debug patch to try to debug bug https://bugzilla.xamarin.com/show_bug.cgi?id=14834

gistfile1.txt

diff --git a/mono/metadata/sgen-descriptor.h b/mono/metadata/sgen-descriptor.h
index cd53a86..178353a 100644
--- a/mono/metadata/sgen-descriptor.h
+++ b/mono/metadata/sgen-descriptor.h
@@ -172,22 +172,36 @@ sgen_gc_descr_has_references (mword desc)
 
 #ifdef __GNUC__
 #define OBJ_BITMAP_FOREACH_PTR(desc,obj)       do {    \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR\n"); \
 		/* there are pointers */        \
 		void **_objptr = (void**)(obj); \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR2\n"); \
 		gsize _bmap = (desc) >> 16;     \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR3\n"); \
 		_objptr += OBJECT_HEADER_WORDS; \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR4\n"); \
 		{ \
 			int _index = __builtin_ctz (_bmap); \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR5\n"); \
 			_objptr += _index; \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR6\n"); \
 			_bmap >>= (_index + 1);				\
+g_print ("___in oBJ_BITMAP_FOREACH_PTR7\n"); \
 			HANDLE_PTR (_objptr, (obj));		\
+g_print ("___in oBJ_BITMAP_FOREACH_PTR8\n"); \
 			_objptr ++;							\
-			} \
+			g_print ("___in oBJ_BITMAP_FOREACH_PTR9\n"); \
+} \
 		while (_bmap) { \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR10\n"); \
 			int _index = __builtin_ctz (_bmap); \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR11\n"); \
 			_objptr += _index; \
+g_print ("___in oBJ_BITMAP_FOREACH_PTR12\n"); \
 			_bmap >>= (_index + 1);				\
+g_print ("___in oBJ_BITMAP_FOREACH_PTR13\n"); \
 			HANDLE_PTR (_objptr, (obj));		\
+g_print ("___in oBJ_BITMAP_FOREACH_PTR14\n"); \
 			_objptr ++;							\
 		}										\
 	} while (0)
diff --git a/mono/metadata/sgen-gc.c b/mono/metadata/sgen-gc.c
index da5e372..d8d1449 100644
--- a/mono/metadata/sgen-gc.c
+++ b/mono/metadata/sgen-gc.c
@@ -659,41 +659,55 @@ is_xdomain_ref_allowed (gpointer *ptr, char *obj, MonoDomain *domain)
 static void
 check_reference_for_xdomain (gpointer *ptr, char *obj, MonoDomain *domain)
 {
+g_print ("___in HANDLE_PTRd1\n");
 	MonoObject *o = (MonoObject*)(obj);
+g_print ("___in HANDLE_PTRd2\n");
 	MonoObject *ref = (MonoObject*)*(ptr);
+g_print ("___in HANDLE_PTRd3\n");
 	int offset = (char*)(ptr) - (char*)o;
+g_print ("___in HANDLE_PTRd4\n");
 	MonoClass *class;
 	MonoClassField *field;
 	char *str;
-
+g_print ("___in HANDLE_PTRd5\n");
 	if (!ref || ref->vtable->domain == domain)
 		return;
+g_print ("___in HANDLE_PTRd6\n");
 	if (is_xdomain_ref_allowed (ptr, obj, domain))
 		return;
 
+g_print ("___in HANDLE_PTRd7\n");
 	field = NULL;
+g_print ("___in HANDLE_PTRd8\n");
 	for (class = o->vtable->klass; class; class = class->parent) {
+g_print ("___in HANDLE_PTRd9\n");
 		int i;
 
 		for (i = 0; i < class->field.count; ++i) {
+g_print ("___in HANDLE_PTRd10\n");
 			if (class->fields[i].offset == offset) {
+g_print ("___in HANDLE_PTRd11\n");
 				field = &class->fields[i];
+g_print ("___in HANDLE_PTRd12\n");
 				break;
 			}
 		}
 		if (field)
 			break;
 	}
+g_print ("___in HANDLE_PTRd13\n");
 
