Avoid creating diff position when line-code is nil

app/services/discussions/capture_diff_note_position_service.rb

@@ -19,13 +19,16 @@ module Discussions
       position = result[:position]
       return unless position
 
+      line_code = position.line_code(project.repository)
+      return unless line_code
+
       # Currently position data is copied across all notes of a discussion
       # It makes sense to store a position only for the first note instead
       # Within the newly introduced table we can start doing just that
       DiffNotePosition.create_or_update_for(discussion.notes.first,
         diff_type: :head,
         position: position,
-        line_code: position.line_code(project.repository))
+        line_code: line_code)
     end
 
     private

changelogs/unreleased/id-fix-nil-line-codes-for-diff-positions.yml

+---
+title: Avoid creating diff position when line-code is nil
+merge_request: 40089
+author:
+type: fixed

spec/services/discussions/capture_diff_note_position_service_spec.rb

@@ -29,18 +29,33 @@ RSpec.describe Discussions::CaptureDiffNotePositionService do
     end
   end
 
-  context 'when position tracer returned nil position' do
+  context 'when position tracer returned position' do
     let!(:note) { create(:diff_note_on_merge_request) }
     let(:paths) { ['files/any_file.txt'] }
 
-    it 'does not create diff note position' do
+    before do
       expect(note.noteable).to receive(:merge_ref_head).and_return(double.as_null_object)
       expect_next_instance_of(Gitlab::Diff::PositionTracer) do |tracer|
-        expect(tracer).to receive(:trace).and_return({ position: nil })
+        expect(tracer).to receive(:trace).and_return({ position: position })
+      end
+    end
+
+    context 'which is nil' do
+      let(:position) { nil }
+
+      it 'does not create diff note position' do
+        expect(subject.execute(note.discussion)).to eq(nil)
+        expect(note.diff_note_positions).to be_empty
+      end
     end
 
+    context 'which does not have a corresponding line' do
+      let(:position) { double(line_code: nil) }
+
+      it 'does not create diff note position' do
         expect(subject.execute(note.discussion)).to eq(nil)
         expect(note.diff_note_positions).to be_empty
       end
     end
+  end
 end