syscall: 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: Id85cff17328ecbf6d09dd52fbeeaa691dbe69b75
Reviewed-on: https://go-review.googlesource.com/108338Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/syscall/syscall_linux.go

@@ -541,16 +541,18 @@ func Recvmsg(fd int, p, oob []byte, flags int) (n, oobn int, recvflags int, from
 	}
 	var dummy byte
 	if len(oob) > 0 {
+		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 && len(p) == 0 {
+			if sockType != SOCK_DGRAM {
 				iov.Base = &dummy
 				iov.SetLen(1)
 			}
+		}
 		msg.Control = &oob[0]
 		msg.SetControllen(len(oob))
 	}
@@ -593,16 +595,18 @@ func SendmsgN(fd int, p, oob []byte, to Sockaddr, flags int) (n int, err error)
 	}
 	var dummy byte
 	if len(oob) > 0 {
+		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 && len(p) == 0 {
+			if sockType != SOCK_DGRAM {
 				iov.Base = &dummy
 				iov.SetLen(1)
 			}
+		}
 		msg.Control = &oob[0]
 		msg.SetControllen(len(oob))
 	}