Fixed SetReadDeadline before connCheck

there's no need to set conn's readline if CheckConnLiveness is false,
and the ReadTimeout shall work with rawConn.Read inside conncheck.
buffer.fill will do SetReadDeadline if needed

Author: zjj

packets.go

@@ -110,14 +110,13 @@ func (mc *mysqlConn) writePacket(data []byte) error {
 			conn = mc.rawConn
 		}
 		var err error
-		// If this connection has a ReadTimeout which we've been setting on
-		// reads, reset it to its default value before we attempt a non-blocking
-		// read, otherwise the scheduler will just time us out before we can read
-		if mc.cfg.ReadTimeout != 0 {
-			err = conn.SetReadDeadline(time.Time{})
-		}
-		if err == nil && mc.cfg.CheckConnLiveness {
-			err = connCheck(conn)
+		if mc.cfg.CheckConnLiveness {
+			if mc.cfg.ReadTimeout != 0 {
+				err = conn.SetReadDeadline(time.Now().Add(mc.cfg.ReadTimeout))
+			}
+			if err == nil {
+				err = connCheck(conn)
+			}
 		}
 		if err != nil {
 			errLog.Print("closing bad idle connection: ", err)