use atomic variables (matrixorigin#20995)

问题：可能的解释是响应client时间耗时，持有了锁。同时rpc 等待锁，而没响应。
修改：dbname,username,改为 原子变量。
相关：其他变量没有与网络锁关联，无需修改。

Approved by: @qingxinhome, @sukki37

pkg/frontend/mysql_protocol.go

@@ -249,14 +249,14 @@ type MysqlProtocolImpl struct {
 	maxClientPacketSize uint32
 
 	//the user of the client
-	username string
+	username atomic.Value
 
 	// opaque authentication response data generated by Authentication Method
 	// indicated by the plugin name field.
 	authResponse []byte
 
 	//the default database for the client
-	database string
+	database atomic.Value
 
 	// Connection attributes are key-value pairs that application programs
 	// can pass to the server at connect time.
@@ -523,15 +523,15 @@ func (mp *MysqlProtocolImpl) SetSequenceID(value uint8) {
 }
 
 func (mp *MysqlProtocolImpl) GetDatabaseName() string {
-	mp.m.Lock()
-	defer mp.m.Unlock()
-	return mp.database
+	val := mp.database.Load()
+	if val == nil {
+		return ""
+	}
+	return val.(string)
 }
 
 func (mp *MysqlProtocolImpl) SetDatabaseName(s string) {
-	mp.m.Lock()
-	defer mp.m.Unlock()
-	mp.database = s
+	mp.database.Store(s)
 }
 
 func (mp *MysqlProtocolImpl) GetAuthString() []byte {
@@ -547,15 +547,15 @@ func (mp *MysqlProtocolImpl) GetAuthResponse() []byte {
 }
 
 func (mp *MysqlProtocolImpl) GetUserName() string {
-	mp.m.Lock()
-	defer mp.m.Unlock()
-	return mp.username
+	val := mp.username.Load()
+	if val == nil {
+		return ""
+	}
+	return val.(string)
 }
 
 func (mp *MysqlProtocolImpl) SetUserName(s string) {
-	mp.m.Lock()
-	defer mp.m.Unlock()
-	mp.username = s
+	mp.username.Store(s)
 }
 
 const defaultTcp4PackageSize = 1<<14 - 66
@@ -1521,8 +1521,8 @@ func (mp *MysqlProtocolImpl) HandleHandshake(ctx context.Context, payload []byte
 		}
 
 		mp.maxClientPacketSize = resp41.maxPacketSize
-		mp.username = resp41.username
-		mp.database = resp41.database
+		mp.username.Store(resp41.username)
+		mp.database.Store(resp41.database)
 		mp.connectAttrs = resp41.connectAttrs
 	} else {
 		var resp320 response320
@@ -1543,8 +1543,8 @@ func (mp *MysqlProtocolImpl) HandleHandshake(ctx context.Context, payload []byte
 		mp.charset = "utf8mb4"
 
 		mp.maxClientPacketSize = resp320.maxPacketSize
-		mp.username = resp320.username
-		mp.database = resp320.database
+		mp.username.Store(resp320.username)
+		mp.database.Store(resp320.database)
 	}
 	return false, nil
 }
@@ -1560,7 +1560,7 @@ func (mp *MysqlProtocolImpl) Authenticate(ctx context.Context) error {
 	ses.Debugf(ctx, "authenticate user")
 	if err := mp.authenticateUser(ctx, mp.authResponse); err != nil {
 		ses.Errorf(ctx, "authenticate user failed.error:%v", err)
-		errorCode, sqlState, msg := RewriteError(err, mp.username)
+		errorCode, sqlState, msg := RewriteError(err, mp.GetUserName())
 		ses.timestampMap[TSSendErrPacketStart] = time.Now()
 		err2 := mp.sendErrPacket(errorCode, sqlState, msg)
 		ses.timestampMap[TSSendErrPacketEnd] = time.Now()