merge-recursive: provide pair of `unpack_trees_{start,finish}()`

Rename `git_merge_trees()` to `unpack_trees_start()` and extract the
call to `discard_index()` into a new function `unpack_trees_finish()`.
As a result, these are called early resp. late in `merge_trees()`,
making the resource handling clearer. A later commit will expand on
that, teaching `..._finish()` to free more memory. (So rather than
moving the FIXME-comment, just drop it, since it will be addressed soon
enough.)

Also call `..._finish()` when `merge_trees()` returns early.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -337,10 +337,10 @@ static void init_tree_desc_from_tree(struct tree_desc *desc, struct tree *tree)
 	init_tree_desc(desc, tree->buffer, tree->size);
 }
 
-static int git_merge_trees(struct merge_options *o,
-			   struct tree *common,
-			   struct tree *head,
-			   struct tree *merge)
+static int unpack_trees_start(struct merge_options *o,
+			      struct tree *common,
+			      struct tree *head,
+			      struct tree *merge)
 {
 	int rc;
 	struct tree_desc t[3];
@@ -379,6 +379,11 @@ static int git_merge_trees(struct merge_options *o,
 	return rc;
 }
 
+static void unpack_trees_finish(struct merge_options *o)
+{
+	discard_index(&o->orig_index);
+}
+
 struct tree *write_tree_from_memory(struct merge_options *o)
 {
 	struct tree *result = NULL;
@@ -3088,13 +3093,14 @@ int merge_trees(struct merge_options *o,
 		return 1;
 	}
 
-	code = git_merge_trees(o, common, head, merge);
+	code = unpack_trees_start(o, common, head, merge);
 
 	if (code != 0) {
 		if (show(o, 4) || o->call_depth)
 			err(o, _("merging of trees %s and %s failed"),
 			    oid_to_hex(&head->object.oid),
 			    oid_to_hex(&merge->object.oid));
+		unpack_trees_finish(o);
 		return -1;
 	}
 
@@ -3147,20 +3153,15 @@ cleanup:
 
 		hashmap_free(&o->current_file_dir_set, 1);
 
-		if (clean < 0)
+		if (clean < 0) {
+			unpack_trees_finish(o);
 			return clean;
+		}
 	}
 	else
 		clean = 1;
 
-	/* Free the extra index left from git_merge_trees() */
-	/*
-	 * FIXME: Need to also free data allocated by
-	 * setup_unpack_trees_porcelain() tucked away in o->unpack_opts.msgs,
-	 * but the problem is that only half of it refers to dynamically
-	 * allocated data, while the other half points at static strings.
-	 */
-	discard_index(&o->orig_index);
+	unpack_trees_finish(o);
 
 	if (o->call_depth && !(*result = write_tree_from_memory(o)))
 		return -1;