Pin object ivars while they are being copied in a hash

When transitioning an object to TOO_COMPLEX we copy all its
ivar in a table, but if GC (and compaction) trigger in the middle
of the transition, the old `iv_ptr` might still be the canonical
ivar list and will be updated by the GC, but the reference we
copied in the table will be outdated.

So we must pin these reference until they are all copied and
the object `iv_ptr` is pointing to the table.

To do this I use a custom TypedData that simply mark the new
table with `rb_mark_tbl` so references are pinned.
.

internal/variable.h

@@ -47,7 +47,8 @@ VALUE rb_mod_set_temporary_name(VALUE, VALUE);
 
 struct gen_ivtbl;
 int rb_gen_ivtbl_get(VALUE obj, ID id, struct gen_ivtbl **ivtbl);
-void rb_obj_copy_ivs_to_hash_table(VALUE obj, st_table *table);
+VALUE rb_obj_copy_ivs_to_hash_table(VALUE obj, st_table *table);
+void rb_obj_copy_ivs_to_hash_table_complete(VALUE ivar_pinner);
 void rb_obj_convert_to_too_complex(VALUE obj, st_table *table);
 void rb_evict_ivars_to_hash(VALUE obj);
 

object.c

@@ -326,8 +326,9 @@ rb_obj_copy_ivar(VALUE dest, VALUE obj)
         shape_to_set_on_dest = rb_shape_rebuild_shape(initial_shape, src_shape);
         if (UNLIKELY(rb_shape_id(shape_to_set_on_dest) == OBJ_TOO_COMPLEX_SHAPE_ID)) {
             st_table * table = rb_st_init_numtable_with_size(src_num_ivs);
-            rb_obj_copy_ivs_to_hash_table(obj, table);
+            VALUE ivar_pinner = rb_obj_copy_ivs_to_hash_table(obj, table);
             rb_obj_convert_to_too_complex(dest, table);
+            rb_obj_copy_ivs_to_hash_table_complete(ivar_pinner);
 
             return;
         }

variable.c

@@ -1422,8 +1422,9 @@ rb_evict_ivars_to_hash(VALUE obj)
     st_table *table = st_init_numtable_with_size(rb_ivar_count(obj));
 
     // Evacuate all previous values from shape into id_table
-    rb_obj_copy_ivs_to_hash_table(obj, table);
+    VALUE ivar_pinner = rb_obj_copy_ivs_to_hash_table(obj, table);
     rb_obj_convert_to_too_complex(obj, table);
+    rb_obj_copy_ivs_to_hash_table_complete(ivar_pinner);
 
     RUBY_ASSERT(rb_shape_obj_too_complex(obj));
 }
@@ -1647,10 +1648,39 @@ rb_obj_copy_ivs_to_hash_table_i(ID key, VALUE val, st_data_t arg)
     return ST_CONTINUE;
 }
 
-void
+static void
+ivar_pinner_mark(void *data) {
+    st_table *table = (st_table *)data;
+    if (!data) {
+        return;
+    }
+    // rb_mark_tbl pin references
+    rb_mark_tbl(table);
+}
+
+static const rb_data_type_t ivars_pinner_data_type = {
+    .wrap_struct_name = "ivar_pinner",
+    .function = {
+        .dmark = ivar_pinner_mark,
+        .dfree = NULL,
+        .dsize = NULL,
+        .dcompact = NULL,
+    },
+    .flags = RUBY_TYPED_FREE_IMMEDIATELY,
+};
+
+VALUE
 rb_obj_copy_ivs_to_hash_table(VALUE obj, st_table *table)
 {
+    VALUE ivar_pinner = TypedData_Wrap_Struct(0, &ivars_pinner_data_type, table);
     rb_ivar_foreach(obj, rb_obj_copy_ivs_to_hash_table_i, (st_data_t)table);
+    return ivar_pinner;
+}
+
+void
+rb_obj_copy_ivs_to_hash_table_complete(VALUE ivar_pinner)
+{
+    RTYPEDDATA_DATA(ivar_pinner) = NULL;
 }
 
 static VALUE *