Object.as_json is too optimistic and causes errors in 3rd party code.

[ioquatix]

This monkey patch is dangerous:

rails/activesupport/lib/active_support/core_ext/object/json.rb

Lines 58 to 66 in 82054e8

	class Object
	def as_json(options = nil) # :nodoc:
	if respond_to?(:to_hash)
	to_hash.as_json(options)
	else
	instance_values.as_json(options)
	end
	end
	end

Specifically, I've seen this issue crop up a number of times. A casual search yields issues that appear to be related:

• Async::Container::Process, SystemStackError: stack level too deep socketry/async-http#157
• Stack level too deep (SystemStackError) then raise exception in Sync. Then try to show warn in console socketry/async-http#156
• Stack level too deep #40781
• SystemStackError: stack level too deep in as_json #16112
• Circular data structures raise SystemStackError when serializing them with #to_json. #51202
• ActionController::Base#to_json fails with SystemStackOverflow. #43040
• active_support/json doesn't handle RSpec::Core::Example properly #28996
• 5.2.0.beta2 doesnt load in dev mode. #31313
• Stack Level Too Deep in ActionDispatch Reloader #11631

I don't think as_json should be monkey patched into Object like this. The default behaviour of using to_s is safer. Alternatively, objects that refer to themselves (circular references) should be handled correctly.

[ioquatix]

I guess it was introduced here: e567a5e

cc @jeremy you have worked on that code quite a bit, do you have any opinion?

[ioquatix]

A safer default might be this:

 class Object 
   def as_json(options = nil) # :nodoc: 
     if respond_to?(:to_hash) 
       to_hash.as_json(options) 
     else 
       self.to_s
     end 
   end 
 end 

[Earlopain]

I've also seen this reported to sidekiq a few times:

• Sidekiq::Config#as_json causes StackLevelTooDeep error sidekiq/sidekiq#6000
• SystemStackError when starting Sidekiq when using a Json log formatter  sidekiq/sidekiq#5920
• JSON formatter throws SystemStackError sidekiq/sidekiq#5858 (maybe?)

This was eventually fixed by implementing a custom to_hash in sidekiq/sidekiq#6198

[ioquatix]

The problem with to_hash is it makes an object implicitly convertible (equivalent to) a Hash. It would have made more sense to use to_h (explicitly convertible).

See https://zverok.space/blog/2016-01-18-implicit-vs-expicit.html for a good summary of the differences.