class ActionDispatch::DebugLocks

def blocked_by?(victim, blocker, all_threads)
  return false if victim.equal?(blocker)
  case victim[:sleeper]
  when :start_sharing
    blocker[:exclusive] ||
      (!victim[:waiting] && blocker[:compatible] && !blocker[:compatible].include?(false))
  when :start_exclusive
    blocker[:sharing] > 0 ||
      blocker[:exclusive] ||
      (blocker[:compatible] && !blocker[:compatible].include?(victim[:purpose]))
  when :yield_shares
    blocker[:exclusive]
  when :stop_exclusive
    blocker[:exclusive] ||
      victim[:compatible] &&
      victim[:compatible].include?(blocker[:purpose]) &&
      all_threads.all? { |other| !other[:compatible] || blocker.equal?(other) || other[:compatible].include?(blocker[:purpose]) }
  end
end

def call(env)
  req = ActionDispatch::Request.new env
  if req.get?
    path = req.path_info.chomp("/")
    if path == @path
      return render_details(req)
    end
  end
  @app.call(env)
end

def initialize(app, path = "/rails/locks")
  @app = app
  @path = path
end

def render_details(req)
  threads = ActiveSupport::Dependencies.interlock.raw_state do |raw_threads|
    # The Interlock itself comes to a complete halt as long as this block is
    # executing. That gives us a more consistent picture of everything, but creates
    # a pretty strong Observer Effect.
    #
    # Most directly, that means we need to do as little as possible in this block.
    # More widely, it means this middleware should remain a strictly diagnostic tool
    # (to be used when something has gone wrong), and not for any sort of general
    # monitoring.
    raw_threads.each.with_index do |(thread, info), idx|
      info[:index] = idx
      info[:backtrace] = thread.backtrace
    end
    raw_threads
  end
  str = threads.map do |thread, info|
    if info[:exclusive]
      lock_state = +"Exclusive"
    elsif info[:sharing] > 0
      lock_state = +"Sharing"
      lock_state << " x#{info[:sharing]}" if info[:sharing] > 1
    else
      lock_state = +"No lock"
    end
    if info[:waiting]
      lock_state << " (yielded share)"
    end
    msg = +"Thread #{info[:index]} [0x#{thread.__id__.to_s(16)} #{thread.status || 'dead'}]  #{lock_state}\n"
    if info[:sleeper]
      msg << "  Waiting in #{info[:sleeper]}"
      msg << " to #{info[:purpose].to_s.inspect}" unless info[:purpose].nil?
      msg << "\n"
      if info[:compatible]
        compat = info[:compatible].map { |c| c == false ? "share" : c.to_s.inspect }
        msg << "  may be pre-empted for: #{compat.join(', ')}\n"
      end
      blockers = threads.values.select { |binfo| blocked_by?(info, binfo, threads.values) }
      msg << "  blocked by: #{blockers.map { |i| i[:index] }.join(', ')}\n" if blockers.any?
    end
    blockees = threads.values.select { |binfo| blocked_by?(binfo, info, threads.values) }
    msg << "  blocking: #{blockees.map { |i| i[:index] }.join(', ')}\n" if blockees.any?
    msg << "\n#{info[:backtrace].join("\n")}\n" if info[:backtrace]
  end.join("\n\n---\n\n\n")
  [200, { Rack::CONTENT_TYPE => "text/plain; charset=#{ActionDispatch::Response.default_charset}",
          Rack::CONTENT_LENGTH => str.size.to_s }, [str]]
end