Avoid clearing thread local handles for already unloaded loader

[alexey-zakharov]

Motivation:

Fix crash in ThreadLocalBlock::FreeTLM code on thread exit which happens during Assembly unloading when Assembly contains a class with a ThreadStatic variable.

Details:

There seem to be a race condition between LoaderAllocator cleanup during garbage collection and thread locals cleanup on thread exit. Racing stacks are the following:

ALC/LoaderAllocator cleanup

 	coreclr.dll!EEToProfInterfaceImpl::ModuleUnloadStarted(unsigned __int64 moduleId) Line 3735	C++	Symbols loaded.
 	coreclr.dll!ProfControlBlock::DoProfilerCallbackHelper<int (__cdecl*)(ProfilerInfo *),long (__cdecl*)(EEToProfInterfaceImpl *,unsigned __int64),unsigned __int64>(ProfilerInfo * pProfilerInfo, int(*)(ProfilerInfo *) condition, HRESULT(*)(EEToProfInterfaceImpl *, unsigned __int64) callback, HRESULT * pHR, unsigned __int64 <args_0>) Line 284	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ProfControlBlock::DoOneProfilerIteration(ProfilerInfo *) Line 199	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ProfControlBlock::IterateProfilers(ProfilerCallbackType) Line 207	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ProfControlBlock::DoProfilerCallback(ProfilerCallbackType) Line 295	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ProfControlBlock::ModuleUnloadStarted(unsigned __int64) Line 691	C++	Symbols loaded.
 	coreclr.dll!Module::Destruct() Line 662	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ClassLoader::FreeModules() Line 1884	C++	Symbols loaded.
 	coreclr.dll!ClassLoader::~ClassLoader() Line 1946	C++	Symbols loaded.
 	coreclr.dll!Assembly::Terminate(int) Line 311	C++	Symbols loaded.
 	coreclr.dll!Assembly::~Assembly() Line 244	C++	Symbols loaded.
 	coreclr.dll!Assembly::`scalar deleting destructor'(unsigned int __flags)	C++	Symbols loaded.
 	coreclr.dll!DomainAssembly::~DomainAssembly() Line 91	C++	Symbols loaded.
 	coreclr.dll!DomainAssembly::`scalar deleting destructor'(unsigned int __flags)	C++	Symbols loaded.
>	coreclr.dll!LoaderAllocator::GCLoaderAllocators(LoaderAllocator * pOriginalLoaderAllocator) Line 570	C++	Symbols loaded.
 	coreclr.dll!LoaderAllocator::Destroy(QCall::LoaderAllocatorHandle pLoaderAllocator) Line 702	C++	Symbols loaded.
 	coreclr.dll!LoaderAllocator_Destroy(QCall::LoaderAllocatorHandle pLoaderAllocator) Line 718	C++	Symbols loaded.
 	System.Private.CoreLib.dll!00007ffa9c4308f9()	Unknown	No symbols loaded.
 	coreclr.dll!FastCallFinalizeWorker() Line 26	Unknown	Symbols loaded.
 	coreclr.dll!MethodTable::CallFinalizer(Object * obj) Line 4908	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!CallFinalizer(Object *) Line 75	C++	Symbols loaded.
 	coreclr.dll!FinalizerThread::FinalizeAllObjects() Line 110	C++	Symbols loaded.
 	coreclr.dll!FinalizerThread::FinalizerThreadWorker(void * args) Line 354	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ManagedThreadBase_DispatchInner(ManagedThreadCallState *) Line 7222	C++	Symbols loaded.
 	coreclr.dll!ManagedThreadBase_DispatchMiddle(ManagedThreadCallState * pCallState) Line 7266	C++	Symbols loaded.
 	coreclr.dll!ManagedThreadBase_DispatchOuter(ManagedThreadCallState * pCallState) Line 7425	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ManagedThreadBase_NoADTransition(void(*)(void *)) Line 7494	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!ManagedThreadBase::FinalizerBase(void(*)(void *)) Line 7513	C++	Symbols loaded.
 	coreclr.dll!FinalizerThread::FinalizerThreadStart(void * args) Line 403	C++	Symbols loaded.
 	kernel32.dll!BaseThreadInitThunk()	Unknown	Symbols loaded.
 	ntdll.dll!RtlUserThreadStart()	Unknown	Symbols loaded.

Thread exit with threadlocal cleanup

>	coreclr.dll!LoaderAllocator::SetHandleValue(unsigned __int64 handle, Object *) Line 992	C++	Symbols loaded.
 	coreclr.dll!LoaderAllocator::FreeHandle(unsigned __int64 handle) Line 884	C++	Symbols loaded.
 	coreclr.dll!ThreadLocalBlock::FreeTLM(unsigned __int64 i, int isThreadShuttingdown) Line 59	C++	Symbols loaded.
 	coreclr.dll!ThreadLocalBlock::FreeTable() Line 93	C++	Symbols loaded.
 	[Inline Frame] coreclr.dll!Thread::DeleteThreadStaticData() Line 7704	C++	Symbols loaded.
 	coreclr.dll!Thread::OnThreadTerminate(int holdingLock) Line 2956	C++	Symbols loaded.
 	coreclr.dll!DestroyThread(Thread * th) Line 924	C++	Symbols loaded.
 	coreclr.dll!ThreadNative::KickOffThread(void * pass) Line 239	C++	Symbols loaded.
 	kernel32.dll!BaseThreadInitThunk()	Unknown	Symbols loaded.
 	ntdll.dll!RtlUserThreadStart()	Unknown	Symbols loaded.

and exception thrown with the following details

Exception thrown: read access violation.
**loaderAllocator** was nullptr.

loaderAllocator value from the LOADERALLOCATORREF loaderAllocator = (LOADERALLOCATORREF)ObjectFromHandle(m_hLoaderAllocatorObjectHandle); call is NULL which makes sense, because finalizer thread just disposed data for some of the same LoaderAllocator assemblies.

Note: enabling COR_PRF_MONITOR_MODULE_LOADS CLR profiler flag increases chances for the race condition as it slows down a bit complete loader destruction.

Changes:

I've looked at the following options as a solution to the problem:

• Null check for loaderAllocator
• Wait for GC/Finalizers done before killing thread
• Skip cleaning up thread locals for unloaded loaders

I think skipping unloaded loaded is the best option wrt performance/correctness considerations as loader deletion will cleanup all internal data, we won't access memory which may be already returned to pool/system and we won't lock other threads.