Bug #19394
opencvars in instance of cloned class point to source class's cvars even after class_variable_set on clone
Description
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
Updated by byroot (Jean Boussier) 4 months ago
- Related to Feature #17763: Implement cache for cvars added
Updated by byroot (Jean Boussier) 4 months ago
Based on the timeline and description, this is likely a result of [Feature #17763]. cc @eileencodes (Eileen Uchitelle) & @tenderlovemaking (Aaron Patterson)
Updated by eileencodes (Eileen Uchitelle) 4 months ago
I will take a look, thanks for the ping Jean.
Updated by eileencodes (Eileen Uchitelle) 4 months ago
I've pushed a fix up to github https://github.com/ruby/ruby/pull/7265. We added a cref to the cvar cache that we can check if it's the same as the cref we have. If they are different we shouldn't read from the cache.
Updated by eileencodes (Eileen Uchitelle) 4 months ago
The fix ended up being a bit more involved. We needed two PRs.
One to copy the CVAR table on clone: https://github.com/ruby/ruby/pull/7275
And one to check the cref and not read from the cache if they differ (ie is the cloned class): https://github.com/ruby/ruby/pull/7265
Both will need to be backported to 3.2.x and 3.1.x when merged.