Optimize assignment_external_tools LTI placement

Closes: CORE-3167

Currently: we have a feature where people can register an LTI tool to
show up on any page load to:
a) Assignment create and edit.
b) Discussion topic create and edit.
c) Quiz create and edit.
So that means that on every page load to any of those pages (some of the
most common pages in canvas) we do an additional XHR request to see
what tools are registered to that Course/Acccount/User to see if we 
should show anything in that LTI placement on the page.

That takes a non-trivial amount of time and I wanted to see if I could
help in 2 ways:
1. the xhr request to see what tools are registered represents a good
   chunk of the total requests we handle on our app servers, can we
   help that?
2. fetching the XHR counts against our New Relic load times, can we do
   it sooner so it is ready sooner?

To help #1 I just put an expires_in 10.minutes on the http response for
that controller action because it is something that does not change
that often. For anyone trying to test adding new ones and see it show up
immediately, they just need to do a “hard refresh” in the browser 
(command+shift+R)

To help #2 I used prefetch_xhr to send the fetch request for the data
sooner, meaning the complete page should load faster for users and this 
should never be the thing we are waiting on for newRelic to consider 
the page loaded.

Test Plan:
1.  Configure a new LTI at the account / course level with a 
    assignment_edit launch.
2.  Verify that the launch happens in three different contexts:
    a) Assignment create and edit.
    b) Discussion topic create and edit.
    c) Quiz create and edit.
3.  For each context above (#2) verify the launch happens as it did 
    before.
4.  Look in your browser network panel. You should see that the request
    To: …/lti_apps/launch_definitions?placements%5B%5D=assignment_view
    Is of type: “Fetch” instead of “XHR” and it happens sooner in the 
    Waterfall.
5.  If you hit the page again, you should see that the “time” is < 1ms
    because it was loaded from the browser’s cache
6.  Review api docs at:
    doc/api/file.assignment_external_tools.html
    experiment with options and verify apps work as expected.

Change-Id: I2986fa281d1f85d4e544b763647b8e3137d42e00
Reviewed-on: https://gerrit.instructure.com/199384
Reviewed-by: Mark Valentine <mvalentine@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins
QA-Review: Trevor Byington <tbyington@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

app/controllers/lti/lti_apps_controller.rb

@@ -52,7 +52,11 @@ module Lti
             named_context_url(@context, :api_v1_context_launch_definitions_url, include_host: true),
             pagination_args
           )
-          format.json { render :json => AppLaunchCollator.launch_definitions(launch_defs, placements) }
+          format.json do
+            cancel_cache_buster
+            expires_in 10.minutes
+            render :json => AppLaunchCollator.launch_definitions(launch_defs, placements)
+          end
         end
       end
     end

app/helpers/application_helper.rb

@@ -1195,6 +1195,15 @@ module ApplicationHelper
     super(attachment, uuid || attachment.uuid, url_options)
   end
 
+  def prefetch_assignment_external_tools
+    content_tag(:div, id: 'assignment_external_tools') do
+      prefetch_xhr(api_v1_course_launch_definitions_path(
+        @context,
+        'placements[]' => 'assignment_view'
+      ))
+    end
+  end
+
   if CANVAS_RAILS5_1
     # this is for rails 5.1. it can be deleted when we upgrade to RAILS_5.2. rails 5.2 includes a preload_link_tag method
     def preload_link_tag(source, options = {})

app/jsx/assignments/AssignmentExternalTools.js

@@ -23,6 +23,7 @@ import ReactDOM from 'react-dom'
 import I18n from 'i18n!moderated_grading'
 import 'compiled/jquery.rails_flash_notifications'
 import iframeAllowances from '../external_apps/lib/iframeAllowances'
+import {asJson, getPrefetchedXHR} from '@instructure/js-utils'
 
 class AssignmentExternalTools extends React.Component {
   constructor(props) {
@@ -61,30 +62,17 @@ class AssignmentExternalTools extends React.Component {
     return null;
   }
 
-  getTools() {
+  async getTools() {
-    const self = this;
+    const url = encodeURI(`${this.getDefinitionsUrl()}?placements[]=${this.props.placement}`)
-    const toolsUrl = this.getDefinitionsUrl();
-    const data = {
-      'placements[]': this.props.placement
-    };
 
-    $.ajax({
+    try {
-      type: 'GET',
+      const request = getPrefetchedXHR(url) || fetch(url, {headers: {Accept: 'application/json'}})
-      url: toolsUrl,
+      const tools = await asJson(request)
-      data,
+      tools.forEach(t => (t.launch = this.getLaunch(t)))
-      success: $.proxy(function(data) {
+      this.setState({tools})
-        const tool_data = data
+    } catch (e) {
-        for (let i = 0; i < data.length; i++) {
+      $.flashError(I18n.t('Error retrieving assignment external tools'))
-          tool_data[i].launch = this.getLaunch(data[i]);
     }
-        this.setState({
-          tools: tool_data
-        });
-      }, self),
-      error(_xhr) {
-        $.flashError(I18n.t('Error retrieving assignment external tools'));
-      }
-    });
   }
 
   getDefinitionsUrl() {

app/views/discussion_topics/show.html.erb

@@ -355,7 +355,7 @@
 
   </div>
 
-  <div id="assignment_external_tools"></div>
+  <%= prefetch_assignment_external_tools %>
 
   <% if @headers != false %>
     <div id="module_sequence_footer"></div>

app/views/quizzes/quizzes/_quiz_edit_details.erb

@@ -301,5 +301,5 @@ content_is_locked = !!js_env.dig(:MASTER_COURSE_DATA, "is_master_course_child_co
       </div>
     </div>
   <% end %>
-  <div id="assignment_external_tools"></div>
+  <%= prefetch_assignment_external_tools %>
 </div>

app/views/quizzes/quizzes/show.html.erb

@@ -383,7 +383,7 @@
     <%= render 'cant_go_back_warning' %>
   <% end %>
 
-  <div id="assignment_external_tools"></div>
+  <%= prefetch_assignment_external_tools %>
 
   <div id="module_sequence_footer"></div>