EM_Task/CoreUObject/Private/UObject/PackageReload.cpp

// Copyright Epic Games, Inc. All Rights Reserved.

#include "UObject/PackageReload.h"
#include "UObject/ReferenceChainSearch.h"
#include "UObject/UObjectHash.h"
#include "UObject/GCObject.h"
#include "Misc/AssetRegistryInterface.h"
#include "Misc/ScopedSlowTask.h"
#include "UObject/Package.h"
#include "UObject/Linker.h"
#include "Templates/Casts.h"
#include "UObject/UObjectIterator.h"

namespace PackageReloadInternal
{

/**
 * Reference to an existing package that prevents it being GC'd while we're still using it (via FExistingPackageReferences).
 * Once we're done with it, we clear out the strong reference and use the weak reference to verify that it was purged correctly via GC.
 */
struct FExistingPackageReference
{
    FExistingPackageReference(UPackage* InPackage)
        : RawRef(InPackage), StrongRef(InPackage), WeakRef(InPackage)
    {
    }

    UPackage* RawRef;
    UPackage* StrongRef;
    TWeakObjectPtr<UPackage> WeakRef;
};

/**
 * Array wrapper that prevents the packages inside the FExistingPackageReference instances being GC'd while we're still using them.
 */
struct FExistingPackageReferences: public FGCObject
{
    virtual void AddReferencedObjects(FReferenceCollector& Collector) override
    {
        for (FExistingPackageReference& Ref: Refs)
        {
            // Note: We deliberate don't ARO RawRef here
            Collector.AddReferencedObject(Ref.StrongRef);
        }
    }

    TArray<FExistingPackageReference> Refs;
};

/**
 * Reference to an replacement package that prevents it being GC'd while we're still using it (via FNewPackageReferences).
 * This also includes the event data used when broadcasting package reload events for this package.
 */
struct FNewPackageReference
{
    FNewPackageReference(UPackage* InPackage)
        : Package(InPackage), EventData()
    {
    }

    UPackage* Package;
    TSharedPtr<FPackageReloadedEvent> EventData;
};

/**
 * Array wrapper that prevents the packages inside the FNewPackageReferences instances being GC'd while we're still using them.
 */
struct FNewPackageReferences: public FGCObject
{
    virtual void AddReferencedObjects(FReferenceCollector& Collector) override
    {
        for (FNewPackageReference& Ref: Refs)
        {
            Collector.AddReferencedObject(Ref.Package);
            if (Ref.EventData)
            {
                Ref.EventData->AddReferencedObjects(Collector);
            }
        }
    }

    TArray<FNewPackageReference> Refs;
};

/**
 * Used to map objects from the old package, to objects in the new package, including the index of the package being reloaded.
 */
struct FObjectAndPackageIndex
{
    FObjectAndPackageIndex(UObject* InObject, const int32 InPackageIndex)
        : Object(InObject), PackageIndex(InPackageIndex)
    {
    }

    UObject* Object;
    int32 PackageIndex;
};

/**
 * Custom archive type used to re-point any in-memory references to objects in the old package to objects in the new package, or null if there is no replacement object.
 */
class FReplaceObjectReferencesArchive: public FArchiveUObject
    , public FReferenceCollector
{
public:
    FReplaceObjectReferencesArchive(UObject* InPotentialReferencer, const TMap<UObject*, FObjectAndPackageIndex>& InOldObjectToNewData, const TArray<FExistingPackageReference>& InExistingPackages, const TArray<FNewPackageReference>& InNewPackages)
        : PotentialReferencer(InPotentialReferencer), OldObjectToNewData(InOldObjectToNewData), ExistingPackages(InExistingPackages), NewPackages(InNewPackages)
    {
        ArIsObjectReferenceCollector         = true;
        ArIsModifyingWeakAndStrongReferences = true;
        ArIgnoreOuterRef                     = true;
        ArNoDelta                            = true;
    }

    virtual ~FReplaceObjectReferencesArchive()
    {
    }

    virtual FString GetArchiveName() const override
    {
        return TEXT("FReplaceObjectReferencesArchive");
    }

    virtual void HandleObjectReference(UObject*& Object, const UObject* ReferencingObject, const FProperty* ReferencingProperty) override
    {
        (*this) << Object;
    }

    virtual void HandleObjectReferences(UObject** InObjects, const int32 ObjectNum, const UObject* InReferencingObject, const FProperty* InReferencingProperty) override
    {
        for (int32 ObjectIndex = 0; ObjectIndex < ObjectNum; ++ObjectIndex)
        {
            UObject*& Object = InObjects[ObjectIndex];
            (*this) << Object;
        }
    }

    virtual bool IsIgnoringArchetypeRef() const override
    {
        return false;
    }

    virtual bool IsIgnoringTransient() const override
    {
        return false;
    }

