IfAPI: Add iterator to complement if_foreach()

Summary:
Sometimes an if_foreach() callback can be trivial, or need a lot of
outer context.  In this case a regular `for` loop makes more sense.  To
keep things hidden in the new API, use an opaque `if_iter` structure
that can still be instantiated on the stack.  The current implementation
uses just a single pointer out of the 4 alotted to the opaque context,
and the cleanup does nothing, but may be used in the future.

Reviewed by:	melifaro
Sponsored by:	Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39138

1 files changed, 36 insertions, 0 deletions

diff --git a/sys/net/if.c b/sys/net/if.c
index f3ef822178ff..ff942bceb090 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -4600,6 +4600,42 @@ if_foreach_sleep(if_foreach_match_t match_cb, void *match_arg, if_foreach_cb_t c
 	return (error);
 }
 
+
+/*
+ * Uses just 1 pointer of the 4 available in the public struct.
+ */
+if_t
+if_iter_start(struct if_iter *iter)
+{
+	if_t ifp;
+
+	NET_EPOCH_ASSERT();
+
+	bzero(iter, sizeof(*iter));
+	ifp = CK_STAILQ_FIRST(&V_ifnet);
+	if (ifp != NULL)
+		iter->context[0] = CK_STAILQ_NEXT(ifp, if_link);
+	else
+		iter->context[0] = NULL;
+	return (ifp);
+}
+
+if_t
+if_iter_next(struct if_iter *iter)
+{
+	if_t cur_ifp = iter->context[0];
+
+	if (cur_ifp != NULL)
+		iter->context[0] = CK_STAILQ_NEXT(cur_ifp, if_link);
+	return (cur_ifp);
+}
+
+void
+if_iter_finish(struct if_iter *iter)
+{
+	/* Nothing to do here for now. */
+}
+
 u_int
 if_foreach_lladdr(if_t ifp, iflladdr_cb_t cb, void *cb_arg)
 {