fix memory leak of grpc resolver (#4490)

Co-authored-by: nk <kui.niu@akuvox.com>

zrpc/resolver/internal/discovbuilder.go

@@ -38,7 +38,7 @@ func (b *discovBuilder) Build(target resolver.Target, cc resolver.ClientConn, _
 	sub.AddListener(update)
 	update()
 
-	return &nopResolver{cc: cc}, nil
+	return &nopResolver{cc: cc, closeFunc: func() { sub.Close() }}, nil
 }
 
 func (b *discovBuilder) Scheme() string {

zrpc/resolver/internal/resolver.go

@@ -37,10 +37,14 @@ func register() {
 }
 
 type nopResolver struct {
-	cc resolver.ClientConn
+	cc        resolver.ClientConn
+	closeFunc func()
 }
 
 func (r *nopResolver) Close() {
+	if r.closeFunc != nil {
+		r.closeFunc()
+	}
 }
 
 func (r *nopResolver) ResolveNow(_ resolver.ResolveNowOptions) {

zrpc/resolver/internal/resolver_test.go

@@ -18,6 +18,20 @@ func TestNopResolver(t *testing.T) {
 	})
 }
 
+func TestNopResolver_Close(t *testing.T) {
+	var isChanged bool
+	r := nopResolver{}
+	r.Close()
+	assert.False(t, isChanged)
+	r = nopResolver{
+		closeFunc: func() {
+			isChanged = true
+		},
+	}
+	r.Close()
+	assert.True(t, isChanged)
+}
+
 type mockedClientConn struct {
 	state resolver.State
 	err   error