Gracefully restart unicorn using Server::Starter

start_server.sh

start_server --status-file=/path/to/app/log/start_server.stat \
  --port=10080 --signal-on-hup=CONT --dir=/path/to/app -- \
  bundle exec --keep-file-descriptors unicorn -c config/unicorn.conf.rb config.ru

unicorn.conf.rb

worker_processes  4
preload_app true

APP_ROOT = File.expand_path('../..', __FILE__)
status_file = File.join(APP_ROOT, 'log/start_server.stat')

if ENV.key?('SERVER_STARTER_PORT')
  fds = ENV['SERVER_STARTER_PORT'].split(';').map { |x|
    path_or_port, fd = x.split('=', 2)
    fd
  }
  ENV['UNICORN_FD'] = fds.join(',')
  ENV.delete('SERVER_STARTER_PORT')
else
  listen ENV['PORT'] || '10080'
end

before_fork do |server, worker|
  defined?(ActiveRecord::Base) and ActiveRecord::Base.connection.disconnect!

  # Throttle the master from forking too quickly by sleeping.  Due
  # to the implementation of standard Unix signal handlers, this
  # helps (but does not completely) prevent identical, repeated signals
  # from being lost when the receiving process is busy.
  sleep 1
end

after_fork do |server, worker|
  defined?(ActiveRecord::Base) and ActiveRecord::Base.establish_connection

  begin
    # This allows a new master process to incrementally
    # phase out the old master process with SIGTTOU to avoid a
    # thundering herd (especially in the "preload_app false" case)
    # when doing a transparent upgrade.  The last worker spawned
    # will then kill off the old master process with a SIGQUIT.
    pids = File.readlines(status_file).map {|_| _.chomp.split(':') }.to_h
    old_gen = ENV['SERVER_STARTER_GENERATION'].to_i - 1
    if old_pid = pids[old_gen.to_s]
      sig = (worker.nr + 1) >= server.worker_processes ? :QUIT : :TTOU
      Process.kill(sig, old_pid.to_i)
    end
  rescue Errno::ENOENT, Errno::ESRCH => e
    $stderr.puts "#{e.class} #{e.message}"
  end
end