    FArchive& operator<<(UObject*& ObjRef)
    {
        if (ObjRef && ObjRef != PotentialReferencer)
        {
            UObject* NewObject = nullptr;
            TSharedPtr<FPackageReloadedEvent> PackageEventData;
            if (GetNewObjectAndEventData(ObjRef, NewObject, PackageEventData))
            {
                ObjRef = NewObject;
                PackageEventData->AddObjectReferencer(PotentialReferencer);
            }
        }

        return *this;
    }

    bool GetNewObjectAndEventData(UObject* InOldObject, UObject*& OutNewObject, TSharedPtr<FPackageReloadedEvent>& OutEventData) const
    {
        const FObjectAndPackageIndex* ObjectAndPackageIndexPtr = OldObjectToNewData.Find(InOldObject);

        // Only fix-up references to objects outside of the potential referencer package, as internal object references will be orphaned automatically
        if (ObjectAndPackageIndexPtr && PotentialReferencer->GetOutermost() != ExistingPackages[ObjectAndPackageIndexPtr->PackageIndex].RawRef)
        {
            OutNewObject = ObjectAndPackageIndexPtr->Object;
            OutEventData = NewPackages[ObjectAndPackageIndexPtr->PackageIndex].EventData;
            return true;
        }

        return false;
    }

    UObject* PotentialReferencer;
    const TMap<UObject*, FObjectAndPackageIndex>& OldObjectToNewData;
    const TArray<FExistingPackageReference>& ExistingPackages;
    const TArray<FNewPackageReference>& NewPackages;
};

/**
 * Given a package, mark it and all its sub-objects with the RF_NewerVersionExists flag so that other systems can detect that they're being replaced.
 */
void MarkPackageReplaced(UPackage* InPackage)
{
    InPackage->SetFlags(RF_NewerVersionExists);
    ForEachObjectWithPackage(InPackage, [](UObject* InSubObject)
                             {
                                 InSubObject->SetFlags(RF_NewerVersionExists);
                                 return true;  // continue
                             });
}

/**
 * Given a package, remove the RF_NewerVersionExists flag from it and all its sub-objects.
 */
void ClearPackageReplaced(UPackage* InPackage)
{
    InPackage->ClearFlags(RF_NewerVersionExists);
    ForEachObjectWithPackage(InPackage, [](UObject* InSubObject)
                             {
                                 InSubObject->ClearFlags(RF_NewerVersionExists);
                                 return true;  // continue
                             });
}

/**
 * Given an object, put it into a state where a GC may purge it (assuming there are no external references).
 */
void MakeObjectPurgeable(UObject* InObject)
{
    if (InObject->IsRooted())
    {
        InObject->RemoveFromRoot();
    }
    InObject->ClearFlags(RF_Public | RF_Standalone);
}

/**
 * Given a package, put it into a state where a GC may purge it (assuming there are no external references).
 */
void MakePackagePurgeable(UPackage* InPackage)
{
    MakeObjectPurgeable(InPackage);
    ForEachObjectWithPackage(InPackage, [](UObject* InObject)
                             {
                                 MakeObjectPurgeable(InObject);
                                 return true;  // continue
                             });
}

/**
 * Given an object, dump anything that is still externally referencing it to the log.
 */
void DumpExternalReferences(UObject* InObject, UPackage* InPackage)
{
    TArray<FString> ExternalRefDumps;

    {
        FReferenceChainSearch ObjectRefChains(InObject, EReferenceChainSearchMode::Default);
        for (const FReferenceChainSearch::FReferenceChain* ObjectRefChain: ObjectRefChains.GetReferenceChains())
        {
            for (int32 NodeIndex = 0; NodeIndex < ObjectRefChain->Num(); ++NodeIndex)
            {
                const FReferenceChainSearch::FGraphNode* ObjectRefChainLink = ObjectRefChain->GetNode(NodeIndex);
                const bool bIsExternalRef                                   = ObjectRefChainLink->Object->GetOutermost() != InPackage;
                if (bIsExternalRef)
                {
                    ExternalRefDumps.Emplace(ObjectRefChainLink->Object->GetFullName());
                }
            }
        }
    }

    if (ExternalRefDumps.Num() > 0)
    {
        UE_LOG(LogUObjectGlobals, Display, TEXT("ReloadPackage external references for '%s'."), *InObject->GetPathName());
        for (const FString& ExternalRefDump: ExternalRefDumps)
        {
            UE_LOG(LogUObjectGlobals, Display, TEXT("    %s"), *ExternalRefDump);
        }
    }
}

