StreamingGeneration: Actor desc mutator phase now writes ActorDescView directly, breaking the cluster-invariant check in ValidateContainerInstanceDescriptors

This report is from an EPS case:

We hit a regression upgrading our project from 5.7 to 5.8. Map Check (and any other path that ends up calling UWorldPartition::CheckForErrors) assert inside FWorldPartitionStreamingGenerator::ValidateContainerInstanceDescriptors, on the RuntimeGrid check():

for (const FGuid& ActorGuid : Cluster)
{

const FStreamingGenerationActorDescView& ActorDescView =

ContainerCollectionInstanceDescriptor.ActorDescViewMap->FindByGuidChecked(ActorGuid);

check(ActorDescView.GetRuntimeGrid() == ReferenceActorDescView.GetRuntimeGrid());

check(ActorDescView.GetIsSpatiallyLoaded() == ReferenceActorDescView.GetIsSpatiallyLoaded());

check(ActorDescView.GetContentBundleGuid() == ReferenceActorDescView.GetContentBundleGuid());

check(ActorDescView.GetExternalDataLayerAsset()== ReferenceActorDescView.GetExternalDataLayerAsset());

}

What changed between 5.7 and 5.8

Phase ordering inside PreparationPhase did not change. The mutator is still called between two

ValidateContainerInstanceDescriptors() passes. Clusters are still built in UpdateContainerDescriptor → GenerateObjectsClusters from references only. What changed is what the mutator-apply loop writes into.

5.7:

FContainerCollectionInstanceDescriptor::FPerInstanceData PerInstanceData =
ContainerCollectionInstanceDescriptor.GetPerInstanceData(ActorGuid);

if (ActorDescViewMutator.RuntimeGrid.IsSet())
{
    PerInstanceData.RuntimeGrid = ActorDescViewMutator.RuntimeGrid.GetValue();
}

ContainerCollectionInstanceDescriptor.AddPerInstanceData(ActorGuid, PerInstanceData);

Mutator writes a per-instance side-table. FStreamingGenerationActorDescView::GetRuntimeGrid() keeps returning the pre-mutation value, so the second ValidateContainerInstanceDescriptors pass sees a uniform grid per cluster and passes.

5.8 (current):

if (FStreamingGenerationActorDescView* ActorDescView =
ContainerCollectionInstanceDescriptor.ActorDescViewMap->FindByGuid(ActorGuid))
{
   if (ActorDescViewMutator.RuntimeGrid.IsSet())
   {
      ActorDescView->SetRuntimeGrid(ActorDescViewMutator.RuntimeGrid.GetValue());
   }
// ... same pattern for bIsSpatiallyLoaded, bIsHLODRelevant, HLODLayer
}

FPerInstanceData, AddPerInstanceData, GetPerInstanceData were removed; the mutator now writes the shared ActorDescView directly. The second ValidateContainerInstanceDescriptors pass now reads the post-mutation grid and trips the invariant whenever the mutator override touched only some of a ref-connected cluster.

Workaround:

We have made a small internal change that re-splits each ref-built cluster by the four invariant keys right after MutateContainerINstanceDescriptors, before the second validate:

1. if (MutateContainerInstanceDescriptors(...))
   {
      SplitClustersByMutatedAttributes(); // bucket each cluster by (RuntimeGrid, bIsSpatiallyLoaded, ContentBundleGuid, ExternalDataLayerAsset)
      ValidateContainerInstanceDescriptors();
   }