UnrealEngineUWP/Engine/Source/Developer/DerivedDataCache/Private/DerivedDataCacheStoreAsync.cpp

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

#include "Async/AsyncWork.h"
#include "DerivedDataBackendInterface.h"
#include "DerivedDataCachePrivate.h"
#include "DerivedDataCacheStore.h"
#include "DerivedDataCacheUsageStats.h"
#include "DerivedDataRequest.h"
#include "DerivedDataRequestOwner.h"
#include "DerivedDataValueId.h"
#include "Experimental/Async/LazyEvent.h"
#include "FileBackedDerivedDataBackend.h"
#include "Memory/SharedBuffer.h"
#include "Misc/ScopeLock.h"
#include "ProfilingDebugging/CookStats.h"
#include "Stats/Stats.h"
#include "Tasks/Task.h"

namespace UE::DerivedData::CacheStore::Memory
{
FFileBackedDerivedDataBackend* CreateMemoryDerivedDataBackend(const TCHAR* Name, int64 MaxCacheSize, bool bCanBeDisabled);
} // UE::DerivedData::CacheStore::Memory

namespace UE::DerivedData::CacheStore::AsyncPut
{

/** 
 * Thread safe set helper
**/
struct FThreadSet
{
	FCriticalSection	SynchronizationObject;
	TSet<FString>		FilesInFlight;

	void Add(const FString& Key)
	{
		FScopeLock ScopeLock(&SynchronizationObject);
		check(Key.Len());
		FilesInFlight.Add(Key);
	}
	void Remove(const FString& Key)
	{
		FScopeLock ScopeLock(&SynchronizationObject);
		FilesInFlight.Remove(Key);
	}
	bool Exists(const FString& Key)
	{
		FScopeLock ScopeLock(&SynchronizationObject);
		return FilesInFlight.Contains(Key);
	}
	bool AddIfNotExists(const FString& Key)
	{
		FScopeLock ScopeLock(&SynchronizationObject);
		check(Key.Len());
		if (!FilesInFlight.Contains(Key))
		{
			FilesInFlight.Add(Key);
			return true;
		}
		return false;
	}
};

/** 
 * A backend wrapper that coordinates async puts. This means that a Get will hit an in-memory cache while the async put is still in flight.
**/
class FDerivedDataBackendAsyncPutWrapper : public FDerivedDataBackendInterface
{
public:

	/**
	 * Constructor
	 *
	 * @param	InInnerBackend		Backend to use for storage, my responsibilities are about async puts
	 * @param	bCacheInFlightPuts	if true, cache in-flight puts in a memory cache so that they hit immediately
	 */
	FDerivedDataBackendAsyncPutWrapper(FDerivedDataBackendInterface* InInnerBackend, bool bCacheInFlightPuts);

	/** Return a name for this interface */
	virtual FString GetName() const override
	{
		return FString::Printf(TEXT("AsyncPutWrapper (%s)"), *InnerBackend->GetName());
	}

	/** return true if this cache is writable **/
	virtual bool IsWritable() const override;

	/** Returns a class of speed for this interface **/
	virtual ESpeedClass GetSpeedClass() const override;

	/** Return true if hits on this cache should propagate to lower cache level. */
	virtual bool BackfillLowerCacheLevels() const override;

	/**
	 * Synchronous test for the existence of a cache item
	 *
	 * @param	CacheKey	Alphanumeric+underscore key of this cache item
	 * @return				true if the data probably will be found, this can't be guaranteed because of concurrency in the backends, corruption, etc
	 */
	virtual bool CachedDataProbablyExists(const TCHAR* CacheKey) override;

	/**
	 * Synchronous test for the existence of multiple cache items
	 *
	 * @param	CacheKeys	Alphanumeric+underscore key of the cache items
	 * @return				A bit array with bits indicating whether the data for the corresponding key will probably be found
	 */
	virtual TBitArray<> CachedDataProbablyExistsBatch(TConstArrayView<FString> CacheKeys) override;

	/**
	 * Attempt to make sure the cached data will be available as optimally as possible.
	 *
	 * @param	CacheKeys	Alphanumeric+underscore keys of the cache items
	 * @return				true if the data will probably be found in a fast backend on a future request.
	 */
	virtual TBitArray<> TryToPrefetch(TConstArrayView<FString> CacheKeys) override;

