syscall: hoist Getsockname out of NetlinkRIB loops

Calling Getsockname once to fetch the Pid field from the *SockaddrNetlink
is necessary, but this data will remain static for the rest of the netlink
socket's lifetime. Moving this call and type assertion outside of the inner
loops will remove a number of unnecessary system calls.

Change-Id: I7e7e81866af1a31fccdaaf7531efd6cc4cbb8926
Reviewed-on: https://go-review.googlesource.com/c/go/+/336369
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

1 files changed, 12 insertions, 13 deletions

diff --git a/src/syscall/netlink_linux.go b/src/syscall/netlink_linux.go
index 0937ff797a..2d810705bf 100644
--- a/src/syscall/netlink_linux.go
+++ b/src/syscall/netlink_linux.go
@@ -55,14 +55,22 @@ func NetlinkRIB(proto, family int) ([]byte, error) {
 		return nil, err
 	}
 	defer Close(s)
-	lsa := &SockaddrNetlink{Family: AF_NETLINK}
-	if err := Bind(s, lsa); err != nil {
+	sa := &SockaddrNetlink{Family: AF_NETLINK}
+	if err := Bind(s, sa); err != nil {
 		return nil, err
 	}
 	wb := newNetlinkRouteRequest(proto, 1, family)
-	if err := Sendto(s, wb, 0, lsa); err != nil {
+	if err := Sendto(s, wb, 0, sa); err != nil {
 		return nil, err
 	}
+	lsa, err := Getsockname(s)
+	if err != nil {
+		return nil, err
+	}
+	lsanl, ok := lsa.(*SockaddrNetlink)
+	if !ok {
+		return nil, EINVAL
+	}
 	var tab []byte
 	rbNew := make([]byte, Getpagesize())
 done:
@@ -82,16 +90,7 @@ done:
 			return nil, err
 		}
 		for _, m := range msgs {
-			lsa, err := Getsockname(s)
-			if err != nil {
-				return nil, err
-			}
-			switch v := lsa.(type) {
-			case *SockaddrNetlink:
-				if m.Header.Seq != 1 || m.Header.Pid != v.Pid {
-					return nil, EINVAL
-				}
-			default:
+			if m.Header.Seq != 1 || m.Header.Pid != lsanl.Pid {
 				return nil, EINVAL
 			}
 			if m.Header.Type == NLMSG_DONE {