merge-ort: add new merge_ort_generic() function

merge-recursive.[ch] have three entry points:
  * merge_trees()
  * merge_recursive()
  * merge_recursive_generic()
merge-ort*.[ch] only has equivalents for the first two.  Add an
equivalent for the final entry point, so we can switch callers to
use it and remove merge-recursive.[ch].

While porting it over, finally fix the issue with the label for the
ancestor (used when merge.conflictStyle=diff3 as a conflict label).
merge-recursive.c has traditionally not allowed callers to set that
label, but I have found that problematic for years.

(Side note: This function was initially part of the merge-ort rewrite,
but reviewers questioned the ancestor label funnyness which I was
never really happy with anyway.  It resulted in me jettisoning it and
hoping at the time that I would eventually be able to force the existing
callers to use some other API.  That worked with `git stash`, as per
874cf2a604 (stash: apply stash using 'merge_ort_nonrecursive()',
2022-05-10), but this API is the most reasonable one for `git am` and
`git merge-recursive`, if we can just allow them some freedom over the
ancestor label.)

The merge_recursive_generic() function did not know whether it was being
invoked by `git stash`, `git merge-recursive`, or `git am`, and the
choice of meaningful ancestor label, when there is a unique ancestor,
varies for these different callers:

  * git am: ancestor is a constructed "fake ancestor" that user knows
            nothing about and has no access to.  (And is different than
            the normal thing we mean by a "virtual merge base" which is
            the merging of merge bases.)
  * git merge-recursive: ancestor might be a tree, but at least it
                         was one specified by the user (if they invoked
                         merge-recursive directly)
  * git stash: ancestor was the commit serving as the stash base

Thus, using a label like "constructed merge base" (as
merge_recursive_generic() does) presupposes that `git am` is the only
caller; it is incorrect for other callers.  This label has thrown me off
more than once.  Allow the caller to override when there is a unique
merge base.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort-wrappers.c

@@ -1,9 +1,13 @@
 #include "git-compat-util.h"
 #include "gettext.h"
 #include "hash.h"
+#include "hex.h"
+#include "lockfile.h"
 #include "merge-ort.h"
 #include "merge-ort-wrappers.h"
 #include "read-cache-ll.h"
+#include "repository.h"
+#include "tag.h"
 #include "tree.h"
 
 #include "commit.h"
@@ -64,3 +68,63 @@ int merge_ort_recursive(struct merge_options *opt,
 
 	return tmp.clean;
 }
+
+static struct commit *get_ref(struct repository *repo,
+			      const struct object_id *oid,
+			      const char *name)
+{
+	struct object *object;
+
+	object = deref_tag(repo, parse_object(repo, oid),
+			   name, strlen(name));
+	if (!object)
+		return NULL;
+	if (object->type == OBJ_TREE)
+		return make_virtual_commit(repo, (struct tree*)object, name);
+	if (object->type != OBJ_COMMIT)
+		return NULL;
+	if (repo_parse_commit(repo, (struct commit *)object))
+		return NULL;
+	return (struct commit *)object;
+}
+
+int merge_ort_generic(struct merge_options *opt,
+		      const struct object_id *head,
+		      const struct object_id *merge,
+		      int num_merge_bases,
+		      const struct object_id *merge_bases,
+		      struct commit **result)
+{
+	int clean;
+	struct lock_file lock = LOCK_INIT;
+	struct commit *head_commit = get_ref(opt->repo, head, opt->branch1);
+	struct commit *next_commit = get_ref(opt->repo, merge, opt->branch2);
+	struct commit_list *ca = NULL;
+
+	if (merge_bases) {
+		int i;
+		for (i = 0; i < num_merge_bases; ++i) {
+			struct commit *base;
+			if (!(base = get_ref(opt->repo, &merge_bases[i],
+					     oid_to_hex(&merge_bases[i]))))
+				return error(_("Could not parse object '%s'"),
+					     oid_to_hex(&merge_bases[i]));
+			commit_list_insert(base, &ca);
+		}
+	}
+
+	repo_hold_locked_index(opt->repo, &lock, LOCK_DIE_ON_ERROR);
+	clean = merge_ort_recursive(opt, head_commit, next_commit, ca,
+				    result);
+	free_commit_list(ca);
+	if (clean < 0) {
+		rollback_lock_file(&lock);
+		return clean;
+	}
+
+	if (write_locked_index(opt->repo->index, &lock,
+			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
+		return error(_("Unable to write index."));
+
+	return clean ? 0 : 1;
+}

merge-ort-wrappers.h

@@ -22,4 +22,16 @@ int merge_ort_recursive(struct merge_options *opt,
 			const struct commit_list *ancestors,
 			struct commit **result);
 
+/*
+ * rename-detecting three-way merge.  num_merge_bases must be at least 1.
+ * Recursive ancestor consolidation will be performed if num_merge_bases > 1.
+ * Wrapper mimicking the old merge_recursive_generic() function.
+ */
+int merge_ort_generic(struct merge_options *opt,
+		      const struct object_id *head,
+		      const struct object_id *merge,
+		      int num_merge_bases,
+		      const struct object_id *merge_bases,
+		      struct commit **result);
+
 #endif

merge-ort.c

@@ -4878,9 +4878,9 @@ static inline void set_commit_tree(struct commit *c, struct tree *t)
 	c->maybe_tree = t;
 }
 
-static struct commit *make_virtual_commit(struct repository *repo,
-					  struct tree *tree,
-					  const char *comment)
+struct commit *make_virtual_commit(struct repository *repo,
+				   struct tree *tree,
+				   const char *comment)
 {
 	struct commit *commit = alloc_commit_node(repo);
 
@@ -5186,6 +5186,8 @@ static void merge_ort_internal(struct merge_options *opt,
 		ancestor_name = "empty tree";
 	} else if (merge_bases) {
 		ancestor_name = "merged common ancestors";
+	} else if (opt->ancestor) {
+		ancestor_name = opt->ancestor;
 	} else {
 		strbuf_add_unique_abbrev(&merge_base_abbrev,
 					 &merged_merge_bases->object.oid,
@@ -5275,8 +5277,13 @@ void merge_incore_recursive(struct merge_options *opt,
 {
 	trace2_region_enter("merge", "incore_recursive", opt->repo);
 
-	/* We set the ancestor label based on the merge_bases */
-	assert(opt->ancestor == NULL);
+	/*
+	 * We set the ancestor label based on the merge_bases...but we
+	 * allow one exception through so that builtin/am can override
+	 * with its constructed fake ancestor.
+	 */
+	assert(opt->ancestor == NULL ||
+	       (merge_bases && !merge_bases->next));
 
 	trace2_region_enter("merge", "merge_start", opt->repo);
 	merge_start(opt, result);

merge-ort.h

@@ -44,6 +44,11 @@ struct merge_result {
 	unsigned _properly_initialized;
 };
 
+/* Mostly internal function also used by merge-ort-wrappers.c */
+struct commit *make_virtual_commit(struct repository *repo,
+				   struct tree *tree,
+				   const char *comment);
+
 /*
  * rename-detecting three-way merge with recursive ancestor consolidation.
  * working tree and index are untouched.