Loading debug gem breaks blankslate gem

Your environment

ruby -v: ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux]
rdbg -v: rdbg 1.6.2

Describe the bug

The debug gem undefines and redefines singleton_method_added.

The blankslate gem provides a "abstract base class with no predefined methods" except for some core Ruby builtins. "BlankSlate is useful as a base class when writing classes that depend upon method_missing (e.g. dynamic proxies)", so it only works if singleton_method_added is available. However, it hides singleton_method_added because it's redefined by the debug gem.

This problem does not occur when the debug gem is not required.

To Reproduce

require 'debug'
require 'blankslate'

BlankSlate.new.singleton_method_added

Buggy output -- the method isn't defined:

Traceback (most recent call last):
repro.rb:4:in `<main>': undefined method `singleton_method_added' for #<BlankSlate:0x000055a654b12fb0> (NoMethodError)

Expected behavior

The method is defined, but private:

Traceback (most recent call last):
repro.rb:3:in `<main>': private method `singleton_method_added' called for #<BlankSlate:0x000055d758dcd540> (NoMethodError)

Workaround

We can work around the bug by explicilty unhiding this method, or by requiring blankslate before debug:

BlankSlate.reveal(:singleton_method_added)

Additional context

The problem is caused by this code in the debug gem:

debug/lib/debug/session.rb

Lines 1686 to 1689 in 19b4dde

    
           class ::BasicObject 
        
             undef singleton_method_added 
        
             def singleton_method_added mid; end 
        
           end

Why does the debug gem redefine this method?

This problem breaks one of Stripe's services in development because we start a debug session when it starts up, before it requires the blankslate gem. We are working around the issue, but it would be great if it could be fixed!

Thank you for the report.
I didn't know the blankslate but could you make a smaller repro-code without the blankslate gem?
Otherwise I'll try to make it.

minitest also breaks, see minitest/minitest#929

Here is blankslate torn all the way down to a minimal repro:

#!/usr/bin/env ruby -vw

class BlankSlate
  def self.hide(name)
    return unless instance_methods.include? name
    undef_method name
  end

  hide :singleton_method_added
end

begin
  BlankSlate.new.singleton_method_added
rescue NoMethodError => e
  p e.message.include?("private method") ? :GOOD : :BAD
else
  p :BAD
end

__END__
% ruby -rdebug debug_blankslate.rb
:BAD
% ruby debug_blankslate.rb
:GOOD

run w/o ruby -rdebug to output GOOD and run with ruby -rdebug to output BAD...

It essentially boils down to "is singleton_method_added defined?"

for minitest/mock, not too dissimilar:

#!/usr/bin/env ruby -vw

class Mock
  overridden_methods = %w[
    ===
    class
    inspect
    instance_eval
    instance_variables
    object_id
    public_send
    respond_to_missing?
    send
    to_s
  ]

  instance_methods.each do |m|
    undef_method m unless overridden_methods.include?(m.to_s) || m =~ /^__/
  end

  def method_missing sym, *args, **kwargs, &block # :nodoc:
    raise NoMethodError, "unmocked method %p" % [sym]
  end
end

foo = Mock.new
def foo.bar = 'very awesome foo'

p :GOOD

I think in my case I'm going to add singleton_method_added to overridden_methods if DEBUGGER__ is defined.

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Loading `debug` gem breaks `blankslate` gem #757

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	class ::BasicObject
	undef singleton_method_added
	def singleton_method_added mid; end
	end

Loading debug gem breaks blankslate gem #757

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jvilk-stripe commented on Sep 21, 2022

ko1 commented on Oct 4, 2022

dentarg commented on Apr 4, 2023

zenspider commented on Nov 10, 2023

zenspider commented on Nov 10, 2023

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Loading `debug` gem breaks `blankslate` gem #757