internal: raw request sending functions

Auth requests are sent before connection event loop initialization.
It would be useful to be able to send IPROTO_ID requests in the same
way.

Part of #120

connection.go

@@ -581,43 +581,63 @@ func pack(h *smallWBuf, enc *encoder, reqid uint32,
 	return
 }
 
-func (conn *Connection) writeAuthRequest(w *bufio.Writer, scramble []byte) (err error) {
+func (conn *Connection) writeRequest(w *bufio.Writer, req Request) error {
 	var packet smallWBuf
-	req := newAuthRequest(conn.opts.User, string(scramble))
-	err = pack(&packet, newEncoder(&packet), 0, req, ignoreStreamId, conn.Schema)
+	err := pack(&packet, newEncoder(&packet), 0, req, ignoreStreamId, conn.Schema)
 
 	if err != nil {
-		return errors.New("auth: pack error " + err.Error())
+		return fmt.Errorf("pack error: %w", err)
 	}
-	if err := write(w, packet.b); err != nil {
-		return errors.New("auth: write error " + err.Error())
+	if err = write(w, packet.b); err != nil {
+		return fmt.Errorf("write error: %w", err)
 	}
 	if err = w.Flush(); err != nil {
-		return errors.New("auth: flush error " + err.Error())
+		return fmt.Errorf("flush error: %w", err)
 	}
-	return
+	return err
+}
+
+func (conn *Connection) writeAuthRequest(w *bufio.Writer, scramble []byte) error {
+	req := newAuthRequest(conn.opts.User, string(scramble))
+
+	err := conn.writeRequest(w, req)
+	if err != nil {
+		return fmt.Errorf("auth: %w", err)
+	}
+
+	return nil
 }
 
-func (conn *Connection) readAuthResponse(r io.Reader) (err error) {
+func (conn *Connection) readResponse(r io.Reader) (Response, error) {
 	respBytes, err := conn.read(r)
 	if err != nil {
-		return errors.New("auth: read error " + err.Error())
+		return Response{}, fmt.Errorf("read error: %w", err)
 	}
+
 	resp := Response{buf: smallBuf{b: respBytes}}
 	err = resp.decodeHeader(conn.dec)
 	if err != nil {
-		return errors.New("auth: decode response header error " + err.Error())
+		return resp, fmt.Errorf("decode response header error: %w", err)
 	}
 	err = resp.decodeBody()
 	if err != nil {
 		switch err.(type) {
 		case Error:
-			return err
+			return resp, err
 		default:
-			return errors.New("auth: decode response body error " + err.Error())
+			return resp, fmt.Errorf("decode response body error: %w", err)
 		}
 	}
-	return
+	return resp, nil
+}
+
+func (conn *Connection) readAuthResponse(r io.Reader) error {
+	_, err := conn.readResponse(r)
+	if err != nil {
+		return fmt.Errorf("auth: %w", err)
+	}
+
+	return nil
 }
 
 func (conn *Connection) createConnection(reconnect bool) (err error) {