use lock instead of once to create default client (#341)

Signed-off-by: zhangchao <zchao9100@gmail.com>

client/client.go

@@ -51,9 +51,9 @@ const (
 
 var (
 	logger               = log.New(os.Stdout, "", 0)
+	lock                 = &sync.Mutex{}
 	_             Client = (*GRPCClient)(nil)
 	defaultClient Client
-	doOnce        sync.Once
 )
 
 // Client is the interface for Dapr client implementation.
@@ -209,16 +209,21 @@ func NewClient() (client Client, err error) {
 	if port == "" {
 		port = daprPortDefault
 	}
-	var onceErr error
+	if defaultClient != nil {
-	doOnce.Do(func() {
+		return defaultClient, nil
+	}
+	lock.Lock()
+	defer lock.Unlock()
+	if defaultClient != nil {
+		return defaultClient, nil
+	}
 	c, err := NewClientWithPort(port)
 	if err != nil {
-			onceErr = fmt.Errorf("error creating default client: %w", err)
+		return nil, fmt.Errorf("error creating default client: %w", err)
 	}
 	defaultClient = c
-	})
 
-	return defaultClient, onceErr
+	return defaultClient, nil
 }
 
 // NewClientWithPort instantiates Dapr using specific gRPC port.