discourse/lib/hijack.rb

# frozen_string_literal: true

require "method_profiler"

# This module allows us to hijack a request and send it to the client in the deferred job queue
# For cases where we are making remote calls like onebox or proxying files and so on this helps
# free up a unicorn worker while the remote IO is happening
module Hijack
  def hijack(info: nil, &blk)
    controller_class = self.class

    if hijack = request.env["rack.hijack"]
      request.env["discourse.request_tracker.skip"] = true
      request_tracker = request.env["discourse.request_tracker"]

      # in the past unicorn would recycle env, this is not longer the case
      env = request.env

      # rack may clean up tempfiles unless we trick it and take control
      tempfiles = env[Rack::RACK_TEMPFILES]
      env[Rack::RACK_TEMPFILES] = nil
      request_copy = ActionDispatch::Request.new(env)

      transfer_timings = MethodProfiler.transfer

      scheduled = Concurrent::Promises.resolvable_event

      begin
        Scheduler::Defer.later(
          "hijack #{params["controller"]} #{params["action"]} #{info}",
          force: false,
          &scheduled.method(:resolve)
        )
      rescue WorkQueue::WorkQueueFull
        return render plain: "", status: 503
      end

      # duplicate headers so other middleware does not mess with it
      # on the way down the stack
      original_headers = response.headers.dup

      io = hijack.call

      scheduled.on_resolution! do
        MethodProfiler.start(transfer_timings)
        begin
          Thread.current[Logster::Logger::LOGSTER_ENV] = env
          # do this first to confirm we have a working connection
          # before doing any work
          io.write "HTTP/1.1 "

          # this trick avoids double render, also avoids any litter that the controller hooks
          # place on the response
          instance = controller_class.new
          response = ActionDispatch::Response.new
          instance.response = response

          instance.request = request_copy
          original_headers&.each { |k, v| instance.response.headers[k] = v }

          view_start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
          begin
            instance.instance_eval(&blk)
          rescue => e
            # TODO we need to reuse our exception handling in ApplicationController
            Discourse.warn_exception(
              e,
              message: "Failed to process hijacked response correctly",
              env: env,
            )
          end
          view_runtime = Process.clock_gettime(Process::CLOCK_MONOTONIC) - view_start

          instance.status = 500 unless instance.response_body || response.committed?

          response.commit!

          body = response.body

          headers = response.headers
          # add cors if needed
          if cors_origins = env[Discourse::Cors::ORIGINS_ENV]
            Discourse::Cors.apply_headers(cors_origins, env, headers)
          end

          headers["Content-Type"] ||= response.content_type || "text/plain"
          headers["Content-Length"] = body.bytesize
          headers["Connection"] = "close"

          headers["Discourse-Logged-Out"] = "1" if env[Auth::DefaultCurrentUserProvider::BAD_TOKEN]

          status_string = Rack::Utils::HTTP_STATUS_CODES[response.status.to_i] || "Unknown"
          io.write "#{response.status} #{status_string}\r\n"

          timings = MethodProfiler.stop
          if timings && duration = timings[:total_duration]
            headers["X-Runtime"] = "#{"%0.6f" % duration}"
          end

          headers.each { |name, val| io.write "#{name}: #{val}\r\n" }

          io.write "\r\n"
          io.write body
        rescue Errno::EPIPE, IOError
          # happens if client terminated before we responded, ignore
          io = nil
        ensure
          if Rails.configuration.try(:lograge).try(:enabled)
            if timings
              db_runtime = 0
              db_runtime = timings[:sql][:duration] if timings[:sql]

              subscriber = Lograge::LogSubscribers::ActionController.new
              payload =
                ActiveSupport::HashWithIndifferentAccess.new(
                  controller: self.class.name,
                  action: action_name,
                  params: request.filtered_parameters,
                  headers: request.headers,
                  format: request.format.ref,
                  method: request.request_method,
                  path: request.fullpath,
                  view_runtime: view_runtime * 1000.0,
                  db_runtime: db_runtime * 1000.0,
                  timings: timings,
                  status: response.status,
                )

              event =
                ActiveSupport::Notifications::Event.new(
                  "hijack",
                  Time.now,
                  Time.now + timings[:total_duration],
                  "",
                  payload,
                )
              subscriber.process_action(event)
            end
          end

          MethodProfiler.clear
          Thread.current[Logster::Logger::LOGSTER_ENV] = nil

          begin
            io.close if io
          rescue StandardError
            nil
          end

          if request_tracker
            status =
              begin
                response.status
              rescue StandardError
                500
              end
            request_tracker.log_request_info(env, [status, headers || {}, []], timings)
          end

          tempfiles&.each(&:close!)
        end
      end

      # not leaked out, we use 418 ... I am a teapot to denote that we are hijacked
      render plain: "", status: 418
    else
      blk.call
    end
  end
end