/**
 * Given a package, validate and prepare it for reload.
 * @return The package to be reloaded, or null if the given package isn't valid to be reloaded.
 */
UPackage* ValidateAndPreparePackageForReload(UPackage* InExistingPackage)
{
    // We can't reload memory-only packages
    if (InExistingPackage->HasAnyPackageFlags(PKG_InMemoryOnly))
    {
        UE_LOG(LogUObjectGlobals, Warning, TEXT("ReloadPackage cannot reload '%s' as it is marked PKG_InMemoryOnly."), *InExistingPackage->GetName());
        return nullptr;
    }

    // Make sure the package has finished loading before we try and unload it again
    if (!InExistingPackage->IsFullyLoaded())
    {
        FlushAsyncLoading();
        InExistingPackage->FullyLoad();
    }
    ResetLoaders(InExistingPackage);

    return InExistingPackage;
}

/**
 * Given a package, reload it from disk.
 * @return The package that was reloaded, or null if the given package couldn't be reloaded.
 */
UPackage* LoadReplacementPackage(UPackage* InExistingPackage, const uint32 InLoadFlags)
{
    if (!InExistingPackage)
    {
        return nullptr;
    }

    const FString ExistingPackageName = InExistingPackage->GetName();

    // Rename the old package, and then load the new one in its place
    const ERenameFlags PkgRenameFlags = REN_ForceNoResetLoaders | REN_DoNotDirty | REN_DontCreateRedirectors | REN_NonTransactional | REN_SkipGeneratedClasses;
    InExistingPackage->Rename(*MakeUniqueObjectName(Cast<UPackage>(InExistingPackage->GetOuter()), UPackage::StaticClass(), *FString::Printf(TEXT("%s_DEADPACKAGE"), *InExistingPackage->GetName())).ToString(), nullptr, PkgRenameFlags);
    MarkPackageReplaced(InExistingPackage);
    UPackage* NewPackage = LoadPackage(Cast<UPackage>(InExistingPackage->GetOuter()), *ExistingPackageName, InLoadFlags);
    if (!NewPackage)
    {
        UE_LOG(LogUObjectGlobals, Warning, TEXT("ReloadPackage cannot reload '%s' as the new package failed to load. The old package will be restored."), *ExistingPackageName);

        // Make sure that the failed load attempt hasn't left any objects behind that would prevent the rename
        if (UPackage* FailedPackage = FindObject<UPackage>(Cast<UPackage>(InExistingPackage->GetOuter()), *ExistingPackageName))
        {
            FailedPackage->Rename(*MakeUniqueObjectName(Cast<UPackage>(FailedPackage->GetOuter()), UPackage::StaticClass(), *FString::Printf(TEXT("%s_DEADPACKAGE"), *FailedPackage->GetName())).ToString(), nullptr, PkgRenameFlags);
            MakePackagePurgeable(FailedPackage);
        }

        // Failed to load the new package, give the old package its original name and bail!
        InExistingPackage->Rename(*ExistingPackageName, nullptr, PkgRenameFlags);
        ClearPackageReplaced(InExistingPackage);
        return nullptr;
    }

    // Make sure the package has finished loading before we try and find things from inside it
    if (!NewPackage->IsFullyLoaded())
    {
        FlushAsyncLoading();
        NewPackage->FullyLoad();
    }

    return NewPackage;
}

/**
 * Given an old and new package, generate the event payload data needed to fix-up references to objects from the old package to the corresponding objects in the new package.
 * @return The event payload data, or null if either given package is invalid.
 */
TSharedPtr<FPackageReloadedEvent> GeneratePackageReloadEvent(UPackage* InExistingPackage, UPackage* InNewPackage)
{
    TSharedPtr<FPackageReloadedEvent> PackageReloadedEvent;

