Create pidfile before loading the application. Thanks to this monit/god knows that process exists and will not try to run other instance event when initialization takes a long time.

Gemfile_puma_single_worker_pidfile_patch.rb

gem "puma", require: false, platform: :jruby

# Issue: https://github.com/puma/puma/issues/785

begin
  require "puma/single"

  class ::Puma::Single
    alias unpatched_run run

    def run
      unless daemon?
        @cli.write_pid
      end

      unpatched_run
    end
  end
rescue LoadError => exception
  warn "#{exception.class}: #{exception.message}"
end