-	if (ref->vtable->klass == mono_defaults.string_class)
-		str = mono_string_to_utf8 ((MonoString*)ref);
+	if (ref->vtable->klass == mono_defaults.string_class) {g_print ("___in HANDLE_PTRd14\n");
+		str = mono_string_to_utf8 ((MonoString*)ref);}
 	else
 		str = NULL;
 	g_print ("xdomain reference in %p (%s.%s) at offset %d (%s) to %p (%s.%s) (%s)  -  pointed to by:\n",
 			o, o->vtable->klass->name_space, o->vtable->klass->name,
 			offset, field ? field->name : "",
 			ref, ref->vtable->klass->name_space, ref->vtable->klass->name, str ? str : "");
+g_print ("___in HANDLE_PTRd15\n");
 	mono_gc_scan_for_specific_ref (o, TRUE);
+g_print ("___in HANDLE_PTRd16\n");
 	if (str)
 		g_free (str);
 }
@@ -713,10 +727,13 @@ static gboolean scan_object_for_specific_ref_precise = TRUE;
 
 #undef HANDLE_PTR
 #define HANDLE_PTR(ptr,obj) do {		\
+	g_print ("___in HANDLE_PTRa1\n"); \
 	if ((MonoObject*)*(ptr) == key) {	\
+	g_print ("___in HANDLE_PTRa2\n"); \
 	g_print ("found ref to %p in object %p (%s) at offset %td\n",	\
 			key, (obj), safe_name ((obj)), ((char*)(ptr) - (char*)(obj))); \
 	}								\
+	g_print ("___in HANDLE_PTRa3\n"); \
 	} while (0)
 
 static void
@@ -1189,7 +1206,10 @@ sgen_drain_gray_stack (int max_objs, ScanCopyContext ctx)
 			if (!obj)
 				return TRUE;
 			SGEN_LOG (9, "Precise gray object scan %p (%s)", obj, safe_name (obj));
-			scan_func (obj, queue);
+			if (scan_func == NULL)
+				g_error ("Invalid current generation %d", current_collection_generation);
+			else
+				scan_func (obj, queue);
 		}
 	} else {
 		int i;
@@ -4391,11 +4411,16 @@ void mono_gc_wbarrier_value_copy_bitmap (gpointer _dest, gpointer _src, int size
 #ifdef SGEN_BINARY_PROTOCOL
 #undef HANDLE_PTR
 #define HANDLE_PTR(ptr,obj) do {					\
+	g_print ("___in HANDLE_PTRb1\n"); \
 		gpointer o = *(gpointer*)(ptr);				\
+	g_print ("___in HANDLE_PTRb2\n"); \
 		if ((o)) {						\
+	g_print ("___in HANDLE_PTRb3\n"); \
 			gpointer d = ((char*)dest) + ((char*)(ptr) - (char*)(obj)); \
+	g_print ("___in HANDLE_PTRb4\n"); \
 			binary_protocol_wbarrier (d, o, (gpointer) LOAD_VTABLE (o)); \
 		}							\
+	g_print ("___in HANDLE_PTRb5\n"); \
 	} while (0)
 
 static void
@@ -4482,15 +4507,24 @@ typedef struct {
 
 #undef HANDLE_PTR
 #define HANDLE_PTR(ptr,obj)	do {	\
+	g_print ("___in HANDLE_PTRc1\n"); \
 		if (*(ptr)) {	\
+	g_print ("___in HANDLE_PTRc2\n"); \
 			if (hwi->count == REFS_SIZE) {	\
+	g_print ("___in HANDLE_PTRc3\n"); \
 				hwi->callback ((MonoObject*)start, mono_object_class (start), hwi->called? 0: size, hwi->count, hwi->refs, hwi->offsets, hwi->data);	\
+	g_print ("___in HANDLE_PTRc4\n"); \
 				hwi->count = 0;	\
+	g_print ("___in HANDLE_PTRc5\n"); \
 				hwi->called = 1;	\
+	g_print ("___in HANDLE_PTRc6\n"); \
 			}	\
+	g_print ("___in HANDLE_PTRc7\n"); \
 			hwi->offsets [hwi->count] = (char*)(ptr)-(char*)start;	\
+	g_print ("___in HANDLE_PTRc8\n"); \
 			hwi->refs [hwi->count++] = *(ptr);	\
 		}	\
+	g_print ("___in HANDLE_PTRc9\n"); \
 	} while (0)
 