    if (InExistingPackage && InNewPackage)
    {
        TMap<UObject*, UObject*> RedirectedObjectsMap;
        RedirectedObjectsMap.Emplace(InExistingPackage, InNewPackage);
        InExistingPackage->BuildSubobjectMapping(InNewPackage, RedirectedObjectsMap);

        for (const auto& ObjectMappingPair: RedirectedObjectsMap)
        {
            UObject* ExistingObject = ObjectMappingPair.Key;
            UObject* NewObject      = ObjectMappingPair.Value;

            if (NewObject)
            {
                // Pass on the root-set state from the old object to the new one
                if (ExistingObject->IsRooted())
                {
                    NewObject->AddToRoot();
                }

                // Pass on some important flags to the new object
                {
                    const EObjectFlags FlagsToPassToNewObject = ExistingObject->GetMaskedFlags(RF_Public | RF_Standalone | RF_Transactional);
                    NewObject->SetFlags(FlagsToPassToNewObject);
                }
            }
            else if (ExistingObject->HasAnyFlags(RF_Transient))
            {
                UE_LOG(LogUObjectGlobals, Display, TEXT("ReloadPackage failed to find a replacement object for '%s' (transient) in the new package '%s'. Any existing references to this object will be nulled out."), *ExistingObject->GetPathName(InExistingPackage), *InNewPackage->GetName());
            }
            else
            {
                UE_LOG(LogUObjectGlobals, Warning, TEXT("ReloadPackage failed to find a replacement object for '%s' in the new package '%s'. Any existing references to this object will be nulled out."), *ExistingObject->GetPathName(InExistingPackage), *InNewPackage->GetName());
            }
        }

        PackageReloadedEvent = MakeShared<FPackageReloadedEvent>(InExistingPackage, InNewPackage, MoveTemp(RedirectedObjectsMap));
    }

    return PackageReloadedEvent;
}

void SortPackagesForReload(const FName PackageName, TSet<FName>& ProcessedPackages, TArray<UPackage*>& SortedPackagesToReload, const TMap<FName, UPackage*>& AllPackagesToReload, IAssetRegistryInterface& InAssetRegistry)
{
    ProcessedPackages.Add(PackageName);

    TArray<FName> PackageDependencies;
    InAssetRegistry.GetDependencies(PackageName, PackageDependencies, UE::AssetRegistry::EDependencyCategory::Package, UE::AssetRegistry::EDependencyQuery::Hard);

    // Recursively go through processing each new dependency until we run out
    for (const FName& Dependency: PackageDependencies)
    {
        if (!ProcessedPackages.Contains(Dependency))
        {
            SortPackagesForReload(Dependency, ProcessedPackages, SortedPackagesToReload, AllPackagesToReload, InAssetRegistry);
        }
    }

    // Add this package to the sorted array now that its dependencies have been processed
    if (AllPackagesToReload.Contains(PackageName))
    {
        SortedPackagesToReload.Emplace(AllPackagesToReload[PackageName]);
    }
}

}  // namespace PackageReloadInternal

void SortPackagesForReload(TArray<UPackage*>& PackagesToReload)
{
    // We need to sort the packages to reload so that dependencies are reloaded before the assets that depend on them
    if (PackagesToReload.Num() > 1)
    {
        IAssetRegistryInterface* AssetRegistry = IAssetRegistryInterface::GetPtr();
        checkf(AssetRegistry, TEXT("SortPackagesForReload requires the asset registry to perform dependency analysis, but no asset registry is available."));

        TSet<FName> ProcessedPackages;
        ProcessedPackages.Reserve(PackagesToReload.Num());

        TArray<UPackage*> SortedPackagesToReload;
        SortedPackagesToReload.Reserve(PackagesToReload.Num());

        TMap<FName, UPackage*> AllPackagesToReload;
        AllPackagesToReload.Reserve(PackagesToReload.Num());
        for (UPackage* PackageToReload: PackagesToReload)
        {
            AllPackagesToReload.Emplace(PackageToReload->GetFName(), PackageToReload);
        }

        for (UPackage* PackageToReload: PackagesToReload)
        {
            if (!ProcessedPackages.Contains(PackageToReload->GetFName()))
            {
                PackageReloadInternal::SortPackagesForReload(PackageToReload->GetFName(), ProcessedPackages, SortedPackagesToReload, AllPackagesToReload, *AssetRegistry);
            }
        }

