Ruby

This unexpected change in behavior happens between Ruby 3.0.x and 3.1.x. In Ruby >= 3.1, when a class with a cvar is cloned (or duped), the cvar in instances of the cloned class continues to point to the source class’s cvar after the clone has its cvar updated with class_variable_set. In Ruby < 3.1, the cloned class instance points to the updated cvar, as expected.

It seems likely that this is a bug in the cvar cache introduced in Ruby 3.1.

Repro:

class Foo @@bar = 'bar' def print_bar puts "#{self.class.name} (from instance): #{@@bar} #{@@bar.object_id}" end end foo_bar = Foo.class_variable_get(:@@bar) puts "Foo (class_variable_get): #{foo_bar} #{foo_bar.object_id}" Foo.new.print_bar FooClone = Foo.clone FooClone.class_variable_set(:@@bar, 'bar_clone') foo_clone_bar = FooClone.class_variable_get(:@@bar) puts "FooClone (class_variable_get): #{foo_clone_bar} #{foo_clone_bar.object_id}" FooClone.new.print_bar 

Ruby 3.0.5:

Foo (class_variable_get): bar 60 Foo (from instance): bar 60 FooClone (class_variable_get): bar_clone 80 FooClone (from instance): bar_clone 80 

Ruby 3.1.3, 3.2.0:

Foo (class_variable_get): bar 60 Foo (from instance): bar 60 FooClone (class_variable_get): bar_clone 80 FooClone (from instance): bar 60 

Something similar happens when there are multiple clones and a cvar that the source class does not have defined is set on the clones. In this case, the cvars in instances of the clones all point to the first clone’s cvar.

Repro:

class Foo def print_bar puts "#{self.class.name} (from instance): #{@@bar} #{@@bar.object_id}" end end Foo1 = Foo.clone Foo2 = Foo.clone Foo3 = Foo.clone Foo1.class_variable_set(:@@bar, 'bar1') Foo2.class_variable_set(:@@bar, 'bar2') Foo3.class_variable_set(:@@bar, 'bar3') foo1_bar = Foo1.class_variable_get(:@@bar) foo2_bar = Foo2.class_variable_get(:@@bar) foo3_bar = Foo3.class_variable_get(:@@bar) puts "Foo1 (class_variable_get): #{foo1_bar} #{foo1_bar.object_id}" puts "Foo2 (class_variable_get): #{foo2_bar} #{foo2_bar.object_id}" puts "Foo3 (class_variable_get): #{foo3_bar} #{foo3_bar.object_id}" Foo1.new.print_bar Foo2.new.print_bar Foo3.new.print_bar 

Ruby 3.0.5:

Foo1 (class_variable_get): bar1 60 Foo2 (class_variable_get): bar2 80 Foo3 (class_variable_get): bar3 100 Foo1 (from instance): bar1 60 Foo2 (from instance): bar2 80 Foo3 (from instance): bar3 100 

Ruby 3.1.3, 3.2.0:

Foo1 (class_variable_get): bar1 60 Foo2 (class_variable_get): bar2 80 Foo3 (class_variable_get): bar3 100 Foo1 (from instance): bar1 60 Foo2 (from instance): bar1 60 Foo3 (from instance): bar1 60