gracefully handle missing sourcedid for grade passback

fixes PLAT-1356

test plan:
send a grade passback with no sourcedid
canvas shouldn't return a 500

Change-Id: I21436d679e05a5fa837167b163212a729c10a3a8
Reviewed-on: https://gerrit.instructure.com/72998
Tested-by: Jenkins
QA-Review: August Thornton <august@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>

lib/basic_lti/basic_outcomes.rb

@@ -40,6 +40,7 @@ module BasicLTI
 
     def self.decode_source_id(tool, sourceid)
       tool.shard.activate do
+        raise InvalidSourceId, 'Invalid sourcedid' if sourceid.blank?
         md = sourceid.match(SOURCE_ID_REGEX)
         raise InvalidSourceId, 'Invalid sourcedid' unless md
         new_encoding = [md[1], md[2], md[3], md[4]].join('-')
@@ -159,13 +160,15 @@ module BasicLTI
       end
 
       def handle_request(tool)
+        #check if we recognize the xml structure
+        return false unless operation_ref_identifier
         # verify the lis_result_sourcedid param, which will be a canvas-signed
         # tuple of (course, assignment, user) to ensure that only this launch of
         # the tool is attempting to modify this data.
         source_id = self.sourcedid
 
         begin
-          course, assignment, user = BasicLTI::BasicOutcomes.decode_source_id(tool, source_id) if source_id
+          course, assignment, user = BasicLTI::BasicOutcomes.decode_source_id(tool, source_id)
         rescue InvalidSourceId => e
           self.code_major = 'failure'
           self.description = e.to_s

spec/lib/basic_lti/basic_outcomes_spec.rb

@@ -114,6 +114,16 @@ describe BasicLTI::BasicOutcomes do
       expect(described_class.decode_source_id(tool, source_id)).to eq [@course, assignment, @user]
     end
 
+    it 'throws Invalid sourcedid if sourcedid is nil' do
+      expect{described_class.decode_source_id(tool, nil)}.
+          to raise_error(BasicLTI::BasicOutcomes::InvalidSourceId, 'Invalid sourcedid')
+    end
+
+    it 'throws Invalid sourcedid if sourcedid is empty' do
+      expect{described_class.decode_source_id(tool, "")}.
+          to raise_error(BasicLTI::BasicOutcomes::InvalidSourceId, 'Invalid sourcedid')
+    end
+
     it 'throws Invalid sourcedid if no signature' do
       missing_signature = source_id.split('-')[0..3].join('-')
       expect{described_class.decode_source_id(tool, missing_signature)}.