? From my testing it seems to do the
job, but I don't understand the details "refs_resolve_ref_unsafe"...

 revision.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/revision.c b/revision.c
index 2424c9bd67..786e1a3e89 100644
--- a/revision.c
+++ b/revision.c
@@ -1967,17 +1967,23 @@ static void prepare_show_merge(struct rev_info *revs)
 	struct commit *head, *other;
 	struct object_id oid;
 	const char **prune = NULL;
+	const char *other_head;
 	int i, prune_num = 1; /* counting terminating NULL */
 	struct index_state *istate = revs->repo->index;
 
 	if (repo_get_oid(the_repository, "HEAD", &oid))
 		die("--merge without HEAD?");
 	head = lookup_commit_or_die(&oid, "HEAD");
-	if (repo_get_oid(the_repository, "MERGE_HEAD", &oid))
+	other_head = refs_resolve_ref_unsafe(get_main_ref_store(the_repository),
+					     "MERGE_HEAD",
+					     RESOLVE_REF_READING,
+					     &oid,
+					     NULL);
+	if (!other_head)
 		die("--merge without MERGE_HEAD?");
-	other = lookup_commit_or_die(&oid, "MERGE_HEAD");
+	other = lookup_commit_or_die(&oid, other_head);
 	add_pending_object(revs, &head->object, "HEAD");
-	add_pending_object(revs, &other->object, "MERGE_HEAD");
+	add_pending_object(revs, &other->object, other_head);
 	bases = repo_get_merge_bases(the_repository, head, other);
 	add_rev_cmdline_list(revs, bases, REV_CMD_MERGE_BASE, UNINTERESTING | BOTTOM);
 	add_pending_commit_list(revs, bases, UNINTERESTING | BOTTOM);
-- 
2.43.0.284.g6c31128a96