Fixes draft note race condition with whitespace changes

Fixes a race condition when draft notes exist on a line in a whitespace
diff but would end up getting an incorrect `line_code` set.
This is fixed by saving the `line_code` property into the database
to better match what the notes table does.

Changelog: fixed

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/340815

app/controllers/projects/merge_requests/drafts_controller.rb

@@ -92,7 +92,8 @@ class Projects::MergeRequests::DraftsController < Projects::MergeRequests::Appli
       :commit_id,
       :note,
       :position,
-      :resolve_discussion
+      :resolve_discussion,
+      :line_code
     ).tap do |h|
       # Old FE version will still be sending `draft_note[commit_id]` as 'undefined'.
       # That can result to having a note linked to a commit with 'undefined' ID

app/models/draft_note.rb

@@ -25,6 +25,7 @@ class DraftNote < ApplicationRecord
   validates :merge_request_id, presence: true
   validates :author_id, presence: true, uniqueness: { scope: [:merge_request_id, :discussion_id] }, if: :discussion_id?
   validates :discussion_id, allow_nil: true, format: { with: /\A\h{40}\z/ }
+  validates :line_code, length: { maximum: 255 }, allow_nil: true
 
   scope :authored_by, ->(u) { where(author_id: u.id) }
 
@@ -89,7 +90,11 @@ class DraftNote < ApplicationRecord
   end
 
   def line_code
-    @line_code ||= diff_file&.line_code_for_position(original_position)
+    super.presence || find_line_code
+  end
+
+  def find_line_code
+    write_attribute(:line_code, diff_file&.line_code_for_position(original_position))
   end
 
   def publish_params

db/migrate/20211122103051_add_line_code_to_draft_notes.rb

+# frozen_string_literal: true
+
+class AddLineCodeToDraftNotes < Gitlab::Database::Migration[1.0]
+  # rubocop:disable Migration/AddLimitToTextColumns
+  # limit is added in db/migrate/20211124095704_add_draft_notes_line_code_text_limit.rb
+  def change
+    add_column :draft_notes, :line_code, :text
+  end
+  # rubocop:enable Migration/AddLimitToTextColumns
+end

db/migrate/20211124095704_add_draft_notes_line_code_text_limit.rb

+# frozen_string_literal: true
+
+class AddDraftNotesLineCodeTextLimit < Gitlab::Database::Migration[1.0]
+  disable_ddl_transaction!
+
+  def up
+    add_text_limit :draft_notes, :line_code, 255
+  end
+
+  def down
+    remove_text_limit :draft_notes, :line_code
+  end
+end

db/schema_migrations/20211122103051

+674a44e70291d6ed04318a5f6b639d216f2c26c43d15cb00e59b06cc6f6cc401
\ No newline at end of file

db/schema_migrations/20211124095704

+1f5ed9e7af3f56d0e11d1a2bb78a7430ce05af49c8102d1c75c8ff84ae4e1c6d
\ No newline at end of file

db/structure.sql

@@ -13687,7 +13687,9 @@ CREATE TABLE draft_notes (
     "position" text,
     original_position text,
     change_position text,
-    commit_id bytea
+    commit_id bytea,
+    line_code text,
+    CONSTRAINT check_c497a94a0e CHECK ((char_length(line_code) <= 255))
 );
 
 CREATE SEQUENCE draft_notes_id_seq

spec/models/draft_note_spec.rb

@@ -20,6 +20,28 @@ RSpec.describe DraftNote do
     it { is_expected.to delegate_method(:file_identifier_hash).to(:diff_file).allow_nil }
   end
 
+  describe '#line_code' do
+    describe 'stored line_code' do
+      let(:draft_note) { build(:draft_note, merge_request: merge_request, line_code: '1234567890') }
+
+      it 'returns stored line_code' do
+        expect(draft_note.line_code).to eq('1234567890')
+      end
+    end
+
+    describe 'none stored line_code' do
+      let(:draft_note) { build(:draft_note, merge_request: merge_request) }
+
+      before do
+        allow(draft_note).to receive(:find_line_code).and_return('none stored line_code')
+      end
+
+      it 'returns found line_code' do
+        expect(draft_note.line_code).to eq('none stored line_code')
+      end
+    end
+  end
+
   describe '#diff_file' do
     let(:draft_note) { build(:draft_note, merge_request: merge_request) }
 

spec/services/draft_notes/create_service_spec.rb

@@ -92,7 +92,7 @@ RSpec.describe DraftNotes::CreateService do
 
         expect(merge_request).to receive_message_chain(:diffs, :clear_cache)
 
-        create_draft(note: 'This is a test')
+        create_draft(note: 'This is a test', line_code: '123')
       end
     end
 
@@ -104,7 +104,7 @@ RSpec.describe DraftNotes::CreateService do
 
         expect(merge_request).not_to receive(:diffs)
 
-        create_draft(note: 'This is a test')
+        create_draft(note: 'This is a test', line_code: '123')
       end
     end
   end