 static void
diff --git a/mono/metadata/sgen-minor-scan-object.h b/mono/metadata/sgen-minor-scan-object.h
index 7a34dfd..2526e6b 100644
--- a/mono/metadata/sgen-minor-scan-object.h
+++ b/mono/metadata/sgen-minor-scan-object.h
@@ -39,16 +39,27 @@ extern long long stat_scan_object_called_nursery;
 
 #undef HANDLE_PTR
 #define HANDLE_PTR(ptr,obj)	do {	\
+g_print ("___in HANDLE_PTRe1\n");	\
 		void *__old = *(ptr);	\
+g_print ("___in HANDLE_PTRe2\n");	\
 		void *__copy;		\
+g_print ("___in HANDLE_PTRe3\n");	\
 		SGEN_OBJECT_LAYOUT_STATISTICS_MARK_BITMAP ((obj), (ptr)); \
+g_print ("___in HANDLE_PTRe4\n");	\
 		if (__old) {	\
+g_print ("___in HANDLE_PTRe5\n");	\
 			PARALLEL_COPY_OBJECT ((ptr), queue);	\
+g_print ("___in HANDLE_PTRe6\n");	\
 			__copy = *(ptr);	\
+g_print ("___in HANDLE_PTRe7\n");	\
 			SGEN_COND_LOG (9, __old != __copy, "Overwrote field at %p with %p (was: %p)", (ptr), *(ptr), __old);	\
+g_print ("___in HANDLE_PTRe8\n");	\
 			if (G_UNLIKELY (sgen_ptr_in_nursery (__copy) && !sgen_ptr_in_nursery ((ptr)))) \
+g_print ("___in HANDLE_PTRe9\n");	\
 				sgen_add_to_global_remset ((ptr), __copy); \
+g_print ("___in HANDLE_PTRe10\n");	\
 		}	\
+g_print ("___in HANDLE_PTRe11\n");	\
 	} while (0)
 
 /*
@@ -91,12 +102,22 @@ PARALLEL_SCAN_VTYPE (char *start, mword desc, SgenGrayQueue *queue BINARY_PROTOC
 #undef HANDLE_PTR
 /* Global remsets are handled in SERIAL_COPY_OBJECT_FROM_OBJ */
 #define HANDLE_PTR(ptr,obj)	do {	\
-		void *__old = *(ptr);	\
+g_print ("___in HANDLE_PTRf1\n");	\
+if (!ptr) {				\
+g_print ("___in HANDLE_PTRfUCK1\n");	\
+} else { g_print ("___in HANDLE_PTRfit's good!1\n"); }					\
+		void *__old = (void*)*(ptr);	\
+g_print ("___in HANDLE_PTRf2\n");	\
 		SGEN_OBJECT_LAYOUT_STATISTICS_MARK_BITMAP ((obj), (ptr)); \
+g_print ("___in HANDLE_PTRf3\n");	\
 		if (__old) {	\
+g_print ("___in HANDLE_PTRf4\n");	\
 			SERIAL_COPY_OBJECT_FROM_OBJ ((ptr), queue);	\
+g_print ("___in HANDLE_PTRf5\n");	\
 			SGEN_COND_LOG (9, __old != *(ptr), "Overwrote field at %p with %p (was: %p)", (ptr), *(ptr), __old); \
+g_print ("___in HANDLE_PTRf6\n");	\
 		}	\
+g_print ("___in HANDLE_PTRf7\n");	\
 	} while (0)
 
 static void