Deadlock with Sequel 5.52 and ActiveSupport Core Extensions under Ruby < 3.1

[rgalanakis]

I was originally going to file this as a bug, but I couldn't in good conscience file a bug against Sequel involving Active Support core extensions, and I don't think it has a good solution other than 'do not install the activesupport class core extensions'. I figure it was a nice writeup and is good reference so didn't want to get rid of it and figured the mailing list is a good place. And if you can think of a solution to make Sequel subclasses and ActiveSupport core extensions work well together pre-Ruby 3.1, that'd be swell (though it actually wouldn't affect any of our current projects so nbd to me).

Complete Description of Issue

Sequel's Subclasses plugin was recently updated to make use of Ruby 3.1's Object.subclasses method in 185efe5

It did this by removing the Sequel-land subclasses method, so descendants calls the built-in subclasses method. It checks whether to use this based on Object.respond_to?(:subclasses).

If ActiveSupport core extensions are installed, they will also install subclasses/descendants methods. This code has churned recently due to the Classes.descendants changes (I saw you were involved in the discussion), but I think it's still an issue at HEAD.

Under Ruby 3.1, this isn't a problem. ActiveSupport doesn't install either subclasses nor descendants; Sequel doesn't install subclasses; Sequel descendants uses Ruby's built-in subclasses.

Under earlier Ruby, Sequel 5.51 and earlier, and ActiveSupport, this also wasn't an issue- ActiveSupport installs its methods, then using the subclasses plugin would replace both of them with Sequel's implementation.

However under earlier Ruby, Sequel 5.52, and ActiveSupport, what happens is:

• ActiveSupport installs subclasses and descendants methods
• Sequel checks Object.respond_to?(:subclasses). It does (because the core extensions). Sequel only overrides descendants.
• Sequel's descendants calls ActiveSupport's subclasses. ActiveSupport subclasses calls descendants, which goes back into Sequel's descendants, causing a deadlock in Sequel.synchronize (it would eventually stack overflow if not for that).

Simplest Possible Self-Contained Example Showing the Bug

require "active_support"
require "active_support/core_ext"
require 'sequel'

module MyProj; end

MyProj::Model = Class.new(Sequel::Model)
MyProj::Model.def_Model(MyProj)
MyProj::Model.plugin(:subclasses)

class MyProj::Model; end
puts MyProj::Model.descendants

Full Backtrace of Exception (if any)

ThreadError: deadlock; recursive locking

  0) Webhookdb::Postgres::ModelUtilities an extended class has a method for fetching a subclass by its full name
     Failure/Error:
       return self.descendents.find do |cl|
         cl.name&.end_with?(classname)
       end

     ThreadError:
       deadlock; recursive locking
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/core.rb:316:in `synchronize'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/core.rb:316:in `synchronize'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/plugins/subclasses.rb:64:in `descendants'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/activesupport-7.0.0/lib/active_support/core_ext/class/subclasses.rb:33:in `subclasses'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/plugins/subclasses.rb:64:in `block in descendants'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/core.rb:316:in `synchronize'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/core.rb:316:in `synchronize'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/sequel-5.52.0/lib/sequel/plugins/subclasses.rb:64:in `descendants'
     # ./lib/webhookdb/postgres/model_utilities.rb:81:in `by_name'
     # ./spec/webhookdb/postgres/model_utilities_spec.rb:65:in `block (2 levels) in <top (required)>'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:262:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:262:in `block in run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:508:in `block in with_around_and_singleton_context_hooks'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:465:in `block in with_around_example_hooks'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:486:in `block in run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:626:in `block in run_around_example_hooks_for'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:350:in `call'
     # ./lib/webhookdb/spec_helpers/postgres.rb:39:in `block in included'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:455:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:455:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:390:in `execute_with'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:628:in `block (2 levels) in run_around_example_hooks_for'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:350:in `call'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/appydays-0.1.2/lib/appydays/loggable/spec_helpers.rb:15:in `block in included'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:455:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:455:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:390:in `execute_with'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:628:in `block (2 levels) in run_around_example_hooks_for'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:350:in `call'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/webmock-3.12.2/lib/webmock/rspec.rb:37:in `block (2 levels) in <top (required)>'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:455:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:455:in `instance_exec'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:390:in `execute_with'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:628:in `block (2 levels) in run_around_example_hooks_for'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:350:in `call'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:629:in `run_around_example_hooks_for'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/hooks.rb:486:in `run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:465:in `with_around_example_hooks'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:508:in `with_around_and_singleton_context_hooks'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example.rb:259:in `run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example_group.rb:644:in `block in run_examples'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example_group.rb:640:in `map'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example_group.rb:640:in `run_examples'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/example_group.rb:606:in `run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:121:in `block (3 levels) in run_specs'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:121:in `map'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:121:in `block (2 levels) in run_specs'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/configuration.rb:2067:in `with_suite_hooks'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:116:in `block in run_specs'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/reporter.rb:74:in `report'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:115:in `run_specs'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:89:in `run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:71:in `run'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/lib/rspec/core/runner.rb:45:in `invoke'
     # /Users/rob/.rbenv/versions/2.7.5/lib/ruby/gems/2.7.0/gems/rspec-core-3.10.1/exe/rspec:4:in `<top (required)>'
     # /Users/rob/.rbenv/versions/2.7.5/bin/rspec:23:in `load'
     # /Users/rob/.rbenv/versions/2.7.5/bin/rspec:23:in `<main>'

[jeremyevans]

Thanks for the report. I suppose I can check whether subclasses is implemented in Ruby to guess if ActiveSupport is involved, and if so, define the Sequel methods anyway (they are only not defined as an optimization). I don't really like making changes to work around issues caused by ActiveSupport, but I don't think there is much choice in this case.