        PackagesToReload = MoveTemp(SortedPackagesToReload);
    }
}

UPackage* ReloadPackage(UPackage* InPackageToReload, const uint32 InLoadFlags)
{
    TArray<UPackage*> ReloadedPackages;

    FReloadPackageData ReloadPackageData(InPackageToReload, InLoadFlags);
    ReloadPackages(TArrayView<FReloadPackageData>(&ReloadPackageData, 1), ReloadedPackages, 1);

    return ReloadedPackages[0];
}

void ReloadPackages(const TArrayView<FReloadPackageData>& InPackagesToReload, TArray<UPackage*>& OutReloadedPackages, int32 InNumPackagesPerBatch)
{
    // Interdependencies between packages (in particular Blueprints) make it unsafe to run this logic in batches.
    // There are a number of edge cases that would have to be addressed if the batching logic were to be re-enabled, but
    // most likely the blueprint reparenting step would have to take in a TMap<UObject*, UObject*> that it could update
    // when it decided that it needed to replace an instance due to hierarchy changes (e.g. class layout changing
    // due to SuperStruct changes). For now, just process assets in a batch size of 1:
    InNumPackagesPerBatch = 1;

    FString Msg;
    Msg.Append(FString::Printf(TEXT("Reloading %d Package(s):"), InPackagesToReload.Num()));
    const int32 MAX_PACKAGES_TO_LOG = 10;
    for (int32 I = 0; I < InPackagesToReload.Num() && I < MAX_PACKAGES_TO_LOG; ++I)
    {
        const FReloadPackageData& ReloadPackageData = InPackagesToReload[I];
        Msg.Append(TEXT("\n"));
        Msg.Append(FString::Printf(TEXT("\tAsset Name: %s"), *ReloadPackageData.PackageToReload->GetName()));
    }
    UE_LOG(LogUObjectGlobals, Log, TEXT("%s"), *Msg);

    FScopedSlowTask ReloadingPackagesSlowTask(InPackagesToReload.Num(), NSLOCTEXT("CoreUObject", "ReloadingPackages", "Reloading Packages"));
    ReloadingPackagesSlowTask.MakeDialog();

    // Cache the current dirty state of all packages so we can restore it after the reload
    TSet<FName> DirtyPackages;
    ForEachObjectOfClass(UPackage::StaticClass(), [&DirtyPackages](UObject* InPackageObj)
                         {
                             UPackage* Package = CastChecked<UPackage>(InPackageObj);
                             if (Package->IsDirty())
                             {
                                 DirtyPackages.Add(Package->GetFName());
                             }
                         },
                         false);

    // Gather up the list of all packages to reload (note: this array may include null packages!)
    PackageReloadInternal::FExistingPackageReferences ExistingPackages;
    ExistingPackages.Refs.Reserve(InPackagesToReload.Num());
    {
        FScopedSlowTask PreparingPackagesForReloadSlowTask(InPackagesToReload.Num(), NSLOCTEXT("CoreUObject", "PreparingPackagesForReload", "Preparing Packages for Reload"));

        for (const FReloadPackageData& PackageToReloadData: InPackagesToReload)
        {
            PreparingPackagesForReloadSlowTask.EnterProgressFrame(1.0f);
            ExistingPackages.Refs.Emplace(PackageReloadInternal::ValidateAndPreparePackageForReload(PackageToReloadData.PackageToReload));
        }

        if (ExistingPackages.Refs.Num() > 0)
        {
            // Run a GC before we start to clean-up any lingering objects that may reference things we're about to reload
            CollectGarbage(GARBAGE_COLLECTION_KEEPFLAGS);
        }
    }

    // Rename the existing packages, load the new packages, then fix-up any references
    PackageReloadInternal::FNewPackageReferences NewPackages;
    NewPackages.Refs.Reserve(ExistingPackages.Refs.Num());
    {
        // Process the packages in batches to avoid consuming too much memory due to a lack of GC
        int32 PackageIndex = 0;
        while (PackageIndex < ExistingPackages.Refs.Num())
        {
            FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::PreBatch, nullptr);

            const int32 BatchStartIndex = PackageIndex;
            for (; PackageIndex < ExistingPackages.Refs.Num(); ++PackageIndex)
            {
                UPackage* ExistingPackage = ExistingPackages.Refs[PackageIndex].RawRef;

                const FText ProgressText  = ExistingPackage ? FText::Format(NSLOCTEXT("CoreUObject", "ReloadingPackagef", "Reloading {0}..."), FText::FromName(ExistingPackage->GetFName())) : NSLOCTEXT("CoreUObject", "ReloadingPackages", "Reloading Packages");
                ReloadingPackagesSlowTask.EnterProgressFrame(1, ProgressText);

