Exception while trying to marshal ActiveRecord/Sequel model, with attachment & derivatives: no _dump_data is defined for class Thread::Mutex

[Igor S. Morozov]

Brief Description

My app has a case when we need to cache an ActiveRecord object, which has a Shrine attachment.

The attachment's uploader works with two plugins: activerecord and derivatives.

This worked fine under 2.19.4, but broke after our upgrade to 3.3.0

Expected behavior

I expect Rails cache to marshal the provided object and put in the cache. I also expect the object to be functioning properly afterwards

Actual behavior

Whenever I try to marshal a newly-created object with an attachment, I get an exception: no _dump_data is defined for class Thread::Mutex (TypeError)

However, if I try to re-fetch the same object and marshal it, it works fine. #reload doesn't work, though.

Simplest self-contained example code to demonstrate issue

Provide a self-contained example code (see the Sequel/ActiveRecord templates) to show the problem, and the full backtrace of any exception.

ActiveRecord

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'rails', '5.2.4.4'
  gem 'shrine', '3.3.0'
  gem 'sqlite3'
end

require 'active_record'
require 'shrine'
require 'shrine/storage/memory'
require 'down'

Shrine.storages = {
  cache: Shrine::Storage::Memory.new,
  store: Shrine::Storage::Memory.new
}

Shrine.plugin :activerecord

class MyUploader < Shrine
  plugin :derivatives
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.connection.create_table(:posts) { |t| t.text :image_data }

class Post < ActiveRecord::Base
  include MyUploader::Attachment(:image)
end

post = Post.create(image: Down.download('http://theoldreader.com/kittens/600/400/'))

Marshal.dump(post)

# Traceback (most recent call last):
#	1: from reproduce_2.rb:36:in `<main>'
# reproduce.rb:36:in `dump': no _dump_data is defined for class Thread::Mutex (TypeError)


# HOWEVER!

Marshal.dump(post.class.find(post.id)) # works just fine

Marshal.dump(post.reload) # => no _dump_data is defined for class Thread::Mutex (TypeError)

Marshal.dump(post.image) # works just fine

Sequel

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'sequel'
  gem 'shrine', '3.3.0'
  gem 'sqlite3'
end

require 'sequel'
require 'shrine'
require 'shrine/storage/memory'
require 'down'

Shrine.storages = {
  cache: Shrine::Storage::Memory.new,
  store: Shrine::Storage::Memory.new
}

Shrine.plugin :sequel

class MyUploader < Shrine
  plugin :derivatives
end

DB = Sequel.sqlite # SQLite memory database
DB.create_table :posts do
  primary_key :id
  String :image_data
end

class Post < Sequel::Model
  include MyUploader::Attachment(:image)
end

post = Post.create(image: Down.download('http://theoldreader.com/kittens/600/400/'))

Marshal.dump(post)
# Traceback (most recent call last):
# 	1: from reproduce_sequel.rb:39:in `<main>'
# reproduce_sequel.rb:39:in `dump': no _dump_data is defined for class Thread::Mutex (TypeError)

System configuration

Ruby version: 2.6.5 and 2.7.2

Shrine version: 3.3.0

[Igor S. Morozov]

As it turns out, this problem also exists with Sequel models. Sorry for being so hasty and not considering it before creating the issue

[Janko Marohnić]

Hey @Morozzzko, thanks for the report 👍

I believe this is happening because the derivatives plugin uses a mutex internally (assigned to an instance variable), which was not the case with the versions plugin. We should probably find a way to exclude it when marshalling.

I'm wondering whether Sequel has some inspiration, as I recall it has some logic for Marshalling, though I suppose it doesn't use mutexes on models directly.

[Igor S. Morozov]

Thanks for the quick reply!

Yes, I've been trying to find the lines which cause the error, and it is indeed the mutex

I'm looking for a solution to exclude it. Thanks for the tip about Sequel! I'll post whatever I find

[Igor S. Morozov]

Sequel hasn't really fixed it, as it turns out. I've managed to marshal ActiveRecord model, but not Sequel model.

However, if I try to marshal _attacher separately, it works now. I think it's good enough for us, since everything works fine on Shrine's side. I'm preparing a PR