Compatibility with puma and Rails - "WARNING: Detected 2 Thread(s) started in app boot:"

[ndbroadbent]

* Operating system:                mac + linux
* Ruby implementation:             Ruby
* `concurrent-ruby` version:       1.1.6
* `concurrent-ruby-ext` installed: no
* `concurrent-ruby-edge` used: no

Hello, I'm trying to debug a thread warning in puma. Here's the issue I opened over there: puma/puma#2237

The problem seems to be that requiring lib/concurrent-ruby/concurrent/atomic/ruby_thread_local_var.rb immediately starts a new Thread.

Here's the stack trace when puma loads my application:

/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomic/ruby_thread_local_var.rb:8:in `<module:Concurrent>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomic/ruby_thread_local_var.rb:4:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomic/thread_local_var.rb:2:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomic/thread_local_var.rb:2:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb:5:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomic/reentrant_read_write_lock.rb:5:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomics.rb:8:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent/atomics.rb:8:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent.rb:6:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/concurrent-ruby-1.1.6/lib/concurrent-ruby/concurrent.rb:6:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-5.2.4.2/lib/active_support/logger_silence.rb:5:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-5.2.4.2/lib/active_support/logger_silence.rb:5:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-5.2.4.2/lib/active_support/logger.rb:3:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-5.2.4.2/lib/active_support/logger.rb:3:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-5.2.4.2/lib/active_support.rb:29:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/activesupport-5.2.4.2/lib/active_support.rb:29:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/railties-5.2.4.2/lib/rails.rb:7:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/railties-5.2.4.2/lib/rails.rb:7:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/railties-5.2.4.2/lib/rails/all.rb:3:in `require'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/railties-5.2.4.2/lib/rails/all.rb:3:in `<top (required)>'
/Users/myuser/code/myapp/config/application.rb:7:in `require'
/Users/myuser/code/myapp/config/application.rb:7:in `<top (required)>'
/Users/myuser/code/myapp/config/environment.rb:4:in `require_relative'
/Users/myuser/code/myapp/config/environment.rb:4:in `<top (required)>'
config.ru:5:in `require_relative'
config.ru:5:in `block in <main>'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rack-2.2.2/lib/rack/builder.rb:116:in `eval'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rack-2.2.2/lib/rack/builder.rb:116:in `new_from_string'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rack-2.2.2/lib/rack/builder.rb:105:in `load_file'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/rack-2.2.2/lib/rack/builder.rb:66:in `parse_file'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/lib/puma/configuration.rb:321:in `load_rackup'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/lib/puma/configuration.rb:246:in `app'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/lib/puma/runner.rb:155:in `load_and_bind'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/lib/puma/cluster.rb:413:in `run'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/lib/puma/launcher.rb:172:in `run'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/lib/puma/cli.rb:80:in `run'
/Users/myuser/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/puma-4.3.3/bin/puma:10:in `<top (required)>'
/Users/myuser/.rbenv/versions/2.6.5/bin/puma:23:in `load'
/Users/myuser/.rbenv/versions/2.6.5/bin/puma:23:in `<main>'

When puma is running in "clustered" mode, it forks a new process for each worker, so the Thread started by the RubyThreadLocalVar class is not running inside any of these worker processes. Is this a problem? My first impression is that this might cause a memory leak because none of the thread-local variables are ever getting released and garbage collected in my puma workers. (But I'm not sure which parts of Rails are using this.)

New threads can be started inside puma.on_worker_boot, so should we doing something like:

puma.on_worker_boot do
  Concurrent::RubyThreadLocalVar.start_thread!
end

Or maybe puma should do this automatically if it detects the concurrent-ruby library?

Thanks for your time!

[nateberkopec]

Should concurrent-ruby be creating threads at require-time? I think this could create risks for anyone using a preforking webserver.

[timriley]

I just noticed this warning in my app running via Puma, too.

I took a look at the latest code in lib/concurrent-ruby/concurrent/atomic/ruby_thread_local_var.rb and it seems like the issue might have been fixed by the changes in #856?

This PR was merged only yesterday! Lucky timing :)

@pitr-ch out of interest, were you aware of this Puma-related issue when working on the changes in that PR?

[pitr-ch]

I was not, indeed a lucky timing :)
Could you test the fix from #856 in your environment?

[MTres19]

I had a very similar if not the same issue and the patch from that pull request does eliminate the warning for concurrent-ruby. I still get the warning for ActiveRecord, but I guess that is a separate issue. Thank you all for your work!

[nateberkopec]

Released, so I think this can be closed.

[pitr-ch]

@nateberkopec please, could you confirm the warning is gone? Then I'll close.

[dentarg]

@pitr-ch I can confirm that I no longer see the warning using concurrent-ruby 1.1.7 (I was using concurrent-ruby 1.1.6 before). My app uses Puma 4.3.5 and Ruby 2.5 (and Sinatra, not Rails).

[pitr-ch]

@dentarg Thank you 👍 for confirming!