unix: avoid extra syscall on send/recvmsg on Linux

By simply rearranging the logic, we avoid the overhead of a superfluous
call to getsockopt. For, if p is already non empty, there's no point
in having to check if we need to attach dummy payload. This has
performance benefits when using send/recvmsg for high speed
communications.

Change-Id: If47ab5322464aad8e6d2603a369e95327223211f
Reviewed-on: https://go-review.googlesource.com/108337
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_linux.go

@@ -944,15 +944,17 @@ func Recvmsg(fd int, p, oob []byte, flags int) (n, oobn int, recvflags int, from
 	}
 	var dummy byte
 	if len(oob) > 0 {
-		var sockType int
-		sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
-		if err != nil {
-			return
-		}
-		// receive at least one normal byte
-		if sockType != SOCK_DGRAM && len(p) == 0 {
-			iov.Base = &dummy
-			iov.SetLen(1)
+		if len(p) == 0 {
+			var sockType int
+			sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
+			if err != nil {
+				return
+			}
+			// receive at least one normal byte
+			if sockType != SOCK_DGRAM {
+				iov.Base = &dummy
+				iov.SetLen(1)
+			}
 		}
 		msg.Control = &oob[0]
 		msg.SetControllen(len(oob))
@@ -996,15 +998,17 @@ func SendmsgN(fd int, p, oob []byte, to Sockaddr, flags int) (n int, err error)
 	}
 	var dummy byte
 	if len(oob) > 0 {
-		var sockType int
-		sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
-		if err != nil {
-			return 0, err
-		}
-		// send at least one normal byte
-		if sockType != SOCK_DGRAM && len(p) == 0 {
-			iov.Base = &dummy
-			iov.SetLen(1)
+		if len(p) == 0 {
+			var sockType int
+			sockType, err = GetsockoptInt(fd, SOL_SOCKET, SO_TYPE)
+			if err != nil {
+				return 0, err
+			}
+			// send at least one normal byte
+			if sockType != SOCK_DGRAM {
+				iov.Base = &dummy
+				iov.SetLen(1)
+			}
 		}
 		msg.Control = &oob[0]
 		msg.SetControllen(len(oob))