merge: introduce {copy|clear}_merge_options()

When mostly the same set of options are to be used to perform
multiple merges, one instance of the merge_options structure may
want to be created and used by copying from the same template
instance.  We saw such a use recently in "git merge-tree".

Let's make the pattern official by introducing copy_merge_options()
as a supported way to make a copy of the structure, and also give
clear_merge_options() to release any resources held by a copied
instance.  Currently we only make a shallow copy, so the former is a
mere structure assignment while the latter is a no-op, but this may
change in the future as the members of merge_options structure
evolve.

Suggested-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: gitster

builtin/merge-tree.c

@@ -425,10 +425,11 @@ static int real_merge(struct merge_tree_options *o,
 {
 	struct commit *parent1, *parent2;
 	struct commit_list *merge_bases = NULL;
-	struct merge_options opt = o->merge_options;
 	struct merge_result result = { 0 };
 	int show_messages = o->show_messages;
+	struct merge_options opt;
 
+	copy_merge_options(&opt, &o->merge_options);
 	parent1 = get_merge_parent(branch1);
 	if (!parent1)
 		help_unknown_ref(branch1, "merge-tree",
@@ -507,6 +508,7 @@ static int real_merge(struct merge_tree_options *o,
 	if (o->use_stdin)
 		putchar(line_termination);
 	merge_finalize(&opt, &result);
+	clear_merge_options(&opt);
 	return !result.clean; /* result.clean < 0 handled above */
 }
 

merge-recursive.c

@@ -3912,6 +3912,22 @@ void init_merge_options(struct merge_options *opt,
 		opt->buffer_output = 0;
 }
 
+/*
+ * For now, members of merge_options do not need deep copying, but
+ * it may change in the future, in which case we would need to update
+ * this, and also make a matching change to clear_merge_options() to
+ * release the resources held by a copied instance.
+ */
+void copy_merge_options(struct merge_options *dst, struct merge_options *src)
+{
+	*dst = *src;
+}
+
+void clear_merge_options(struct merge_options *opt UNUSED)
+{
+	; /* no-op as our copy is shallow right now */
+}
+
 int parse_merge_opt(struct merge_options *opt, const char *s)
 {
 	const char *arg;

merge-recursive.h

@@ -55,6 +55,9 @@ struct merge_options {
 
 void init_merge_options(struct merge_options *opt, struct repository *repo);
 
+void copy_merge_options(struct merge_options *dst, struct merge_options *src);
+void clear_merge_options(struct merge_options *opt);
+
 /* parse the option in s and update the relevant field of opt */
 int parse_merge_opt(struct merge_options *opt, const char *s);