Bug #11916: Fix delegating to 'args' and 'block'

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 2.4.0dev
• Backport: 2.0.0: WONTFIX, 2.1: WONTFIX, 2.2: DONE, 2.3: DONE

[mcmire (Elliot Winkler)]

If you have a class that uses Forwardable to delegate a method to
another object, and the method that returns the delegate object is
called args or block, then Forwardable will fail to work.

Here's a simple example:

class ModelCreator
  extend Forwardable

  attr_reader :args

  def_delegator :args, :model_name

  def initialize(args)
    @args = args
  end
end

ModelCreator.new.model_name

If you run the last line above, then you'll get:

NoMethodError: undefined method `model_name' for []:Array

This error occurs because def_delegator -- as it is written in Ruby --
uses metaprogramming to add methods to the class that will then delegate
to the delegate object. So it's as if we had written:

class ModelCreator
  extend Forwardable

  attr_reader :args

  def model_name(*args, &block)
    args.model_name(*args, &block)
  end

  def initialize(args)
    @args = args
  end
end

As you can see, def_delegator will not only forward the method call
onto the delegate object, it will also forward any arguments provided as
well. It is here that the bug arises: it splats all of the arguments
into a variable which is called args, and because of how variable
scope works in Ruby, it then attempts to call model_name on this
variable and not our delegate object method.

The fix is to call the delegate object method manually using __send__.
(This assumes, of course, that the given receiver is, in fact, the name
of a method and not the name of an instance variable, which is also a
possibility.) We use __send__ because the delegate object method could
be private.

So, that looks like this:

def model_name(*args, &block)
  __send__(:args).model_name(*args, &block)
end

Because def_delegators and delegate use def_delegator internally,
they also get this fix as well.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Closed

Applied in changeset r53381.

---

Forwardable: Fix delegating to 'args' and 'block'

• lib/forwardable.rb (def_instance_delegator) fix delegating to
  'args' and 'block', clashing with local variables in generated
  methods. [ruby-core:72579] [Bug #11916]

• lib/forwardable.rb (def_single_delegator): ditto.

If you have a class that uses Forwardable to delegate a method to
another object, and the method that returns the delegate object is
called args or block, then Forwardable will fail to work.

Here's a simple example:

class ModelCreator
  extend Forwardable

  attr_reader :args

  def_delegator :args, :model_name

  def initialize(args)
    @args = args
  end
end

ModelCreator.new.model_name

If you run the last line above, then you'll get:

NoMethodError: undefined method `model_name' for []:Array

This error occurs because def_delegator -- as it is written in Ruby --
uses metaprogramming to add methods to the class that will then delegate
to the delegate object. So it's as if we had written:

class ModelCreator
  extend Forwardable

  attr_reader :args

  def model_name(*args, &block)
    args.model_name(*args, &block)
  end

  def initialize(args)
    @args = args
  end
end

As you can see, def_delegator will not only forward the method call
onto the delegate object, it will also forward any arguments provided as
well. It is here that the bug arises: it splats all of the arguments
into a variable which is called args, and because of how variable
scope works in Ruby, it then attempts to call model_name on this
variable and not our delegate object method.

The fix is to call the delegate object method manually using __send__.
(This assumes, of course, that the given receiver is, in fact, the name
of a method and not the name of an instance variable, which is also a
possibility.) We use __send__ because the delegate object method could
be private.

So, that looks like this:

def model_name(*args, &block)
  __send__(:args).model_name(*args, &block)
end

Because def_delegators and delegate use def_delegator internally,
they also get this fix as well.

[usa (Usaku NAKAMURA)]

• Backport changed from 2.0.0: UNKNOWN, 2.1: UNKNOWN, 2.2: UNKNOWN, 2.3: UNKNOWN to 2.0.0: WONTFIX, 2.1: REQUIRED, 2.2: REQUIRED, 2.3: REQUIRED

[naruse (Yui NARUSE)]

• Backport changed from 2.0.0: WONTFIX, 2.1: REQUIRED, 2.2: REQUIRED, 2.3: REQUIRED to 2.0.0: WONTFIX, 2.1: REQUIRED, 2.2: REQUIRED, 2.3: DONE

ruby_2_3 r54595 merged revision(s) 53381.

[usa (Usaku NAKAMURA)]

• Backport changed from 2.0.0: WONTFIX, 2.1: REQUIRED, 2.2: REQUIRED, 2.3: DONE to 2.0.0: WONTFIX, 2.1: WONTFIX, 2.2: REQUIRED, 2.3: REQUIRED

naruse-san, it seems that r53382, r53511 and r53512 are also needed for ruby_2_3.

[usa (Usaku NAKAMURA)]

• Backport changed from 2.0.0: WONTFIX, 2.1: WONTFIX, 2.2: REQUIRED, 2.3: REQUIRED to 2.0.0: WONTFIX, 2.1: WONTFIX, 2.2: DONE, 2.3: REQUIRED

ruby_2_2 r54673 merged revision(s) 53381,53382,53511,53512.

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 2.0.0: WONTFIX, 2.1: WONTFIX, 2.2: DONE, 2.3: REQUIRED to 2.0.0: WONTFIX, 2.1: WONTFIX, 2.2: DONE, 2.3: DONE

ruby_2_3 r54711 merged revision(s) 53382,53511,53512.