puma_plugin_ping.rb

# Usage:
# bind "tcp://127.0.0.1:5000"
# activate_control_app "tcp://127.0.0.1:9293", auth_token: "foo"
#
# curl -v http://127.0.0.1:5000/ping
# curl -v http://127.0.0.1:9293/ping/disable?token=foo
# curl -v http://127.0.0.1:9293/ping/enable?token=foo

# https://github.com/puma/puma/blob/master/lib/puma/app/status.rb
require "puma/app/status"

::Puma::App::Status.prepend(Module.new do
  def call(env)
    unless authenticate(env)
      return rack_response(403, 'Invalid auth token', 'text/plain')
    end

    case env['PATH_INFO']
    when /\A\/ping\/disable\z/
      Thread.main[:ping].disable
      return rack_response(200, ::Puma::App::Status::OK_STATUS)
    when /\A\/ping\/enable\z/
      Thread.main[:ping].enable
      return rack_response(200, ::Puma::App::Status::OK_STATUS)
    end

    super
  end
end)

require "io/wait"

class PingMaster
  LOCK_MESSAGE = "!".freeze
  private_constant :LOCK_MESSAGE
  attr_reader :pipe

  def initialize(pipe)
    @pipe = pipe
    @read_ping, @write_ping = pipe
  end

  def enable
    unless enabled?
      @read_ping.gets
    end
  end

  def disable
    if enabled?
      @write_ping.puts(LOCK_MESSAGE)
    end
  end

  def enabled?
    !@read_ping.ready?
  end

  def close
    @write_ping.close
    @read_ping.close
  end
end

class PingWorker
  def initialize(pipe)
    @read, @write = pipe
    @write.close
  end

  def enabled?
    !@read.ready?
  end

  def close
    @write.close
    @read.close
  end
end

before_fork do
  Thread.main[:ping] = PingMaster.new(IO.pipe)

  at_exit do
    Thread.main[:ping].close
  end
end

on_worker_boot do
  Thread.main[:ping] = PingWorker.new(Thread.main[:ping].pipe)
end