Fix object_id for classes in modules in namespace context

Given classes and modules have a different set of fields in every
namespace, we can't store the object_id in fields for them.

Given that some space was freed in `RClass` we can store it there
instead.

Author: byroot

gc.c

@@ -1861,9 +1861,40 @@ static const rb_data_type_t id_to_obj_tbl_type = {
     .flags = RUBY_TYPED_WB_PROTECTED | RUBY_TYPED_FREE_IMMEDIATELY
 };
 
+static VALUE
+class_object_id(VALUE klass)
+{
+    VALUE id = RCLASS(klass)->object_id;
+    if (!id) {
+        unsigned int lock_lev = rb_gc_vm_lock();
+        id = ULL2NUM(next_object_id);
+        next_object_id += OBJ_ID_INCREMENT;
+        VALUE existing_id = RUBY_ATOMIC_VALUE_CAS(RCLASS(klass)->object_id, 0, id);
+        if (existing_id) {
+            id = existing_id;
+        }
+        else if (RB_UNLIKELY(id_to_obj_tbl)) {
+            st_insert(id_to_obj_tbl, id, klass);
+        }
+        rb_gc_vm_unlock(lock_lev);
+    }
+    return id;
+}
+
 static VALUE
 object_id(VALUE obj)
 {
+    switch (BUILTIN_TYPE(obj)) {
+      case T_CLASS:
+      case T_MODULE:
+        // With namespaces, classes and modules have different fields
+        // in different namespaces, so we cannot store the object id
+        // in fields.
+        return class_object_id(obj);
+      default:
+        break;
+    }
+
     VALUE id = Qfalse;
     unsigned int lock_lev;
 
@@ -1896,8 +1927,19 @@ static void
 build_id_to_obj_i(VALUE obj, void *data)
 {
     st_table *id_to_obj_tbl = (st_table *)data;
-    if (rb_shape_obj_has_id(obj)) {
-        st_insert(id_to_obj_tbl, rb_obj_id(obj), obj);
+
+    switch (BUILTIN_TYPE(obj)) {
+      case T_CLASS:
+      case T_MODULE:
+        if (RCLASS(obj)->object_id) {
+            st_insert(id_to_obj_tbl, RCLASS(obj)->object_id, obj);
+        }
+        break;
+      default:
+        if (rb_shape_obj_has_id(obj)) {
+            st_insert(id_to_obj_tbl, rb_obj_id(obj), obj);
+        }
+        break;
     }
 }
 

internal/class.h

@@ -136,6 +136,7 @@ STATIC_ASSERT(shape_max_variations, SHAPE_MAX_VARIATIONS < (1 << (sizeof(((rb_cl
 struct RClass {
     struct RBasic basic;
     st_table *ns_classext_tbl; // ns_object -> (rb_classext_t *)
+    VALUE object_id;
     /*
      * If ns_classext_tbl is NULL, then the prime classext is readable (because no other classext exists).
      * For the check whether writable or not, check flag RCLASS_PRIME_CLASSEXT_WRITABLE