Don't infer TypeURL of ads.Resource with utils.GetTypeURL

See comment inside `ads.Resource.TypeURL`, this could otherwise cause a panic
when `ads.Resource[proto.Message]` is used instead of a hard type like
`ads.Resource[*ads.Endpoint]`.

ads/ads.go

@@ -124,8 +124,12 @@ func (r *Resource[T]) Marshal() (*RawResource, error) {
 
 // TypeURL returns the underlying resource's type URL.
 func (r *Resource[T]) TypeURL() string {
-	var t T
-	return types.APITypePrefix + string(t.ProtoReflect().Descriptor().FullName())
+	// A literal `Resource[proto.Message]` works well when the goal is to store the deserialized
+	// [RawResource]. However, inferring the type URL cannot be done with [utils.GetTypeURL] since it
+	// uses reflection on the generic type parameter, which in this case is just [proto.Message], which
+	// causes a panic. Instead, infer the type directly from the deserialized message, avoiding the panic
+	// if the type parameter is indeed simply proto.Message.
+	return types.APITypePrefix + string(r.Resource.ProtoReflect().Descriptor().FullName())
 }
 
 // UnmarshalRawResource unmarshals the given RawResource and returns a Resource of the corresponding

ads/ads_test.go

@@ -0,0 +1,19 @@
+package ads
+
+import (
+	"testing"
+
+	types "github.com/envoyproxy/go-control-plane/pkg/resource/v3"
+	"github.com/linkedin/diderot/internal/utils"
+	"github.com/stretchr/testify/require"
+	"google.golang.org/protobuf/proto"
+)
+
+func TestTypeURL(t *testing.T) {
+	require.Panics(t, func() {
+		utils.GetTypeURL[proto.Message]()
+	})
+
+	r := NewResource[proto.Message]("foo", "0", new(Endpoint))
+	require.Equal(t, types.EndpointType, r.TypeURL())
+}