Hacky Rack middleware to get backtrace of hung Unicorn workers

application.rb

# config/application.rb
require File.expand_path('../boot', __FILE__)

require 'rails/all'

Bundler.require(:default, Rails.env)

module MyApp
  class Application < Rails::Application
    # configure Rails to inject this middleware. Must be at least after Rails logger!
    require File.expand_path('../../lib/log_before_timeout', __FILE__)
    config.middleware.use LogBeforeTimeout
    
    # to insert at start of chain instead:
    # config.middleware.insert(0, LogBeforeTimeout)

    # to insert before a specific middleware
    #config.middleware.insert_before(ActionDispatch::Static, LogBeforeTimeout)
  end
end

log_before_timeout.rb

# lib/log_before_timeout.rb
class LogBeforeTimeout < Struct.new(:app)
  def call(env)
    thr = Thread.new(Thread.current) do |request_thread|
      sleep(14) # set this to Unicorn timeout - 1
      unless Thread.current[:done]
        # instead of Rails.logger you should be able to use env["rack.logger"], 
        # but I never got this to work (was always nil in my Rails app)
        Rails.logger.warn "about to die from SIGKILL. thread:"
        Rails.logger.warn "#{request_thread.inspect}"
        Rails.logger.warn request_thread.backtrace.join("\n").prepend("\n")
      end
    end
    app.call(env)
  ensure
    thr[:done] = true
    thr.run
  end
end