Skip st_diff setting on LegacyDiffNote during import

- not needed as the import files will have this information

Changelog: fixed

app/models/legacy_diff_note.rb

@@ -13,7 +13,7 @@ class LegacyDiffNote < Note
 
   validates :line_code, presence: true, line_code: true
 
-  before_create :set_diff
+  before_create :set_diff, unless: :skip_setting_st_diff?
 
   def discussion_class(*)
     LegacyDiffDiscussion
@@ -90,6 +90,10 @@ class LegacyDiffNote < Note
     self.st_diff = diff.to_hash if diff
   end
 
+  def skip_setting_st_diff?
+    st_diff.present? && importing? && Feature.enabled?(:skip_legacy_diff_note_callback_on_import, default_enabled: :yaml)
+  end
+
   def diff_for_line_code
     attributes = {
       noteable_type: noteable_type,

config/feature_flags/development/skip_legacy_diff_note_callback_on_import.yml

+---
+name: skip_legacy_diff_note_callback_on_import
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/72897
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/343666
+milestone: '14.5'
+type: development
+group: group::import
+default_enabled: false

spec/models/legacy_diff_note_spec.rb

@@ -8,4 +8,58 @@ RSpec.describe LegacyDiffNote do
 
     it { is_expected.to eq('note') }
   end
+
+  describe 'callbacks' do
+    describe '#set_diff' do
+      let(:note) do
+        build(:legacy_diff_note_on_merge_request, st_diff: '_st_diff_').tap do |record|
+          record.instance_variable_set(:@diff, {})
+        end
+      end
+
+      context 'when not importing' do
+        it 'updates st_diff' do
+          note.save!(validate: false)
+
+          expect(note.st_diff).to eq({})
+        end
+      end
+
+      context 'when importing' do
+        before do
+          note.importing = true
+        end
+
+        it 'does not update st_diff' do
+          note.save!(validate: false)
+
+          expect(note.st_diff).to eq('_st_diff_')
+        end
+
+        context 'when feature flag is false' do
+          before do
+            stub_feature_flags(skip_legacy_diff_note_callback_on_import: false)
+          end
+
+          it 'updates st_diff' do
+            note.save!(validate: false)
+
+            expect(note.st_diff).to eq({})
+          end
+        end
+
+        context 'when st_diff is blank' do
+          before do
+            note.st_diff = nil
+          end
+
+          it 'updates st_diff' do
+            note.save!(validate: false)
+
+            expect(note.st_diff).to eq({})
+          end
+        end
+      end
+    end
+  end
 end