small code cleanup (#1598)

* Go programmers familier with `if err != nil {}` than `if err == nil {}`.
* Update some URLs about MySQL client/server protocol.

Author: methane

connection.go

@@ -400,31 +400,34 @@ func (mc *mysqlConn) query(query string, args []driver.Value) (*textRows, error)
 	}
 	// Send command
 	err := mc.writeCommandPacketStr(comQuery, query)
-	if err == nil {
-		// Read Result
-		var resLen int
-		resLen, err = handleOk.readResultSetHeaderPacket()
-		if err == nil {
-			rows := new(textRows)
-			rows.mc = mc
+	if err != nil {
+		return nil, mc.markBadConn(err)
+	}
 
-			if resLen == 0 {
-				rows.rs.done = true
+	// Read Result
+	var resLen int
+	resLen, err = handleOk.readResultSetHeaderPacket()
+	if err != nil {
+		return nil, mc.markBadConn(err)
+	}
 
-				switch err := rows.NextResultSet(); err {
-				case nil, io.EOF:
-					return rows, nil
-				default:
-					return nil, err
-				}
-			}
+	rows := new(textRows)
+	rows.mc = mc
 
-			// Columns
-			rows.rs.columns, err = mc.readColumns(resLen)
-			return rows, err
+	if resLen == 0 {
+		rows.rs.done = true
+
+		switch err := rows.NextResultSet(); err {
+		case nil, io.EOF:
+			return rows, nil
+		default:
+			return nil, err
 		}
 	}
-	return nil, mc.markBadConn(err)
+
+	// Columns
+	rows.rs.columns, err = mc.readColumns(resLen)
+	return rows, err
 }
 
 // Gets the value of the given MySQL System Variable

packets.go

@@ -524,32 +524,33 @@ func (mc *okHandler) readResultOK() error {
 }
 
 // Result Set Header Packet
-// http://dev.mysql.com/doc/internals/en/com-query-response.html#packet-ProtocolText::Resultset
+// https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_com_query_response.html
 func (mc *okHandler) readResultSetHeaderPacket() (int, error) {
 	// handleOkPacket replaces both values; other cases leave the values unchanged.
 	mc.result.affectedRows = append(mc.result.affectedRows, 0)
 	mc.result.insertIds = append(mc.result.insertIds, 0)
 
 	data, err := mc.conn().readPacket()
-	if err == nil {
-		switch data[0] {
-
-		case iOK:
-			return 0, mc.handleOkPacket(data)
+	if err != nil {
+		return 0, err
+	}
 
-		case iERR:
-			return 0, mc.conn().handleErrorPacket(data)
+	switch data[0] {
+	case iOK:
+		return 0, mc.handleOkPacket(data)
 
-		case iLocalInFile:
-			return 0, mc.handleInFileRequest(string(data[1:]))
-		}
+	case iERR:
+		return 0, mc.conn().handleErrorPacket(data)
 
-		// column count
-		num, _, _ := readLengthEncodedInteger(data)
-		// ignore remaining data in the packet. see #1478.
-		return int(num), nil
+	case iLocalInFile:
+		return 0, mc.handleInFileRequest(string(data[1:]))
 	}
-	return 0, err
+
+	// column count
+	// https://dev.mysql.com/doc/dev/mysql-server/latest/page_protocol_com_query_response_text_resultset.html
+	num, _, _ := readLengthEncodedInteger(data)
+	// ignore remaining data in the packet. see #1478.
+	return int(num), nil
 }
 
 // Error Packet