don't allow setting class variable on module that's frozen [Bug #19341]

Merged: https://github.com/ruby/ruby/pull/7124

test/ruby/test_variable.rb

@@ -155,6 +155,21 @@ class TestVariable < Test::Unit::TestCase
     end
   end
 
+  def test_set_class_variable_on_frozen_object
+    set_cvar = EnvUtil.labeled_class("SetCVar")
+    set_cvar.class_eval "#{<<~"begin;"}\n#{<<~'end;'}"
+    begin;
+      def self.set(val)
+        @@a = val # inline cache
+      end
+    end;
+    set_cvar.set(1) # fill write cache
+    set_cvar.freeze
+    assert_raise(FrozenError) do
+      set_cvar.set(2) # hit write cache, but should check frozen status
+    end
+  end
+
   def test_variable
     assert_instance_of(Integer, $$)
 

variable.c

@@ -3939,6 +3939,7 @@ rb_class_ivar_set(VALUE obj, ID key, VALUE value)
 {
     RUBY_ASSERT(RB_TYPE_P(obj, T_CLASS) || RB_TYPE_P(obj, T_MODULE));
     int found;
+    rb_check_frozen(obj);
 
     RB_VM_LOCK_ENTER();
     {