Copy cvar table on clone

When a class with a class variable is cloned we need to also copy the
cvar cache table from the original table to the clone. I found this bug
while working on fixing [Bug #19379]. While this does not fix that bug
directly it is still a required change to fix another bug revealed by
the fix in #7265

This needs to be backported to 3.2.x and 3.1.x.

Co-authored-by: Aaron Patterson <tenderlove@ruby-lang.org>

class.c

@@ -404,13 +404,44 @@ class_init_copy_check(VALUE clone, VALUE orig)
     }
 }
 
+struct cvc_table_copy_ctx {
+    VALUE clone;
+    struct rb_id_table * new_table;
+};
+
+static enum rb_id_table_iterator_result
+cvc_table_copy(ID id, VALUE val, void *data) {
+    struct cvc_table_copy_ctx *ctx = (struct cvc_table_copy_ctx *)data;
+    struct rb_cvar_class_tbl_entry * orig_entry;
+    orig_entry = (struct rb_cvar_class_tbl_entry *)val;
+
+    struct rb_cvar_class_tbl_entry *ent;
+
+    ent = ALLOC(struct rb_cvar_class_tbl_entry);
+    ent->class_value = ctx->clone;
+    ent->global_cvar_state = orig_entry->global_cvar_state;
+    rb_id_table_insert(ctx->new_table, id, (VALUE)ent);
+
+    return ID_TABLE_CONTINUE;
+}
+
 static void
 copy_tables(VALUE clone, VALUE orig)
 {
     if (RCLASS_CONST_TBL(clone)) {
         rb_free_const_table(RCLASS_CONST_TBL(clone));
         RCLASS_CONST_TBL(clone) = 0;
     }
+    if (RCLASS_CVC_TBL(orig)) {
+        struct rb_id_table *rb_cvc_tbl = RCLASS_CVC_TBL(orig);
+        struct rb_id_table *rb_cvc_tbl_dup = rb_id_table_create(rb_id_table_size(rb_cvc_tbl));
+
+        struct cvc_table_copy_ctx ctx;
+        ctx.clone = clone;
+        ctx.new_table = rb_cvc_tbl_dup;
+        rb_id_table_foreach(rb_cvc_tbl, cvc_table_copy, &ctx);
+        RCLASS_CVC_TBL(clone) = rb_cvc_tbl_dup;
+    }
     RCLASS_M_TBL(clone) = 0;
     if (!RB_TYPE_P(clone, T_ICLASS)) {
         st_data_t id;

test/ruby/test_variable.rb

@@ -33,6 +33,12 @@ def ruler4
     end
   end
 
+  Athena = Gods.clone
+
+  def test_cloned_classes_copy_cvar_cache
+    assert_equal "Cronus", Athena.new.ruler0
+  end
+
   def test_setting_class_variable_on_module_through_inheritance
     mod = Module.new
     mod.class_variable_set(:@@foo, 1)