Memory Leaks With validation_scopes

We had a requirement in our app to ignore few validations by just showing warnings to user and continuing the object to save. The gem validation_scopes seemed to be a right choice and we used it.

We faced memory issues and figured out model.has_warnings? of validation_scopes was causing issue.

Let’s take a rails model class Employee

class Employee < ActiveRecord::Base
  attr_accessible :address, :age, :name
  validates_presence_of :name
  validation_scope :warnings do |s|
    s.validates_presence_of :address
1.9.3p194 :001 > e = => 'Senthil V S')
 => #<Employee id: nil, name: "Senthil V S", age: nil, address: nil, created_at: nil, updated_at: nil>
1.9.3p194 :002 > e.valid?
 => true
1.9.3p194 :003 > e.instance_variables
 => [:@attributes, :@association_cache, :@aggregation_cache, :@attributes_cache,
 :@new_record, :@readonly, :@destroyed, :@marked_for_destruction, :@previously_changed,
 :@changed_attributes, :@mass_assignment_options, :@validation_context, :@errors]

No problem till now. Now I call has_warnings? on this object.

1.9.3p194 :004 > e.has_warnings?
 => true
1.9.3p194 :005 > e.instance_variables
 => [:@attributes, :@association_cache, :@aggregation_cache, :@attributes_cache,
 :@new_record, :@readonly, :@destroyed, :@marked_for_destruction, :@previously_changed,
 :@changed_attributes, :@mass_assignment_options, :@validation_context, :@errors, :@warnings]

Notice a new instance variable(@warnings)gets set. Let’s see what it has.

1.9.3p194 :034 > warning_object = e.instance_variable_get :@warnings
 => #<Employee id: nil, name: "Senthil V S", age: nil, address: nil, created_at: nil, updated_at: nil> 
1.9.3p194 :035 > warning_object.class
 => #<Class:0x007ffeddb0c1e0>
1.9.3p194 :036 > warning_object.class.ancestors
 => [#<Class:0x007ffeddb0c1e0>, ActiveModel::Validations::HelperMethods,
 ActiveModel::Validations, ActiveSupport::Callbacks, #<Class:0x007ffeddba5f70>,
 Delegator, #<Module:0x007ffedcb35eb0>, BasicObject]  
1.9.3p194 :037 > warning_object.__getobj__ == e
 => true

What @warnings has is an object of class defined during runtime by DelegatorClass . The problem is that these runtime created classes don’t get garbage collected.

1.9.3p194 :007 > warning_object_class_id = warning_object.class.object_id
 => 70366308884720
1.9.3p194 :008 > e_object_id = e.object_id
 => 70366307844540

We’ve taken the object_id of the model object and runtime created delegator class(which is an instance of class Class of course). Let’s set the references to nil and run the GC.

1.9.3p194 :014 > local_variables
 => [:e_object_id, :warning_object_class_id, :warning_object, :e, :_]
 1.9.3p194 :009 > warning_object = e = nil
 => nil
1.9.3p194 :010 > GC.start
 => nil

Now let’s use those object_ids to check whether they get GCed.

1.9.3p194 :011 > ObjectSpace._id2ref e_object_id
RangeError: 0x003fff6ec881bc is recycled object
  from (irb):11:in `_id2ref'
 from (irb):11
 from /Users/senthilvs/.rvm/gems/ruby-1.9.3-p194@global/gems/railties-3.2.10/lib/rails/commands/console.rb:47:in `start'
 from /Users/senthilvs/.rvm/gems/ruby-1.9.3-p194@global/gems/railties-3.2.10/lib/rails/commands/console.rb:8:in `start'
  from /Users/senthilvs/.rvm/gems/ruby-1.9.3-p194@global/gems/railties-3.2.10/lib/rails/commands.rb:41:in `<top (required)>'
 from script/rails:6:in `require'
 from script/rails:6:in `<main>'
1.9.3p194 :012 > ObjectSpace._id2ref warning_object_class_id
 => #<Class:0x007ffeddb0c1e0> 

So, the runtime created anonymous classes never get GCed and stay in memory. This the reason why the memory was kept on increasing for every request.

We fixed it by replacing validation_scopes with activemodel-warnings.