                {
                    FPackageReloadedEvent TempReloadEvent(ExistingPackage, nullptr, TMap<UObject*, UObject*>());
                    FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::PrePackageLoad, &TempReloadEvent);
                }

                check(NewPackages.Refs.Num() == PackageIndex);
                NewPackages.Refs.Emplace(PackageReloadInternal::LoadReplacementPackage(ExistingPackage, InPackagesToReload[PackageIndex].LoadFlags));

                UPackage* NewPackage                     = NewPackages.Refs[PackageIndex].Package;
                NewPackages.Refs[PackageIndex].EventData = PackageReloadInternal::GeneratePackageReloadEvent(ExistingPackage, NewPackage);

                const bool bEndBatch                     = (PackageIndex == (BatchStartIndex + InNumPackagesPerBatch)) || (ExistingPackage && ExistingPackage->ContainsMap());
                if (bEndBatch)
                {
                    ++PackageIndex;  // We still need to move on-to the next package for the next batch
                    break;
                }
            }

            const int32 NumPackagesInBatch = PackageIndex - BatchStartIndex;

            FScopedSlowTask FixingUpReferencesSlowTask((NumPackagesInBatch * 4) + GUObjectArray.GetObjectArrayNum(), NSLOCTEXT("CoreUObject", "FixingUpReferences", "Fixing-Up References"));

            // Pre-pass to notify things that the package old package is about to be fixed-up
            TMap<UObject*, PackageReloadInternal::FObjectAndPackageIndex> OldObjectToNewData;
            for (int32 BatchPackageIndex = BatchStartIndex; BatchPackageIndex < PackageIndex; ++BatchPackageIndex)
            {
                FixingUpReferencesSlowTask.EnterProgressFrame(1.0f);

                PackageReloadInternal::FNewPackageReference& NewPackageData = NewPackages.Refs[BatchPackageIndex];
                if (NewPackageData.EventData.IsValid())
                {
                    FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::PrePackageFixup, NewPackageData.EventData.Get());
                    FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::OnPackageFixup, NewPackageData.EventData.Get());

                    // Build up the mapping of old objects to the package index that contains them; this is needed to track per-package references correctly
                    OldObjectToNewData.Reserve(OldObjectToNewData.Num() + NewPackageData.EventData->GetRepointedObjects().Num());
                    for (const auto& ObjectMappingPair: NewPackageData.EventData->GetRepointedObjects())
                    {
                        OldObjectToNewData.Add(ObjectMappingPair.Key, PackageReloadInternal::FObjectAndPackageIndex(ObjectMappingPair.Value, BatchPackageIndex));
                    }
                }
            }

            // Main pass to go through and fix-up any references pointing to data from the old package to point to data from the new package
            // todo: multi-thread this like FHotReloadModule::ReplaceReferencesToReconstructedCDOs?
            for (FThreadSafeObjectIterator ObjIter(UObject::StaticClass(), false, RF_NoFlags, EInternalObjectFlags::PendingKill); ObjIter; ++ObjIter)
            {
                UObject* PotentialReferencer = *ObjIter;

                // Mutating the old versions of classes can result in us replacing the SuperStruct pointer, which results
                // in class layout change and subsequently crashes because instances will not match this new class layout:
                UClass* AsClass = Cast<UClass>(PotentialReferencer);
                if (!AsClass)
                {
                    AsClass = PotentialReferencer->GetTypedOuter<UClass>();
                }

                if (AsClass)
                {
                    if (AsClass->HasAnyClassFlags(CLASS_NewerVersionExists) ||
                        AsClass->HasAnyFlags(RF_NewerVersionExists))
                    {
                        continue;
                    }
                }

                FixingUpReferencesSlowTask.EnterProgressFrame(1.0f);

                PackageReloadInternal::FReplaceObjectReferencesArchive ReplaceRefsArchive(PotentialReferencer, OldObjectToNewData, ExistingPackages.Refs, NewPackages.Refs);
                PotentialReferencer->Serialize(ReplaceRefsArchive);                                                  // Deal with direct references during Serialization
                PotentialReferencer->GetClass()->CallAddReferencedObjects(PotentialReferencer, ReplaceRefsArchive);  // Deal with indirect references via AddReferencedObjects
            }

