Add Allowed File Extensions Variable Expansion

Added the `com.instructure.Assignment.allowedFileExtensions` variable
extensions. If an assignment is present in the context of the current
LTI placement/launch and has online_upload present as a submission_type,
this new expansion variable will provide a comma separated list of
allowed file extensions that can be used for submitting to the
assignment.

closes INTEROP-6909

flag = none

test-plan:
* Install the LTI 1.3 Test Tool with at least the homework submission
  placement. Make sure that you give set it up to use the new
  custom variable when you're configuring things.
* Create an assignment with a file upload submission type. Limit the
  file types to whatever you want. Note that this must be a comma
  separated list.
* Masquerade as a student and try to submit to the assignment through
  the tool. Ensure that the in the
  "https://purl.imsglobal.org/spec/lti/claim/custom" section, you can
  see that the variable was properly substituted with the list of valid
  file types.
* Create another assignment that also uses file upload, but don't limit
  the file types.
* Go to submit as a student and make sure that in the
  "https://purl.imsglobal.org/spec/lti/claim/custom" section, the
  variable was expanded to be an empty string, indicating no limits were
  imposed on file upload types.

Change-Id: I4315ce1cfa98b034446cdc5bfc3f61d133412a5c
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/270094
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Evan Battaglia <ebattaglia@instructure.com>
QA-Review: Xander Moffatt <xmoffatt@instructure.com>
Product-Review: Karl Lloyd <karl@instructure.com>

doc/api/tools_variable_substitutions.md

@@ -245,6 +245,18 @@ the `ext_lti_assignment_id` send in various launches and webhooks.
 ```
 "9ae4170c-6b64-444d-9246-0b7dedd5f560"
 ```
+## 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
+returned. If the assignment does not allow file uploads as a submission type, then no
+substitution will be performed.
+
+**Availability**: *when the tool is used to upload a file as an assignment submission*  
+**Launch Parameter**: *com_instructure_assignment_allowed_file_extensions*  
+
+```
+"docx,pdf,txt"
+```
 ## com.instructure.OriginalityReport.id
 The Canvas id of the Originality Report associated
 with the launch.

doc/yard_plugins/lti_variable_expansion_plugin.rb

@@ -128,6 +128,8 @@ class RegisterExpansionHandler < YARD::Handlers::Ruby::Base
         "when launched as an assignment by a student"
       when 'EDITOR_GUARD'
         "when the tool is launched from the editor_button placement"
+      when 'FILE_UPLOAD_GUARD'
+        "when the tool is used to upload a file as an assignment submission"
       end
 
     end.compact

lib/lti/variable_expander.rb

@@ -84,6 +84,7 @@ module Lti
     ROLES_GUARD = -> { @current_user && (@context.is_a?(Course) || @context.is_a?(Account)) }
     CONTENT_TAG_GUARD = -> { @content_tag }
     ASSIGNMENT_GUARD = -> { @assignment }
+    FILE_UPLOAD_GUARD = -> { !!@assignment && @assignment.submission_types.split(',').include?('online_upload') }
     COLLABORATION_GUARD = -> { @collaboration }
     MEDIA_OBJECT_GUARD = -> { @attachment && @attachment.media_object}
     USAGE_RIGHTS_GUARD = -> { @attachment && @attachment.usage_rights}
@@ -263,6 +264,21 @@ module Lti
                        LTI_ASSIGN_ID,
                        default_name: 'com_instructure_assignment_lti_id'
 
+    # 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
+    # substitution will be performed.
+    #
+    # @launch_parameter com_instructure_originality_report_id
+    # @example
+    #   ```
+    #   "docx,pdf,txt"
+    #   ```
+    register_expansion 'com.instructure.Assignment.allowedFileExtensions', [],
+                       -> { @assignment.allowed_extensions.join(',') },
+                       FILE_UPLOAD_GUARD,
+                       default_name: 'com_instructure_assignment_allowed_file_extensions'
+
     # The Canvas id of the Originality Report associated
     # with the launch.
     # @launch_parameter com_instructure_originality_report_id

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.allowedFileExtensions
          com.instructure.Person.name_sortable
          com.instructure.PostMessageToken
          com.instructure.Editor.contents

spec/lib/lti/variable_expander_spec.rb

@@ -104,12 +104,17 @@ module Lti
         root_account,
         account,
         controller,
+        variable_expander_opts
+      )
+    end
+    let(:variable_expander_opts) do
+      {
         current_user: user,
         tool: tool,
         originality_report: originality_report,
         editor_contents: editor_contents,
         editor_selection: editor_selection
-      )
+      }
     end
 
     describe ".deregister_expansion" do
@@ -489,6 +494,46 @@ module Lti
         expect(exp_hash[:test]).to eq 'api/lti/assignments/{assignment_id}/submissions/{submission_id}/originality_report'
       end
 
+      context 'com.instructure.Assignment.allowedFileExtensions' do
+        let(:exp_hash) do
+          {
+            test: expansion
+          }
+        end
+        let(:allowed_extensions) { 'docx,txt,pdf' }
+        let(:course) { course_model }
+        let(:assignment) { assignment_model(context: course) }
+        let(:expansion) { '$com.instructure.Assignment.allowedFileExtensions'}
+        let(:variable_expander_opts) { super().merge(context: course, assignment: assignment) }
+
+        it 'it expands when an assignment with online_upload submission type and extensions is present' do
+          assignment.update!(allowed_extensions: allowed_extensions, submission_types: 'online_upload')
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq allowed_extensions
+        end
+
+        it "doesn't expand if online_uploads is not a submission_type" do
+          assignment.update!(submission_types: 'online_text_entry,online_url')
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq "$com.instructure.Assignment.allowedFileExtensions"
+        end
+
+        it 'expands to an empty string if there are no limits on file types' do
+          assignment.update!(submission_types: 'online_upload,online_text_entry')
+          variable_expander.expand_variables!(exp_hash)
+          expect(exp_hash[:test]).to eq ""
+        end
+
+        context 'no assignment present' do
+          let(:variable_expander_opts) { super().merge(context: course) }
+
+          it "doesn't expand" do
+            variable_expander.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq expansion
+          end
+        end
+      end
+
       it 'has substitution for com.instructure.OriginalityReport.id' do
         exp_hash = {test: '$com.instructure.OriginalityReport.id'}
         variable_expander.expand_variables!(exp_hash)