xds/clusterimpl: update TestChildPolicyChangeOnConfigUpdate to use custom lb policy. (#8730)

Fixes #8703 

### Description
This PR fixes the data race in `TestChildPolicyChangeOnConfigUpdate`.

### Changes
* **Isolated Policy:** Instead of overwriting the global `pick_first`
policy (and triggering a race condition via `Unregister`), the test now
registers a unique custom stub policy named `test_pick_first`.
* **TypedStruct Configuration:** Updated the management server resource
update to use `v3xdsxdstypepb.TypedStruct`and specify the custom
`type.googleapis.com/test_pick_first` policy via TypeURL.


RELEASE NOTES: N/A

Author: Pranjali-2501

internal/xds/balancer/clusterimpl/tests/balancer_test.go

@@ -55,6 +55,7 @@ import (
 	"google.golang.org/protobuf/types/known/durationpb"
 	"google.golang.org/protobuf/types/known/wrapperspb"
 
+	v3xdsxdstypepb "github.com/cncf/xds/go/xds/type/v3"
 	v3clusterpb "github.com/envoyproxy/go-control-plane/envoy/config/cluster/v3"
 	v3corepb "github.com/envoyproxy/go-control-plane/envoy/config/core/v3"
 	v3endpointpb "github.com/envoyproxy/go-control-plane/envoy/config/endpoint/v3"
@@ -64,6 +65,7 @@ import (
 	v3lrspb "github.com/envoyproxy/go-control-plane/envoy/service/load_stats/v3"
 	testgrpc "google.golang.org/grpc/interop/grpc_testing"
 	testpb "google.golang.org/grpc/interop/grpc_testing"
+	"google.golang.org/protobuf/types/known/structpb"
 
 	_ "google.golang.org/grpc/xds"
 )
@@ -1124,6 +1126,8 @@ func (s) TestUpdateLRSServerToNil(t *testing.T) {
 // Test verifies that child policy was updated on receipt of
 // configuration update.
 func (s) TestChildPolicyChangeOnConfigUpdate(t *testing.T) {
+	const customLBPolicy = "test_custom_lb_policy"
+
 	// Create an xDS management server.
 	mgmtServer := e2e.StartManagementServer(t, e2e.ManagementServerOptions{AllowResourceSubset: true})
 	defer mgmtServer.Stop()
@@ -1175,21 +1179,21 @@ func (s) TestChildPolicyChangeOnConfigUpdate(t *testing.T) {
 		t.Fatalf("client.EmptyCall() failed: %v", err)
 	}
 
-	// Register stub pickfirst LB policy so that we can catch config changes.
+	// Register stub customLBPolicy LB policy so that we can catch config changes.
 	pfBuilder := balancer.Get(pickfirst.Name)
-	internal.BalancerUnregister(pfBuilder.Name())
 	lbCfgCh := make(chan serviceconfig.LoadBalancingConfig, 1)
-	var updatedChildPolicy atomic.Pointer[string]
-	stub.Register(pfBuilder.Name(), stub.BalancerFuncs{
+	var updatedChildPolicy atomic.Value
+
+	stub.Register(customLBPolicy, stub.BalancerFuncs{
 		ParseConfig: func(lbCfg json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
 			return pfBuilder.(balancer.ConfigParser).ParseConfig(lbCfg)
 		},
 		Init: func(bd *stub.BalancerData) {
 			bd.ChildBalancer = pfBuilder.Build(bd.ClientConn, bd.BuildOptions)
 		},
 		UpdateClientConnState: func(bd *stub.BalancerData, ccs balancer.ClientConnState) error {
-			name := pfBuilder.Name()
-			updatedChildPolicy.Store(&name)
+			// name := customLBPolicy
+			updatedChildPolicy.Store(customLBPolicy)
 			select {
 			case lbCfgCh <- ccs.BalancerConfig:
 			case <-ctx.Done():
@@ -1201,13 +1205,17 @@ func (s) TestChildPolicyChangeOnConfigUpdate(t *testing.T) {
 			bd.ChildBalancer.Close()
 		},
 	})
-	defer balancer.Register(pfBuilder)
 
-	// Now update the cluster to use "pick_first" as the endpoint picking policy.
+	defer internal.BalancerUnregister(customLBPolicy)
+
+	// Update the cluster to use customLBPolicy as the endpoint picking policy.
 	resources.Clusters[0].LoadBalancingPolicy = &v3clusterpb.LoadBalancingPolicy{
 		Policies: []*v3clusterpb.LoadBalancingPolicy_Policy{{
 			TypedExtensionConfig: &v3corepb.TypedExtensionConfig{
-				TypedConfig: testutils.MarshalAny(t, &v3pickfirstpb.PickFirst{}),
+				TypedConfig: testutils.MarshalAny(t, &v3xdsxdstypepb.TypedStruct{
+					TypeUrl: "type.googleapis.com/" + customLBPolicy,
+					Value:   &structpb.Struct{},
+				}),
 			},
 		}},
 	}
@@ -1217,21 +1225,26 @@ func (s) TestChildPolicyChangeOnConfigUpdate(t *testing.T) {
 
 	select {
 	case <-ctx.Done():
-		t.Fatalf("Timeout waiting for pickfirst child policy config")
+		t.Fatalf("Timeout waiting for child policy config")
 	case <-lbCfgCh:
 	}
 
-	if p := updatedChildPolicy.Load(); p == nil || *p != pfBuilder.Name() {
-		var got string
-		if p != nil {
-			got = *p
-		}
-		t.Fatalf("Unexpected child policy after config update, got %q, want %q", got, pfBuilder.Name())
+	if p, ok := updatedChildPolicy.Load().(string); !ok || p != customLBPolicy {
+		t.Fatalf("Unexpected child policy after config update, got %q (ok: %v), want %q", p, ok, customLBPolicy)
 	}
 
-	// New RPC should still be routed successfully
-	if _, err := client.EmptyCall(ctx, &testpb.Empty{}); err != nil {
-		t.Errorf("EmptyCall() failed after policy update: %v", err)
+	ticker := time.NewTicker(10 * time.Millisecond)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case <-ctx.Done():
+			t.Fatalf("Timeout waiting for successful RPC after policy update.")
+		case <-ticker.C:
+			if _, err := client.EmptyCall(ctx, &testpb.Empty{}); err == nil {
+				return
+			}
+		}
 	}
 }