Eliminate busy loop when receiving messages from non-blocking socket

For a non-blocking socket, it should wait for events first before
receiving messages from the socket, otherwise it would receive empty
message and run into a busy loop.

Signed-off-by: Quan Tian <[email protected]>

nl/nl_linux.go

@@ -629,7 +629,9 @@ func NewNetlinkRequest(proto, flags int) *NetlinkRequest {
 }
 
 type NetlinkSocket struct {
-	fd  int32
+	fd int32
+	// pfd is non nil when the socket is in non-blocking mode, and is used to wait for events on the socket.
+	pfd *unix.PollFd
 	lsa unix.SockaddrNetlink
 	sync.Mutex
 }
@@ -728,17 +730,20 @@ func Subscribe(protocol int, groups ...uint) (*NetlinkSocket, error) {
 		return nil, err
 	}
 
+	var pfd *unix.PollFd
 	// Sometimes (socket_linux.go:SocketGet), Subscribe is used to create a socket
-	// that subscirbed to no groups. So we don't need to set nonblock there.
+	// that subscribes to no groups. So we don't need to set nonblock there.
 	if len(groups) > 0 {
 		if err := unix.SetNonblock(fd, true); err != nil {
 			unix.Close(fd)
 			return nil, err
 		}
+		pfd = &unix.PollFd{Fd: int32(fd), Events: unix.POLLIN}
 	}
 
 	s := &NetlinkSocket{
-		fd: int32(fd),
+		fd:  int32(fd),
+		pfd: pfd,
 	}
 	s.lsa.Family = unix.AF_NETLINK
 
@@ -791,6 +796,13 @@ func (s *NetlinkSocket) Receive() ([]syscall.NetlinkMessage, *unix.SockaddrNetli
 	if fd < 0 {
 		return nil, nil, fmt.Errorf("Receive called on a closed socket")
 	}
+	// The socket is in non-blocking mode.
+	if s.pfd != nil {
+		if _, err := unix.Poll([]unix.PollFd{*s.pfd}, -1); err != nil {
+			return nil, nil, fmt.Errorf("Error polling the socket: %w", err)
+		}
+	}
+
 	var fromAddr *unix.SockaddrNetlink
 	var rb [RECEIVE_BUFFER_SIZE]byte
 	nr, from, err := unix.Recvfrom(fd, rb[:], 0)