	/**
	 * Allows the DDC backend to determine if it wants to cache the provided data. Reasons for returning false could be a slow connection,
	 * a file size limit, etc.
	 */
	virtual bool WouldCache(const TCHAR* CacheKey, TArrayView<const uint8> InData) override;

	/**
	 * Synchronous retrieve of a cache item
	 *
	 * @param	CacheKey	Alphanumeric+underscore key of this cache item
	 * @param	OutData		Buffer to receive the results, if any were found
	 * @return				true if any data was found, and in this case OutData is non-empty
	 */
	virtual bool GetCachedData(const TCHAR* CacheKey, TArray<uint8>& OutData) override;

	/**
	 * Asynchronous, fire-and-forget placement of a cache item
	 *
	 * @param	CacheKey	Alphanumeric+underscore key of this cache item
	 * @param	InData		Buffer containing the data to cache, can be destroyed after the call returns, immediately
	 * @param	bPutEvenIfExists	If true, then do not attempt skip the put even if CachedDataProbablyExists returns true
	 */
	virtual EPutStatus PutCachedData(const TCHAR* CacheKey, TArrayView<const uint8> InData, bool bPutEvenIfExists) override;

	virtual void RemoveCachedData(const TCHAR* CacheKey, bool bTransient) override;

	virtual TSharedRef<FDerivedDataCacheStatsNode> GatherUsageStats() const override;

	virtual bool ApplyDebugOptions(FBackendDebugOptions& InOptions) override;

	virtual void Put(
		TConstArrayView<FCachePutRequest> Requests,
		IRequestOwner& Owner,
		FOnCachePutComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimePut,) Requests, Owner, MoveTemp(OnComplete), &ICacheStore::Put);
	}

	virtual void Get(
		TConstArrayView<FCacheGetRequest> Requests,
		IRequestOwner& Owner,
		FOnCacheGetComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimeGet,) Requests, Owner, MoveTemp(OnComplete), &ICacheStore::Get);
	}

	virtual void PutValue(
		TConstArrayView<FCachePutValueRequest> Requests,
		IRequestOwner& Owner,
		FOnCachePutValueComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimePut,) Requests, Owner, MoveTemp(OnComplete), &ICacheStore::PutValue);
	}

	virtual void GetValue(
		TConstArrayView<FCacheGetValueRequest> Requests,
		IRequestOwner& Owner,
		FOnCacheGetValueComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimeGet,) Requests, Owner, MoveTemp(OnComplete), &ICacheStore::GetValue);
	}

	virtual void GetChunks(
		TConstArrayView<FCacheGetChunkRequest> Requests,
		IRequestOwner& Owner,
		FOnCacheGetChunkComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimeGet,) Requests, Owner, MoveTemp(OnComplete), &ICacheStore::GetChunks);
	}

	virtual void LegacyPut(
		TConstArrayView<FLegacyCachePutRequest> Requests,
		IRequestOwner& Owner,
		FOnLegacyCachePutComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimePut,) Requests, Owner, MoveTemp(OnComplete), &ILegacyCacheStore::LegacyPut);
	}

	virtual void LegacyGet(
		TConstArrayView<FLegacyCacheGetRequest> Requests,
		IRequestOwner& Owner,
		FOnLegacyCacheGetComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimeGet,) Requests, Owner, MoveTemp(OnComplete), &ILegacyCacheStore::LegacyGet);
	}

	virtual void LegacyDelete(
		TConstArrayView<FLegacyCacheDeleteRequest> Requests,
		IRequestOwner& Owner,
		FOnLegacyCacheDeleteComplete&& OnComplete) override
	{
		Execute(COOK_STAT(&FDerivedDataCacheUsageStats::TimePut,) Requests, Owner, MoveTemp(OnComplete), &ILegacyCacheStore::LegacyDelete);
	}

	virtual void LegacyStats(FDerivedDataCacheStatsNode& OutNode) override;

	virtual bool LegacyDebugOptions(FBackendDebugOptions& Options) override
	{
		return InnerBackend->LegacyDebugOptions(Options);
	}

private:
	COOK_STAT(using CookStatsFunction = FCookStats::FScopedStatsCounter (FDerivedDataCacheUsageStats::*)());

	template <typename RequestType, typename OnCompleteType, typename OnExecuteType>
	void Execute(
		COOK_STAT(CookStatsFunction OnAddStats,)
		TConstArrayView<RequestType> Requests,
		IRequestOwner& Owner,
		OnCompleteType&& OnComplete,
		OnExecuteType&& OnExecute);

