[ruby/mmtk] Skip weak references that are special consts

If a reference marked weak becomes a special const, it will crash because
it is not a GC handled object. We should skip special consts here.

ruby/mmtk@870a79426b

Author: peterzhu2118

gc/mmtk/mmtk.c

@@ -342,6 +342,14 @@ rb_mmtk_update_global_tables(int table)
     rb_gc_vm_weak_table_foreach(rb_mmtk_update_table_i, NULL, NULL, true, (enum rb_gc_vm_weak_tables)table);
 }
 
+static bool
+rb_mmtk_special_const_p(MMTk_ObjectReference object)
+{
+    VALUE obj = (VALUE)object;
+
+    return RB_SPECIAL_CONST_P(obj);
+}
+
 // Bootup
 MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_init_gc_worker_thread,
@@ -360,6 +368,7 @@ MMTk_RubyUpcalls ruby_upcalls = {
     rb_mmtk_update_global_tables,
     rb_mmtk_global_tables_count,
     rb_mmtk_update_finalizer_table,
+    rb_mmtk_special_const_p,
 };
 
 // Use max 80% of the available memory by default for MMTk

gc/mmtk/mmtk.h

@@ -68,6 +68,7 @@ typedef struct MMTk_RubyUpcalls {
     void (*update_global_tables)(int tbl_idx);
     int (*global_tables_count)(void);
     void (*update_finalizer_table)(void);
+    bool (*special_const_p)(MMTk_ObjectReference object);
 } MMTk_RubyUpcalls;
 
 typedef struct MMTk_RawVecOfObjRef {

gc/mmtk/src/abi.rs

@@ -313,6 +313,7 @@ pub struct RubyUpcalls {
     pub update_global_tables: extern "C" fn(tbl_idx: c_int),
     pub global_tables_count: extern "C" fn() -> c_int,
     pub update_finalizer_table: extern "C" fn(),
+    pub special_const_p: extern "C" fn(object: ObjectReference) -> bool,
 }
 
 unsafe impl Sync for RubyUpcalls {}

gc/mmtk/src/weak_proc.rs

@@ -134,6 +134,10 @@ impl GCWork<Ruby> for ProcessWeakReferences {
             .expect("Mutators should not be holding the lock.");
 
         for ptr_ptr in weak_references.iter_mut() {
+            if (upcalls().special_const_p)(**ptr_ptr) {
+                continue;
+            }
+
             if !(**ptr_ptr).is_reachable() {
                 **ptr_ptr = crate::binding().weak_reference_dead_value;
             }