            // The above fix-up also repoints the StrongRef in FExistingPackageReference, so we'll fix that up again now to prevent the old package from being GC'd
            for (int32 BatchPackageIndex = BatchStartIndex; BatchPackageIndex < PackageIndex; ++BatchPackageIndex)
            {
                FixingUpReferencesSlowTask.EnterProgressFrame(1.0f);

                ExistingPackages.Refs[BatchPackageIndex].StrongRef = ExistingPackages.Refs[BatchPackageIndex].RawRef;
            }

            // Final pass to clean-up any remaining references prior to GC
            // Note: We do this as a separate pass to preparing the objects for GC as this callback may prematurely invoke a GC that invalidates some data we're working with
            for (int32 BatchPackageIndex = BatchStartIndex; BatchPackageIndex < PackageIndex; ++BatchPackageIndex)
            {
                FixingUpReferencesSlowTask.EnterProgressFrame(1.0f);

                PackageReloadInternal::FNewPackageReference& NewPackageData = NewPackages.Refs[BatchPackageIndex];
                if (NewPackageData.EventData.IsValid())
                {
                    FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::PostPackageFixup, NewPackageData.EventData.Get());
                }
            }

            FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::PostBatchPreGC, nullptr);

            // Purge old packages that have had a replacement package loaded
            for (int32 BatchPackageIndex = BatchStartIndex; BatchPackageIndex < PackageIndex; ++BatchPackageIndex)
            {
                FixingUpReferencesSlowTask.EnterProgressFrame(1.0f);

                UPackage* ExistingPackage = ExistingPackages.Refs[BatchPackageIndex].RawRef;
                UPackage* NewPackage      = NewPackages.Refs[BatchPackageIndex].Package;

                if (ExistingPackage && NewPackage)
                {
                    // Allow the old package to be GC'd
                    PackageReloadInternal::MakePackagePurgeable(ExistingPackage);
                    ExistingPackages.Refs[BatchPackageIndex].StrongRef = nullptr;
                    NewPackages.Refs[BatchPackageIndex].EventData.Reset();
                }
            }
            CollectGarbage(GARBAGE_COLLECTION_KEEPFLAGS);

            FCoreUObjectDelegates::OnPackageReloaded.Broadcast(EPackageReloadPhase::PostBatchPostGC, nullptr);
        }
    }

    // Clean any packages that we dirtied as part of the replacement process
    ForEachObjectOfClass(UPackage::StaticClass(), [&DirtyPackages](UObject* InPackageObj)
                         {
                             UPackage* Package = CastChecked<UPackage>(InPackageObj);
                             if (Package->IsDirty() && !DirtyPackages.Contains(Package->GetFName()))
                             {
                                 Package->SetDirtyFlag(false);
                             }
                         },
                         false);

    // Finalization and error reporting
    OutReloadedPackages.Reserve(ExistingPackages.Refs.Num());
    for (int32 PackageIndex = 0; PackageIndex < ExistingPackages.Refs.Num(); ++PackageIndex)
    {
        UPackage* ExistingPackage = ExistingPackages.Refs[PackageIndex].WeakRef.Get();
        UPackage* NewPackage      = NewPackages.Refs[PackageIndex].Package;

        check(OutReloadedPackages.Num() == PackageIndex);
        OutReloadedPackages.Emplace(NewPackage);

        // Report any old packages that failed to purge
        if (ExistingPackage && NewPackage)
        {
            UE_LOG(LogUObjectGlobals, Warning, TEXT("ReloadPackage failed to purge the old package '%s'. This is unexpected, and likely means that it was still externally referenced."), *ExistingPackage->GetName());

            const bool bDumpExternalReferences = DO_GUARD_SLOW || (WITH_EDITOR && GIsEditor);
            if (bDumpExternalReferences)
            {
                PackageReloadInternal::DumpExternalReferences(ExistingPackage, ExistingPackage);
                // ForEachObjectWithPackage(ExistingPackage, [](UObject* InExistingObject)
                //{
                //	PackageReloadInternal::DumpExternalReferences(InExistingObject, ExistingPackage);
                //	return true; // continue
                // }, true, RF_NoFlags, EInternalObjectFlags::PendingKill);
            }
        }
    }
}