Track when MR merged before code approval rules were synced

Log an error when an MR was merged before the code owner approval
rules could be synced.

ee/app/services/merge_requests/sync_code_owner_approval_rules.rb

@@ -2,13 +2,15 @@
 
 module MergeRequests
   class SyncCodeOwnerApprovalRules
+    AlreadyMergedError = Class.new(StandardError)
+
     def initialize(merge_request, params = {})
       @merge_request = merge_request
       @previous_diff = params[:previous_diff]
     end
 
     def execute
-      return if merge_request.merged?
+      return already_merged if merge_request.merged?
 
       delete_outdated_code_owner_rules
 
@@ -45,5 +47,15 @@ module MergeRequests
       @code_owner_entries ||= Gitlab::CodeOwners
                                 .entries_for_merge_request(merge_request, merge_request_diff: previous_diff)
     end
+
+    def already_merged
+      Gitlab::ErrorTracking.track_exception(
+        AlreadyMergedError.new('MR already merged before code owner approval rules were synced'),
+        merge_request_id: merge_request.id,
+        merge_request_iid: merge_request.iid,
+        project_id: merge_request.project_id
+      )
+      nil
+    end
   end
 end

ee/spec/services/merge_requests/sync_code_owner_approval_rules_spec.rb

@@ -3,11 +3,12 @@
 require 'spec_helper'
 
 RSpec.describe MergeRequests::SyncCodeOwnerApprovalRules do
-  let(:merge_request) { create(:merge_request) }
-  let(:rb_owners) { create_list(:user, 2) }
-  let(:doc_owners) { create_list(:user, 2) }
-  let(:rb_group_owners) { create_list(:group, 2) }
-  let(:doc_group_owners) { create_list(:group, 2) }
+  let_it_be(:merge_request) { create(:merge_request) }
+  let_it_be(:rb_owners) { create_list(:user, 2) }
+  let_it_be(:doc_owners) { create_list(:user, 2) }
+  let_it_be(:rb_group_owners) { create_list(:group, 2) }
+  let_it_be(:doc_group_owners) { create_list(:group, 2) }
+
   let(:rb_entry) { build_entry('*.rb', rb_owners, rb_group_owners) }
   let(:doc_entry) { build_entry('doc/*', doc_owners, doc_group_owners) }
   let(:entries) { [rb_entry, doc_entry] }
@@ -64,5 +65,24 @@ RSpec.describe MergeRequests::SyncCodeOwnerApprovalRules do
       expect(other_rule.reload.users).to eq(rb_owners)
       expect(other_rule.reload.groups).to match_array(rb_group_owners)
     end
+
+    context 'when merge request is already merged' do
+      let(:merge_request) { build(:merge_request, :merged) }
+
+      it 'logs an error' do
+        expect(Gitlab::ErrorTracking)
+          .to receive(:track_exception)
+          .with(
+            instance_of(described_class::AlreadyMergedError),
+            hash_including(
+              merge_request_id: merge_request.id,
+              merge_request_iid: merge_request.iid,
+              project_id: merge_request.project_id
+            )
+          ).and_call_original
+
+        expect(service.execute).to eq(nil)
+      end
+    end
   end
 end