Editor: Allow file retrieval after code run

app/controllers/live_streams_controller.rb

@@ -0,0 +1,70 @@
+# frozen_string_literal: true
+
+class LiveStreamsController < ApplicationController
+  # Including ActionController::Live changes all actions in this controller!
+  # Therefore, it is extracted into a separate controller
+  include ActionController::Live
+
+  def download_submission_file
+    begin
+      @submission = authorize AuthenticatedUrlHelper.retrieve!(Submission, request, force_render_host: false)
+    rescue Pundit::NotAuthorizedError
+      # TODO: Option to disable?
+      # Using the submission ID parameter would allow looking up the corresponding exercise ID
+      # Therefore, we just redirect to the root_path, but actually expect to redirect back (that should work!)
+      redirect_back(fallback_location: root_path, alert: t('exercises.download_file_tree.gone'))
+    end
+
+    desired_file = params[:filename].to_s
+    runner = Runner.for(current_user, @submission.exercise.execution_environment)
+    fallback_location = implement_exercise_path(@submission.exercise)
+    send_runner_file(runner, desired_file, fallback_location)
+  end
+
+  private
+
+  def send_runner_file(runner, desired_file, redirect_fallback = root_path)
+    filename = File.basename(desired_file)
+    send_stream(filename: filename, disposition: 'attachment') do |stream|
+      runner.download_file desired_file do |chunk, overall_size, content_type|
+        unless response.committed?
+          # Disable Rack::ETag, which would otherwise cause the response to be cached
+          # See https://github.com/rack/rack/issues/1619#issuecomment-848460528
+          response.set_header('Last-Modified', Time.now.httpdate)
+          response.set_header('Content-Length', overall_size) if overall_size
+          response.set_header('Content-Type', content_type) if content_type
+          # Commit the response headers immediately, as streaming would otherwise remove the Content-Length header
+          # This will prevent chunked transfer encoding from being used, which is okay as we know the overall size
+          # See https://github.com/rails/rails/issues/18714
+          response.commit!
+        end
+
+        if stream.connected?
+          stream.write chunk
+        else
+          # The client disconnected, so we stop streaming
+          break
+        end
+      end
+    rescue Runner::Error
+      redirect_back(fallback_location: redirect_fallback, alert: t('exercises.download_file_tree.gone'))
+    end
+  end
+
+  # TODO: Taken from Rails 7, remove when upgrading
+  # rubocop:disable all
+  def send_stream(filename:, disposition: "attachment", type: nil)
+    response.headers["Content-Type"] =
+      (type.is_a?(Symbol) ? Mime[type].to_s : type) ||
+        Mime::Type.lookup_by_extension(File.extname(filename).downcase.delete(".")) ||
+        "application/octet-stream"
+
+    response.headers["Content-Disposition"] =
+      ActionDispatch::Http::ContentDisposition.format(disposition: disposition, filename: filename)
+
+    yield response.stream
+  ensure
+    response.stream.close
+  end
+  # rubocop:enable all
+end