Add API to control behavior of thread abort within handler blocks (case 979679).

Unity unloads domains and this is accomplished via thread abort exceptions.
Mono now matches .NET behavior by not interrupting catch/finally blocks with
thread abort exceptions. This can lead to hangs in Unity as domain unload
blocks until all thread are interrupted.

mono/metadata/threads.c

@@ -55,6 +55,8 @@
 #include <mono/utils/w32api.h>
 #include <mono/utils/mono-os-wait.h>
 
+#include <mono/metadata/unity-utils.h>
+
 #ifdef HAVE_SIGNAL_H
 #include <signal.h>
 #endif
@@ -4851,7 +4853,7 @@ async_abort_critical (MonoThreadInfo *info, gpointer ud)
 	gboolean protected_wrapper;
 	gboolean running_managed;
 
-	if (mono_get_eh_callbacks ()->mono_install_handler_block_guard (mono_thread_info_get_suspend_state (info)))
+	if (mono_unity_get_enable_handler_block_guards () && mono_get_eh_callbacks ()->mono_install_handler_block_guard (mono_thread_info_get_suspend_state (info)))
 		return MonoResumeThread;
 
 	/*someone is already interrupting it*/

mono/metadata/unity-utils.c

@@ -1293,3 +1293,17 @@ mono_class_set_allow_gc_aware_layout(mono_bool allow)
 {
 	mono_allow_gc_aware_layout = allow;
 }
+
+static mono_bool enable_handler_block_guards = TRUE;
+
+void
+mono_unity_set_enable_handler_block_guards (mono_bool allow)
+{
+	enable_handler_block_guards = allow;
+}
+
+mono_bool
+mono_unity_get_enable_handler_block_guards (void)
+{
+	return enable_handler_block_guards;
+}

mono/metadata/unity-utils.h

@@ -180,4 +180,10 @@ mono_method_get_method_definition(MonoMethod *method);
 void
 mono_class_set_allow_gc_aware_layout(mono_bool allow);
 
+MONO_API void
+mono_unity_set_enable_handler_block_guards (mono_bool allow);
+
+mono_bool
+mono_unity_get_enable_handler_block_guards (void);
+
 #endif