fix: ensure DNS query name reset in plugin.NS error path (#7142)

Signed-off-by: hansedong <skipiper1314@gmail.com>
Co-authored-by: hansedong <skipiper1314@gmail.com>

plugin/backend_lookup_test.go

@@ -0,0 +1,86 @@
+package plugin
+
+import (
+	"context"
+	"fmt"
+	"testing"
+	"time"
+
+	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+// mockBackend implements ServiceBackend interface for testing
+var _ ServiceBackend = &mockBackend{}
+
+type mockBackend struct {
+	mockServices func(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error)
+}
+
+func (m *mockBackend) Serial(state request.Request) uint32 {
+	return uint32(time.Now().Unix())
+}
+
+func (m *mockBackend) MinTTL(state request.Request) uint32 {
+	return 30
+}
+
+func (m *mockBackend) Services(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error) {
+	return m.mockServices(ctx, state, exact, opt)
+}
+
+func (m *mockBackend) Reverse(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error) {
+	return nil, nil
+}
+
+func (m *mockBackend) Lookup(ctx context.Context, state request.Request, name string, typ uint16) (*dns.Msg, error) {
+	return nil, nil
+}
+
+func (m *mockBackend) IsNameError(err error) bool {
+	return false
+}
+
+func (m *mockBackend) Records(ctx context.Context, state request.Request, exact bool) ([]msg.Service, error) {
+	return nil, nil
+}
+
+func TestNSStateReset(t *testing.T) {
+	// Create a mock backend that always returns error
+	mock := &mockBackend{
+		mockServices: func(ctx context.Context, state request.Request, exact bool, opt Options) ([]msg.Service, error) {
+			return nil, fmt.Errorf("mock error")
+		},
+	}
+	// Create a test request
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeNS)
+	state := request.Request{
+		Req: req,
+		W:   &test.ResponseWriter{},
+	}
+
+	originalName := state.QName()
+	ctx := context.TODO()
+
+	// Call NS function which should fail due to mock error
+	records, extra, err := NS(ctx, mock, "example.org.", state, Options{})
+
+	// Verify error is returned
+	if err == nil {
+		t.Error("Expected error from mock backend, got nil")
+	}
+
+	// Verify query name is reset even when an error occurs
+	if state.QName() != originalName {
+		t.Errorf("Query name not properly reset after error. Expected %s, got %s", originalName, state.QName())
+	}
+
+	// Verify no records are returned
+	if len(records) != 0 || len(extra) != 0 {
+		t.Error("Expected no records returned on error")
+	}
+}