	FDerivedDataCacheUsageStats UsageStats;
	FDerivedDataCacheUsageStats PutSyncUsageStats;

	/** Backend to use for storage, my responsibilities are about async puts **/
	FDerivedDataBackendInterface*					InnerBackend;
	/** Memory based cache to deal with gets that happen while an async put is still in flight **/
	TUniquePtr<FDerivedDataBackendInterface>		InflightCache;
	/** We remember outstanding puts so that we don't do them redundantly **/
	FThreadSet										FilesInFlight;
};

/** 
 * Async task to handle the fire and forget async put
 */
class FCachePutAsyncWorker
{
public:
	/** Cache Key for the put to InnerBackend **/
	FString								CacheKey;
	/** Data for the put to InnerBackend **/
	TArray<uint8>						Data;
	/** Backend to use for storage, my responsibilities are about async puts **/
	FDerivedDataBackendInterface*		InnerBackend;
	/** Memory based cache to clear once the put is finished **/
	FDerivedDataBackendInterface*		InflightCache;
	/** We remember outstanding puts so that we don't do them redundantly **/
	FThreadSet*							FilesInFlight;
	/**If true, then do not attempt skip the put even if CachedDataProbablyExists returns true **/
	bool								bPutEvenIfExists;
	/** Usage stats to track thread times. */
	FDerivedDataCacheUsageStats&        UsageStats;

	/** Constructor
	*/
	FCachePutAsyncWorker(const TCHAR* InCacheKey, TArrayView<const uint8> InData, FDerivedDataBackendInterface* InInnerBackend, bool InbPutEvenIfExists, FDerivedDataBackendInterface* InInflightCache, FThreadSet* InInFilesInFlight, FDerivedDataCacheUsageStats& InUsageStats)
		: CacheKey(InCacheKey)
		, InnerBackend(InInnerBackend)
		, InflightCache(InInflightCache)
		, FilesInFlight(InInFilesInFlight)
		, bPutEvenIfExists(InbPutEvenIfExists)
		, UsageStats(InUsageStats)
	{
		// Only make a copy if it's not going to be available from the Inflight cache
		if (!InInflightCache || !InInflightCache->CachedDataProbablyExists(InCacheKey))
		{
			Data = InData;
		}
		check(InnerBackend);
	}

	bool ShouldAbortForShutdown()
	{
		using ESpeedClass = FDerivedDataBackendInterface::ESpeedClass;
		ESpeedClass SpeedClass = InnerBackend->GetSpeedClass();
		if (SpeedClass == ESpeedClass::Local)
		{
			return false;
		}
		return !GIsBuildMachine && FDerivedDataBackend::Get().IsShuttingDown();
	}
		
	/** Call the inner backend and when that completes, remove the memory cache */
	void DoWork()
	{
		TRACE_CPUPROFILER_EVENT_SCOPE(DDCPut_DoWork);
		COOK_STAT(auto Timer = UsageStats.TimePut());

		if (ShouldAbortForShutdown())
		{
			Abandon();
			return;
		}

		using EPutStatus = FDerivedDataBackendInterface::EPutStatus;
		EPutStatus Status = EPutStatus::NotCached;

		if (!bPutEvenIfExists && InnerBackend->CachedDataProbablyExists(*CacheKey))
		{
			Status = EPutStatus::Cached;
		}
		else
		{
			if (InflightCache && Data.Num() == 0)
			{
				// We verified at construction time that we would be able to get the data from the Inflight cache
				verify(InflightCache->GetCachedData(*CacheKey, Data));
			}
			Status = InnerBackend->PutCachedData(*CacheKey, Data, bPutEvenIfExists);
			COOK_STAT(Timer.AddHit(Data.Num()));
		}

		if (InflightCache)
		{
			// if the data was not cached synchronously, retry
			if (Status != EPutStatus::Cached && Status != EPutStatus::Skipped)
			{
				// retry after a brief wait
				FPlatformProcess::SleepNoStats(0.2f);

