Fix GC for collectible classes (#19892)

There is a subtle bug in the GC that causes access violation when a
LoaderAllocator of a collectible class is pushed to the mark stack and
that class also has partial sub-objects. One example is an array of byte
arrays. In that case, the mark stack is supposed to have the reference
to the instance of the class stored right before the related partial
sub-objects references. But due to the bug, the slot where the instance
of the class was expected to be located was not set to anything.

diff --git a/src/gc/gc.cpp b/src/gc/gc.cpp
index 2f59383..f8946d7 100644 (file)
--- a/src/gc/gc.cpp
+++ b/src/gc/gc.cpp
@@ -17771,6 +17771,11 @@ void gc_heap::mark_object_simple1 (uint8_t* oo, uint8_t* start THREAD_NUMBER_DCL
                             size_t obj_size = size (class_obj);
                             promoted_bytes (thread) += obj_size;
                             *(mark_stack_tos++) = class_obj;
+                            // The code below expects that the oo is still stored in the stack slot that was
+                            // just popped and it "pushes" it back just by incrementing the mark_stack_tos. 
+                            // But the class_obj has just overwritten that stack slot and so the oo needs to
+                            // be stored to the new slot that's pointed to by the mark_stack_tos.
+                            *mark_stack_tos = oo;
                         }
                     }
                 }