Run "OnThreadExiting" during thread finalization. Testing with Mono has already hardened the cases that this makes weird, so we can reliably do it with CoreCLR

Author: jkoritzinsky

src/coreclr/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.cs

@@ -176,7 +176,18 @@ private void Initialize()
         private static partial void Initialize(ObjectHandleOnStack thread);
 
         /// <summary>Clean up the thread when it goes away.</summary>
-        ~Thread() => InternalFinalize(); // Delegate to the unmanaged portion.
+        ~Thread()
+        {
+            // During the actual thread shutdown,
+            // it may not be practical for us to run enough managed code to clean up
+            // any managed code that needs to know when a thread exits.
+            // Instead, run that clean up here when the thread object is finalized
+            // (which is definitely after the thread has exited)
+            OnThreadExiting();
+
+            // Do any additional finalization that's required on the unmanaged side.
+            InternalFinalize();
+        }
 
         [MethodImpl(MethodImplOptions.InternalCall)]
         private extern void InternalFinalize();

src/coreclr/vm/corelib.h

@@ -969,7 +969,6 @@ DEFINE_CLASS(DIRECTONTHREADLOCALDATA, Threading, Thread+DirectOnThreadLocalData)
 DEFINE_CLASS(THREAD,                Threading,              Thread)
 DEFINE_METHOD(THREAD,               START_CALLBACK,                          StartCallback,                               IM_RetVoid)
 DEFINE_METHOD(THREAD,               POLLGC,                                  PollGC,                               NoSig)
-DEFINE_METHOD(THREAD,               ON_THREAD_EXITING,                       OnThreadExiting,                          IM_RetVoid)
 
 #ifdef FEATURE_OBJCMARSHAL
 DEFINE_CLASS(AUTORELEASEPOOL,       Threading,              AutoreleasePool)

src/coreclr/vm/threads.cpp

@@ -2727,15 +2727,6 @@ void Thread::CooperativeCleanup()
 
     GCX_COOP();
 
-    if (!IsGCSpecial())
-    {
-        // Allow managed subsystems to clean up on thread exit.
-        PREPARE_NONVIRTUAL_CALLSITE(METHOD__THREAD__ON_THREAD_EXITING);
-        DECLARE_ARGHOLDER_ARRAY(args, 1);
-        args[ARGNUM_0] = OBJECTREF_TO_ARGHOLDER(GetExposedObject());
-        CALL_MANAGED_METHOD_NORET(args);
-    }
-
     // Clear any outstanding stale EH state that maybe still active on the thread.
 #ifdef FEATURE_EH_FUNCLETS
     ExInfo::PopTrackers((void*)-1);