list: list_swap to exchange elements

This allows deleting and re-inserting an element in place
of the deleted element without branching.
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/list/list.h

@@ -368,6 +368,39 @@ static inline void list_del_from(struct list_head *h, struct list_node *n)
 	list_del(n);
 }
 
+/**
+ * list_swap - swap out an entry from an (unknown) linked list for a new one.
+ * @o: the list_node to replace from the list.
+ * @n: the list_node to insert in place of the old one.
+ *
+ * Note that this leaves @o in an undefined state; it can be added to
+ * another list, but not deleted/swapped again.
+ *
+ * See also:
+ *	list_del()
+ *
+ * Example:
+ *	struct child x1, x2;
+ *	LIST_HEAD(xh);
+ *
+ *	list_add(&xh, &x1.list);
+ *	list_swap(&x1.list, &x2.list);
+ */
+#define list_swap(o, n) list_swap_(o, n, LIST_LOC)
+static inline void list_swap_(struct list_node *o,
+			      struct list_node *n,
+			      const char* abortstr)
+{
+	(void)list_debug_node(o, abortstr);
+	*n = *o;
+	n->next->prev = n;
+	n->prev->next = n;
+#ifdef CCAN_LIST_DEBUG
+	/* Catch use-after-del. */
+	o->next = o->prev = NULL;
+#endif
+}
+
 /**
  * list_entry - convert a list_node back into the structure containing it.
  * @n: the list_node

ccan/list/test/run.c

@@ -19,13 +19,13 @@ static LIST_HEAD(static_list);
 int main(int argc, char *argv[])
 {
 	struct parent parent;
-	struct child c1, c2, c3, *c, *n;
+	struct child c1, c2, c3, x1, *c, *n;
 	unsigned int i;
 	struct list_head list = LIST_HEAD_INIT(list);
 	opaque_t *q, *nq;
 	struct list_head opaque_list = LIST_HEAD_INIT(opaque_list);
 
-	plan_tests(79);
+	plan_tests(84);
 	/* Test LIST_HEAD, LIST_HEAD_INIT, list_empty and check_list */
 	ok1(list_empty(&static_list));
 	ok1(list_check(&static_list, NULL));
@@ -253,5 +253,24 @@ int main(int argc, char *argv[])
 	}
 	ok1(i == 3);
 
+	/* test list_swap */
+	list_swap(&c3.list, &x1.list);
+	ok1(list_check(&parent.children, "list_swap"));
+	i = 0;
+	list_for_each(&parent.children, c, list) {
+		switch (i++) {
+		case 0:
+			ok1(c == &c1);
+			break;
+		case 1:
+			ok1(c == &x1);
+			break;
+		case 2:
+			ok1(c == &c2);
+			break;
+		}
+	}
+	ok1(i == 3);
+
 	return exit_status();
 }