Add $Canvas.assignment.description to LTI launch params

Add support for LTI tools to consume the assignment description
set in canvas.

Test Plan:
 - Specs pass

flag = none

Change-Id: I339d80f91fbe2bf2e9c855f227b9ee015f1bb835
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/275548
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Weston Dransfield <wdransfield@instructure.com>
Reviewed-by: Xander Moffatt <xmoffatt@instructure.com>
QA-Review: Alex Slaughter <aslaughter@instructure.com>
Product-Review: Alex Slaughter <aslaughter@instructure.com>

app/models/assignment.rb

@@ -413,6 +413,7 @@ class Assignment < ActiveRecord::Base
   def secure_params
     body = {}
     body[:lti_assignment_id] = self.lti_context_id || SecureRandom.uuid
+    body[:lti_assignment_description] = self.description
     Canvas::Security.create_jwt(body)
   end
 

doc/api/tools_variable_substitutions.md

@@ -245,6 +245,15 @@ the `ext_lti_assignment_id` send in various launches and webhooks.
 ```
 "9ae4170c-6b64-444d-9246-0b7dedd5f560"
 ```
+## com.instructure.Assignment.description
+The LTI assignment description of an assignment.
+
+**Availability**: *always*  
+**Launch Parameter**: *com_instructure_assignment_description*  
+
+```
+"Example Description"
+```
 ## com.instructure.Assignment.allowedFileExtensions
 A comma separated list of the file extensions that are allowed for submitting to this
 assignment. If there are no limits on what files can be uploaded, an empty string will be
@@ -1077,6 +1086,15 @@ Returns the assignment_id of the assignment that was launched.
 ```
 1234
 ```
+## Canvas.assignment.description
+Returns the assignment_description of the assignment that was launched.
+
+**Availability**: *when launched as an assignment*  
+
+
+```
+"Example Description"
+```
 ## com.instructure.Group.id
 Returns the Canvas id of the group the current user is in if launching
 from a group assignment.

lib/canvas/live_events.rb

@@ -237,6 +237,7 @@ module Canvas::LiveEvents
       updated_at: assignment.updated_at,
       points_possible: assignment.points_possible,
       lti_assignment_id: assignment.lti_context_id,
+      lti_assignment_description: assignment.description,
       lti_resource_link_id: assignment.lti_resource_link_id,
       lti_resource_link_id_duplicated_from: assignment.duplicate_of&.lti_resource_link_id,
       submission_types: assignment.submission_types

lib/lti/security.rb

@@ -130,7 +130,16 @@ module Lti
       secure_params = Canvas::Security.decode_jwt(secure_params)
       secure_params[:lti_assignment_id]
     rescue Canvas::Security::InvalidToken
-      return nil
+      nil
+    end
+
+    def self.decoded_lti_assignment_description(secure_params)
+      return if secure_params.blank?
+
+      secure_params = Canvas::Security.decode_jwt(secure_params)
+      secure_params[:lti_assignment_description]
+    rescue Canvas::Security::InvalidToken
+      nil
     end
   end
 end

lib/lti/variable_expander.rb

@@ -97,6 +97,7 @@ module Lti
     ORIGINALITY_REPORT_GUARD = -> { @originality_report.present? }
     ORIGINALITY_REPORT_ATTACHMENT_GUARD = -> { @originality_report&.attachment.present? }
     LTI_ASSIGN_ID = -> { @assignment.present? || @originality_report.present? || @secure_params.present? }
+    LTI_ASSIGN_DESCRIPTION = -> { @assignment.present? || @originality_report.present? || @secure_params.present? }
     EDITOR_GUARD = -> { @editor_contents.present? }
     STUDENT_ASSIGNMENT_GUARD = -> { @context.is_a?(Course) && @context.user_is_student?(@current_user) && @assignment }
 
@@ -265,6 +266,25 @@ module Lti
                        LTI_ASSIGN_ID,
                        default_name: 'com_instructure_assignment_lti_id'
 
+    # The LTI assignment description of an assignment.
+    # @launch_parameter com_instructure_assignment_lti_description
+    # @example
+    #   ```
+    #   "Example Description"
+    #   ```
+    register_expansion 'com.instructure.Assignment.description', [],
+                       -> do
+                         if @assignment
+                           @assignment.description
+                         elsif @originality_report
+                           @originality_report.submission.assignment.description
+                         elsif @secure_params.present?
+                           Lti::Security.decoded_lti_assignment_description(@secure_params)
+                         end
+                       end,
+                       LTI_ASSIGN_DESCRIPTION,
+                       default_name: 'com_instructure_assignment_description'
+
     # A comma separated list of the file extensions that are allowed for submitting to this
     # assignment. If there are no limits on what files can be uploaded, an empty string will be
     # returned. If the assignment does not allow file uploads as a submission type, then no
@@ -1196,6 +1216,16 @@ module Lti
                        -> { @assignment.id },
                        ASSIGNMENT_GUARD
 
+    # Returns the assignment_description of the assignment that was launched.
+    #
+    # @example
+    #   ```
+    #   "Example Description"
+    #   ```
+    register_expansion 'Canvas.assignment.description', [],
+                       -> { @assignment.description },
+                       ASSIGNMENT_GUARD
+
     # Returns the Canvas id of the group the current user is in if launching
     # from a group assignment.
     #

spec/lib/lti/capabilities_helper_spec.rb

@@ -101,6 +101,7 @@ module Lti
          vnd.Canvas.submission.url
          Context.title
          com.instructure.Assignment.lti.id
+         com.instructure.Assignment.description
          com.instructure.Assignment.allowedFileExtensions
          com.instructure.Person.name_sortable
          com.instructure.PostMessageToken

spec/lib/lti/security_spec.rb

@@ -81,6 +81,19 @@ describe Lti::Security do
       end
     end
 
+    context '#decoded_lti_assignment_description' do
+      it 'returns nil if secure params are invalid' do
+        expect(Lti::Security.decoded_lti_assignment_description('banana')).to be_nil
+      end
+
+      it 'returns the lti assignment description if secure params are valid' do
+        assignment_id = 12
+        body = { lti_assignment_description: assignment_id }
+        secure_params = Canvas::Security.create_jwt(body).to_s
+        expect(Lti::Security.decoded_lti_assignment_description(secure_params)).to eq assignment_id
+      end
+    end
+
     context '.check_and_store_nonce' do
       it 'rejects a used nonce' do
         enable_cache do

spec/lib/lti/variable_expander_spec.rb

@@ -1414,6 +1414,13 @@ module Lti
           expect(exp_hash[:test]).to eq 2015
         end
 
+        it 'has substitution for $Canvas.assignment.description' do
+          allow(assignment).to receive(:description).and_return('desc')
+          exp_hash = { test: '$Canvas.assignment.description' }
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq 'desc'
+        end
+
         it 'has substitution for $Canvas.assignment.title' do
           assignment.title = 'Buy as many ducks as you can'
           exp_hash = { test: '$Canvas.assignment.title' }