Fix truncated_diff_lines for legacy notes without a line42160-error-500-loading-merge-request-undefined-method-index-for-nil-nilclass

It appears that some legacy diff notes on GitLab.com have invalid line codes,
which means truncated_diff_lines won't work. This is just a band-aid solution,
as neither the diff nor the discussion will display correctly for these notes,
but they won't cause a 500 error.

1 files changed, 10 insertions, 0 deletions

diff --git a/spec/models/concerns/discussion_on_diff_spec.rb b/spec/models/concerns/discussion_on_diff_spec.rb
index 2322eb206fb..30572ce9332 100644
--- a/spec/models/concerns/discussion_on_diff_spec.rb
+++ b/spec/models/concerns/discussion_on_diff_spec.rb
@@ -20,6 +20,16 @@ describe DiscussionOnDiff do
         expect(truncated_lines).not_to include(be_meta)
       end
     end
+
+    context "when the diff line does not exist on a legacy diff note" do
+      it "returns an empty array" do
+        legacy_note = LegacyDiffNote.new
+
+        allow(subject).to receive(:first_note).and_return(legacy_note)
+
+        expect(truncated_lines).to eq([])
+      end
+    end
   end
 
   describe '#line_code_in_diffs' do