Add unity function to clear domain specific fields from thread object, as they...

Add unity function to clear domain specific fields from thread object, as they may hold objects from an unloaded domain.

mono/metadata/object-internals.h

@@ -364,6 +364,9 @@ struct _MonoThread {
 	gpointer unused4;
 	gpointer unused5;
 	gpointer unused6;
+	MonoObject *threadstart;
+	int managed_id;
+	MonoObject* principal;
 };
 
 typedef struct {

msvc/mono.def

@@ -46,6 +46,7 @@ mono_unity_set_unhandled_exception_handler
 mono_unity_class_is_interface
 mono_unity_class_is_abstract
 mono_backtrace_from_context
+mono_unity_thread_clear_domain_fields
 
 ;Exports that should have been here, but I dont understand why they're not.
 mono_security_enable_core_clr

unity/unity_utils.c

@@ -8,6 +8,8 @@
 #include <mono/metadata/metadata.h>
 #include <mono/metadata/tabledefs.h>
 #include <mono/metadata/class-internals.h>
+#include <mono/metadata/object-internals.h>
+#include <mono/metadata/threads.h>
 
 #include <glib.h>
 
@@ -143,3 +145,13 @@ unity_mono_install_memory_callbacks(MonoMemoryCallbacks* callbacks)
 {
 	g_mem_set_callbacks (callbacks);
 }
+
+void mono_unity_thread_clear_domain_fields (void)
+{
+	/*
+	 we need to clear fields that may reference objects living in non-root appdomain
+	 since the objects will live but their vtables will be destroyed when domain is torn down.
+	 */
+	MonoThread* thread = mono_thread_current ();
+	thread->principal = NULL;
+}