				if (Status == EPutStatus::Executing && InnerBackend->CachedDataProbablyExists(*CacheKey))
				{
					Status = EPutStatus::Cached;
				}
				else
				{
					if (Data.Num() == 0)
					{
						verify(InflightCache->GetCachedData(*CacheKey, Data));
					}
					Status = InnerBackend->PutCachedData(*CacheKey, Data, /*bPutEvenIfExists*/ false);
				}
			}

			switch (Status)
			{
			case EPutStatus::Skipped:
			case EPutStatus::Cached:
				// remove this from the in-flight cache because the inner cache contains the data or it was intentionally skipped
				InflightCache->RemoveCachedData(*CacheKey, /*bTransient*/ false);
				break;
			case EPutStatus::NotCached:
				UE_LOG(LogDerivedDataCache, Display, TEXT("%s: Put failed, keeping in memory copy %s."), *InnerBackend->GetName(), *CacheKey);
				if (uint32 ErrorCode = FPlatformMisc::GetLastError())
				{
					TCHAR ErrorBuffer[1024];
					FPlatformMisc::GetSystemErrorMessage(ErrorBuffer, 1024, ErrorCode);
					UE_LOG(LogDerivedDataCache, Display, TEXT("Failed to write %s to %s. Error: %u (%s)"), *CacheKey, *InnerBackend->GetName(), ErrorCode, ErrorBuffer);
				}
				break;
			case EPutStatus::Executing:
				UE_LOG(LogDerivedDataCache, Display, TEXT("%s: Put not finished executing, keeping in memory copy %s."), *InnerBackend->GetName(), *CacheKey);
				break;
			default:
				break;
			}
		}

		FilesInFlight->Remove(CacheKey);
		FDerivedDataBackend::Get().AddToAsyncCompletionCounter(-1);
		UE_LOG(LogDerivedDataCache, VeryVerbose, TEXT("%s: Completed AsyncPut of %s."), *InnerBackend->GetName(), *CacheKey);
	}

	FORCEINLINE TStatId GetStatId() const
	{
		RETURN_QUICK_DECLARE_CYCLE_STAT(FCachePutAsyncWorker, STATGROUP_ThreadPoolAsyncTasks);
	}

	/** Indicates to the thread pool that this task is abandonable */
	bool CanAbandon()
	{
		return true;
	}

	/** Abandon routine, we need to remove the item from the in flight cache because something might be waiting for that */
	void Abandon()
	{
		if (InflightCache)
		{
			InflightCache->RemoveCachedData(*CacheKey, /*bTransient=*/ false); // we can remove this from the temp cache, since the real cache will hit now
		}
		FilesInFlight->Remove(CacheKey);
		FDerivedDataBackend::Get().AddToAsyncCompletionCounter(-1);
		UE_LOG(LogDerivedDataCache, VeryVerbose, TEXT("%s: Abandoned AsyncPut of %s."), *InnerBackend->GetName(), *CacheKey);
	}
};

FDerivedDataBackendAsyncPutWrapper::FDerivedDataBackendAsyncPutWrapper(FDerivedDataBackendInterface* InInnerBackend, bool bCacheInFlightPuts)
	: InnerBackend(InInnerBackend)
	, InflightCache(bCacheInFlightPuts ? Memory::CreateMemoryDerivedDataBackend(TEXT("InflightMemoryCache"), /*MaxCacheSize*/ -1, /*bCanBeDisabled*/ false) : nullptr)
{
	check(InnerBackend);
}

/** return true if this cache is writable **/
bool FDerivedDataBackendAsyncPutWrapper::IsWritable() const
{
	return InnerBackend->IsWritable();
}

FDerivedDataBackendInterface::ESpeedClass FDerivedDataBackendAsyncPutWrapper::GetSpeedClass() const
{
	return InnerBackend->GetSpeedClass();
}

bool FDerivedDataBackendAsyncPutWrapper::BackfillLowerCacheLevels() const
{
	return InnerBackend->BackfillLowerCacheLevels();
}

bool FDerivedDataBackendAsyncPutWrapper::CachedDataProbablyExists(const TCHAR* CacheKey)
{
	COOK_STAT(auto Timer = UsageStats.TimeProbablyExists());
	bool Result = (InflightCache && InflightCache->CachedDataProbablyExists(CacheKey)) || InnerBackend->CachedDataProbablyExists(CacheKey);
	COOK_STAT(if (Result) {	Timer.AddHit(0); });

	UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s CachedDataProbablyExists=%d for %s"), *GetName(), Result, CacheKey);
	return Result;
}

TBitArray<> FDerivedDataBackendAsyncPutWrapper::CachedDataProbablyExistsBatch(TConstArrayView<FString> CacheKeys)
{
	COOK_STAT(auto Timer = UsageStats.TimeProbablyExists());

	TBitArray<> Result;
	if (InflightCache)
	{
		Result = InflightCache->CachedDataProbablyExistsBatch(CacheKeys);
		check(Result.Num() == CacheKeys.Num());
		if (Result.CountSetBits() < CacheKeys.Num())
		{
			TBitArray<> InnerResult = InnerBackend->CachedDataProbablyExistsBatch(CacheKeys);
			check(InnerResult.Num() == CacheKeys.Num());
			Result.CombineWithBitwiseOR(InnerResult, EBitwiseOperatorFlags::MaintainSize);
		}
	}
	else
	{
		Result = InnerBackend->CachedDataProbablyExistsBatch(CacheKeys);
		check(Result.Num() == CacheKeys.Num());
	}

	COOK_STAT(if (Result.CountSetBits() == CacheKeys.Num()) { Timer.AddHit(0); });
	UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s CachedDataProbablyExists found %d/%d keys"), *GetName(), Result.CountSetBits(), CacheKeys.Num());
	return Result;
}

TBitArray<> FDerivedDataBackendAsyncPutWrapper::TryToPrefetch(TConstArrayView<FString> CacheKeys)
{
	COOK_STAT(auto Timer = UsageStats.TimePrefetch());

	TBitArray<> Result;
	if (InflightCache)
	{
		Result = InflightCache->CachedDataProbablyExistsBatch(CacheKeys);
	}
	Result = TBitArray<>::BitwiseOR(Result, InnerBackend->TryToPrefetch(CacheKeys), EBitwiseOperatorFlags::MaxSize);

	if (Result.CountSetBits() == CacheKeys.Num())
	{
		COOK_STAT(Timer.AddHit(0));
	}

	return Result;
}

/*
	Determine if we would cache this by asking all our inner layers
*/
bool FDerivedDataBackendAsyncPutWrapper::WouldCache(const TCHAR* CacheKey, TArrayView<const uint8> InData)
{
	return InnerBackend->WouldCache(CacheKey, InData);
}

bool FDerivedDataBackendAsyncPutWrapper::ApplyDebugOptions(FBackendDebugOptions& InOptions)
{
	return InnerBackend->ApplyDebugOptions(InOptions);
}

bool FDerivedDataBackendAsyncPutWrapper::GetCachedData(const TCHAR* CacheKey, TArray<uint8>& OutData)
{
	COOK_STAT(auto Timer = UsageStats.TimeGet());
	if (InflightCache && InflightCache->GetCachedData(CacheKey, OutData))
	{
		COOK_STAT(Timer.AddHit(OutData.Num()));
		UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s CacheHit from InFlightCache on %s"), *GetName(), CacheKey);
		return true;
	}

	bool bSuccess = InnerBackend->GetCachedData(CacheKey, OutData);
	if (bSuccess)
	{
		UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s Cache hit on %s"), *GetName(), CacheKey);
		COOK_STAT(Timer.AddHit(OutData.Num()));
	}
	else
	{
		UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s Cache miss on %s"), *GetName(), CacheKey);
	}
	return bSuccess;
}

FDerivedDataBackendInterface::EPutStatus FDerivedDataBackendAsyncPutWrapper::PutCachedData(const TCHAR* CacheKey, TArrayView<const uint8> InData, bool bPutEvenIfExists)
{
	COOK_STAT(auto Timer = PutSyncUsageStats.TimePut());

	if (!InnerBackend->IsWritable())
	{
		return EPutStatus::NotCached; // no point in continuing down the chain
	}
	const bool bAdded = FilesInFlight.AddIfNotExists(CacheKey);
	if (!bAdded)
	{
		return EPutStatus::Executing; // if it is already on its way, we don't need to send it again
	}
	if (InflightCache)
	{
		if (InflightCache->CachedDataProbablyExists(CacheKey))
		{
			UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s skipping out of key already in in-flight cache %s"), *GetName(), CacheKey);
			return EPutStatus::Executing; // if it is already on its way, we don't need to send it again
		}
		InflightCache->PutCachedData(CacheKey, InData, true); // temp copy stored in memory while the async task waits to complete
		COOK_STAT(Timer.AddHit(InData.Num()));
	}
	
	UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s queueing %s for put"), *GetName(), CacheKey);

	FDerivedDataBackend::Get().AddToAsyncCompletionCounter(1);
	(new FAutoDeleteAsyncTask<FCachePutAsyncWorker>(CacheKey, InData, InnerBackend, bPutEvenIfExists, InflightCache.Get(), &FilesInFlight, UsageStats))->StartBackgroundTask(Private::GCacheThreadPool, EQueuedWorkPriority::Low);

	return EPutStatus::Executing;
}

void FDerivedDataBackendAsyncPutWrapper::RemoveCachedData(const TCHAR* CacheKey, bool bTransient)
{
	if (!InnerBackend->IsWritable())
	{
		return; // no point in continuing down the chain
	}
	while (FilesInFlight.Exists(CacheKey))
	{
		FPlatformProcess::Sleep(0.0f); // this is an exception condition (corruption), spin and wait for it to clear
	}
	if (InflightCache)
	{
		InflightCache->RemoveCachedData(CacheKey, bTransient);
	}
	InnerBackend->RemoveCachedData(CacheKey, bTransient);

	UE_LOG(LogDerivedDataCache, Verbose, TEXT("%s removed %s"), *GetName(), CacheKey)
}

TSharedRef<FDerivedDataCacheStatsNode> FDerivedDataBackendAsyncPutWrapper::GatherUsageStats() const
{
	TSharedRef<FDerivedDataCacheStatsNode> Usage =
		MakeShared<FDerivedDataCacheStatsNode>(TEXT("AsyncPutWrapper"), TEXT(""), InnerBackend->GetSpeedClass() == ESpeedClass::Local);
	Usage->Stats.Add(TEXT("AsyncPut"), UsageStats);
	Usage->Stats.Add(TEXT("AsyncPutSync"), PutSyncUsageStats);

	Usage->Children.Add(InnerBackend->GatherUsageStats());
	if (InflightCache)
	{
		Usage->Children.Add(InflightCache->GatherUsageStats());
	}

	return Usage;
}

void FDerivedDataBackendAsyncPutWrapper::LegacyStats(FDerivedDataCacheStatsNode& OutNode)
{
	OutNode = {TEXT("AsyncPutWrapper"), TEXT(""), InnerBackend->GetSpeedClass() == ESpeedClass::Local};
	OutNode.Stats.Add(TEXT("AsyncPut"), UsageStats);
	OutNode.Stats.Add(TEXT("AsyncPutSync"), PutSyncUsageStats);

	InnerBackend->LegacyStats(OutNode.Children.Add_GetRef(MakeShared<FDerivedDataCacheStatsNode>()).Get());
	if (InflightCache)
	{
		InflightCache->LegacyStats(OutNode.Children.Add_GetRef(MakeShared<FDerivedDataCacheStatsNode>()).Get());
	}
}

class FDerivedDataAsyncWrapperRequest final : public FRequestBase, private IQueuedWork
{
public:
	inline FDerivedDataAsyncWrapperRequest(
		IRequestOwner& InOwner,
		TUniqueFunction<void (IRequestOwner& Owner, bool bCancel)>&& InFunction)
		: Owner(InOwner)
		, Function(MoveTemp(InFunction))
	{
	}

	inline void Start(EPriority Priority)
	{
		Owner.Begin(this);
		DoneEvent.Reset();
		Private::GCacheThreadPool->AddQueuedWork(this, GetPriority(Priority));
	}

	inline void Execute(bool bCancel)
	{
		FScopeCycleCounter Scope(GetStatId(), /*bAlways*/ true);
		Owner.End(this, [this, bCancel]
		{
			Function(Owner, bCancel);
			DoneEvent.Trigger();
		});
		// DO NOT ACCESS ANY MEMBERS PAST THIS POINT!
	}

