merge-ort: add an err() function similar to one from merge-recursive

Various places in merge-recursive used an err() function when it hit
some kind of unrecoverable error.  That code was from the reusable bits
of merge-recursive.c that we liked, such as merge_3way, writing object
files to the object store, reading blobs from the object store, etc.  So
create a similar function to allow us to port that code over, and use it
for when we detect problems returned from collect_merge_info()'s
traverse_trees() call, which we will be adding next.

While we are at it, also add more documentation for the "clean" field
from struct merge_result, particularly since the name suggests a boolean
but it is not quite one and this is our first non-boolean usage.

Signed-off-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: newren

merge-ort.c

@@ -168,12 +168,27 @@ struct conflict_info {
 	unsigned match_mask:3;
 };
 
+static int err(struct merge_options *opt, const char *err, ...)
+{
+	va_list params;
+	struct strbuf sb = STRBUF_INIT;
+
+	strbuf_addstr(&sb, "error: ");
+	va_start(params, err);
+	strbuf_vaddf(&sb, err, params);
+	va_end(params);
+
+	error("%s", sb.buf);
+	strbuf_release(&sb);
+
+	return -1;
+}
+
 static int collect_merge_info(struct merge_options *opt,
 			      struct tree *merge_base,
 			      struct tree *side1,
 			      struct tree *side2)
 {
-	/* TODO: Implement this using traverse_trees() */
 	die("Not yet implemented.");
 }
 
@@ -276,7 +291,19 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
 {
 	struct object_id working_tree_oid;
 
-	collect_merge_info(opt, merge_base, side1, side2);
+	if (collect_merge_info(opt, merge_base, side1, side2) != 0) {
+		/*
+		 * TRANSLATORS: The %s arguments are: 1) tree hash of a merge
+		 * base, and 2-3) the trees for the two trees we're merging.
+		 */
+		err(opt, _("collecting merge info failed for trees %s, %s, %s"),
+		    oid_to_hex(&merge_base->object.oid),
+		    oid_to_hex(&side1->object.oid),
+		    oid_to_hex(&side2->object.oid));
+		result->clean = -1;
+		return;
+	}
+
 	result->clean = detect_and_process_renames(opt, merge_base,
 						   side1, side2);
 	process_entries(opt, &working_tree_oid);

merge-ort.h

@@ -7,7 +7,14 @@ struct commit;
 struct tree;
 
 struct merge_result {
-	/* Whether the merge is clean */
+	/*
+	 * Whether the merge is clean; possible values:
+	 *    1: clean
+	 *    0: not clean (merge conflicts)
+	 *   <0: operation aborted prematurely.  (object database
+	 *       unreadable, disk full, etc.)  Worktree may be left in an
+	 *       inconsistent state if operation failed near the end.
+	 */
 	int clean;
 
 	/*