MB-30072: Respond to DCP_NOOP immediately, in projector dcp consumer

DCP_NOOP message is used for keeping alive the connection between
dcp producer and dcp consumer. If DCP_NOOP is added to receive channel,
it will have to wait for all other events in the receive channel to
be processed, before consumer can repond to it. If this takes more
time than dcp noop interval, producer will wrongly close the connection.
To avoid this, respond to DCP_NOOP immediately, without adding
it to receive channel.

Change-Id: I0c4d540e48ad4ecfbd4d380dd209499f09ff626b

secondary/dcp/transport/client/dcp_feed.go

@@ -294,19 +294,6 @@ func (feed *DcpFeed) handlePacket(
 
 	sendAck := false
 	prefix := feed.logPrefix
-	if pkt.Opcode == transport.DCP_NOOP {
-		noop := &transport.MCResponse{
-			Opcode: transport.DCP_NOOP, Opaque: pkt.Opaque,
-		}
-		if err := feed.conn.TransmitResponse(noop); err != nil {
-			logging.Errorf("%v NOOP.Transmit(): %v", prefix, err)
-		} else {
-			fmsg := "%v responded to NOOP ok ...\n"
-			logging.Tracef(fmsg, prefix)
-		}
-		return "ok" // for NOOP, bytes are not counted for for buffer-ack
-	}
-
 	stream := feed.vbstreams[vb]
 	if stream == nil {
 		feed.stats.TotalSpurious++
@@ -1102,6 +1089,21 @@ loop:
 			break loop
 		}
 
+		// Immediately respond to NOOP and listen for next message.
+		// NOOPs are not accounted for buffer-ack.
+		if pkt.Opcode == transport.DCP_NOOP {
+			noop := &transport.MCResponse{
+				Opcode: transport.DCP_NOOP, Opaque: pkt.Opaque,
+			}
+			if err := feed.conn.TransmitResponse(noop); err != nil {
+				logging.Errorf("%v NOOP.Transmit(): %v", feed.logPrefix, err)
+			} else {
+				fmsg := "%v responded to NOOP ok ...\n"
+				logging.Tracef(fmsg, feed.logPrefix)
+			}
+			continue loop
+		}
+
 		logging.LazyTrace(func() string {
 			return fmt.Sprintf("%v packet received %#v", feed.logPrefix, logging.TagUD(pkt))
 		})