fix broken content notifications when referred from front page

test plan:
* create a wiki page with a broken canvas link
* set the page as the front page of course
* if a student follows the broken link from the
 front page (e.g. /courses/X) it should still
 send the broken link notification to the teacher
 as if they had followed it from the page directly
 (e.g. /courses/X/pages/URL)

closes #KNO-182

Change-Id: Ib4d561997084d7aa167f3a30cf30815f7221b8dc
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/221660
Reviewed-by: Carl Kibler <ckibler@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Carl Kibler <ckibler@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>

app/helpers/broken_link_helper.rb

@@ -19,8 +19,10 @@ module BrokenLinkHelper
   def send_broken_content!
     # call when processing a 4xx error.
     # this will examine the referrer to see if we got here because of a bad link
-    return false unless request.referer
-    record = Context.find_asset_by_url(request.referer)
+    from_url = request.referer
+    return false unless from_url
+    record = Context.find_asset_by_url(from_url)
+    record ||= Context.get_front_wiki_page_for_course_from_url(from_url)
     return false unless record
     body = Nokogiri::HTML(Context.asset_body(record))
     anchor = body.css("a[href$='#{request.fullpath}']").text

app/models/context.rb

@@ -238,6 +238,18 @@ module Context
     nil
   end
 
+  def self.get_front_wiki_page_for_course_from_url(url)
+    params = Rails.application.routes.recognize_path(url)
+    if params[:controller] == "courses" && params[:action] == "show"
+      course = Course.find(params[:id])
+      if course.default_view == "wiki"
+        course.wiki.front_page
+      end
+    end
+  rescue
+    nil
+  end
+
   def self.find_asset_by_url(url)
     object = nil
     params = Rails.application.routes.recognize_path(url)
@@ -245,6 +257,7 @@ module Context
     group = Group.find(params[:group_id]) if params[:group_id]
     user = User.find(params[:user_id]) if params[:user_id]
     context = course || group || user
+
     return nil unless context
     case params[:controller]
     when 'files'

spec/helpers/broken_link_helper_spec.rb

@@ -85,6 +85,15 @@ describe BrokenLinkHelper, type: :controller do
     expect(send_broken_content!).to be true
   end
 
+  it 'should work with wiki pages set to the front page' do
+    wiki_page_model(context: @course, body: "<a href='/test_error'>bad link</a>")
+    @page.set_as_front_page!
+    @course.update_attribute(:default_view, 'wiki')
+    allow(request).to receive(:referer).and_return "/courses/#{@course.id}"
+    allow(request).to receive(:path).and_return "/test_error"
+    expect(send_broken_content!).to be true
+  end
+
   it 'should return true for unpublished content' do
     linked_assignment = @assignment
     assignment_model(course: @course).update_attributes(workflow_state: 'unpublished')