fix ewma panic (#2480)

* fix ewma panic

selector/node/ewma/node.go

@@ -161,7 +161,7 @@ func (n *Node) Pick() selector.DoneFunc {
 			}
 			var netErr net.Error
 			if errors.Is(context.DeadlineExceeded, di.Err) || errors.Is(context.Canceled, di.Err) ||
-				errors.IsServiceUnavailable(di.Err) || errors.IsGatewayTimeout(di.Err) || errors.As(di.Err, netErr) {
+				errors.IsServiceUnavailable(di.Err) || errors.IsGatewayTimeout(di.Err) || errors.As(di.Err, &netErr) {
 				success = 0
 			}
 		}

selector/node/ewma/node_test.go

@@ -2,6 +2,7 @@ package ewma
 
 import (
 	"context"
+	"net"
 	"reflect"
 	"testing"
 	"time"
@@ -100,11 +101,15 @@ func TestDirectErrorHandler(t *testing.T) {
 			Endpoints: []string{"http://127.0.0.1:9090"},
 			Metadata:  map[string]string{"weight": "10"},
 		}))
-
+	errs := []error{
+		context.DeadlineExceeded,
+		context.Canceled,
+		net.ErrClosed,
+	}
 	for i := 0; i < 5; i++ {
 		var err error
 		if i != 0 {
-			err = context.DeadlineExceeded
+			err = errs[i%len(errs)]
 		}
 		done := wn.Pick()
 		if done == nil {