	// IRequest Interface

	inline void SetPriority(EPriority Priority) final
	{
		if (Private::GCacheThreadPool->RetractQueuedWork(this))
		{
			Private::GCacheThreadPool->AddQueuedWork(this, GetPriority(Priority));
		}
	}

	inline void Cancel() final
	{
		if (!DoneEvent.Wait(0))
		{
			if (Private::GCacheThreadPool->RetractQueuedWork(this))
			{
				Abandon();
			}
			else
			{
				FScopeCycleCounter Scope(GetStatId());
				DoneEvent.Wait();
			}
		}
	}

	inline void Wait() final
	{
		if (!DoneEvent.Wait(0))
		{
			if (Private::GCacheThreadPool->RetractQueuedWork(this))
			{
				DoThreadedWork();
			}
			else
			{
				FScopeCycleCounter Scope(GetStatId());
				DoneEvent.Wait();
			}
		}
	}

private:
	static EQueuedWorkPriority GetPriority(EPriority Priority)
	{
		switch (Priority)
		{
		case EPriority::Blocking: return EQueuedWorkPriority::Highest;
		case EPriority::Highest:  return EQueuedWorkPriority::Highest;
		case EPriority::High:     return EQueuedWorkPriority::High;
		case EPriority::Normal:   return EQueuedWorkPriority::Normal;
		case EPriority::Low:      return EQueuedWorkPriority::Low;
		case EPriority::Lowest:   return EQueuedWorkPriority::Lowest;
		default: checkNoEntry();  return EQueuedWorkPriority::Normal;
		}
	}

	// IQueuedWork Interface

	inline void DoThreadedWork() final
	{
		Execute(/*bCancel*/ false);
	}

	inline void Abandon() final
	{
		Execute(/*bCancel*/ true);
	}

	inline TStatId GetStatId() const
	{
		RETURN_QUICK_DECLARE_CYCLE_STAT(FDerivedDataAsyncWrapperRequest, STATGROUP_ThreadPoolAsyncTasks);
	}

private:
	IRequestOwner& Owner;
	TUniqueFunction<void (IRequestOwner& Owner, bool bCancel)> Function;
	FLazyEvent DoneEvent{EEventMode::ManualReset};
};

template <typename RequestType, typename OnCompleteType, typename OnExecuteType>
void FDerivedDataBackendAsyncPutWrapper::Execute(
	COOK_STAT(CookStatsFunction OnAddStats,)
	const TConstArrayView<RequestType> Requests,
	IRequestOwner& Owner,
	OnCompleteType&& OnComplete,
	OnExecuteType&& OnExecute)
{
	auto ExecuteWithStats = [this, COOK_STAT(OnAddStats,) OnExecute](TConstArrayView<RequestType> Requests, IRequestOwner& Owner, OnCompleteType&& OnComplete) mutable
	{
		Invoke(OnExecute, *InnerBackend, Requests, Owner, [this, COOK_STAT(OnAddStats,) OnComplete = MoveTemp(OnComplete)](auto&& Response)
		{
			if (Response.Status == EStatus::Ok)
			{
				COOK_STAT((UsageStats.*OnAddStats)().AddHit(0));
			}
			if (OnComplete)
			{
				OnComplete(MoveTemp(Response));
			}
			FDerivedDataBackend::Get().AddToAsyncCompletionCounter(-1);
		});
	};

	FDerivedDataBackend::Get().AddToAsyncCompletionCounter(Requests.Num());
	if (Owner.GetPriority() == EPriority::Blocking || !Private::GCacheThreadPool)
	{
		return ExecuteWithStats(Requests, Owner, MoveTemp(OnComplete));
	}

	FDerivedDataAsyncWrapperRequest* Request = new FDerivedDataAsyncWrapperRequest(Owner,
		[this, Requests = TArray<RequestType>(Requests), OnComplete = MoveTemp(OnComplete), ExecuteWithStats = MoveTemp(ExecuteWithStats)](IRequestOwner& Owner, bool bCancel) mutable
		{
			if (!bCancel)
			{
				ExecuteWithStats(Requests, Owner, MoveTemp(OnComplete));
			}
			else
			{
				CompleteWithStatus(Requests, MoveTemp(OnComplete), EStatus::Canceled);
			}
		});
	Request->Start(Owner.GetPriority());
}

FDerivedDataBackendInterface* CreateAsyncPutDerivedDataBackend(FDerivedDataBackendInterface* InnerBackend, bool bCacheInFlightPuts)
{
	return new FDerivedDataBackendAsyncPutWrapper(InnerBackend, bCacheInFlightPuts);
}

} // UE::DerivedData::CacheStore::AsyncPut