Use specialized services for faster updates

When we're only updating a single attribute of a merge request, it can
be MUCH faster to use a specialized update service for that specific
attribute (or relationship) than the "update the world" approach of
MergeRequests::UpdateService#execute does by default.

app/services/merge_requests/update_service.rb

@@ -11,6 +11,10 @@ module MergeRequests
     end
 
     def execute(merge_request)
+      updated_merge_request = update_merge_request_with_specialized_service(merge_request)
+
+      return updated_merge_request unless updated_merge_request.nil?
+
       # We don't allow change of source/target projects and source branch
       # after merge request was created
       params.delete(:source_project_id)
@@ -268,6 +272,32 @@ module MergeRequests
     def quick_action_options
       { merge_request_diff_head_sha: params.delete(:merge_request_diff_head_sha) }
     end
+
+    def update_merge_request_with_specialized_service(merge_request)
+      return unless params.delete(:use_specialized_service)
+
+      # If we're attempting to modify only a single attribute, look up whether
+      #   we have a specialized, targeted service we should use instead. We may
+      #   in the future extend this to include specialized services that operate
+      #   on multiple attributes, but for now limit to only single attribute
+      #   updates.
+      #
+      return unless params.keys.length == 1
+
+      updated_merge_request = nil
+
+      case params.keys[0].to_sym
+      when :assignee_ids
+        updated_merge_request = assignees_service.execute(merge_request)
+      end
+
+      updated_merge_request
+    end
+
+    def assignees_service
+      @assignees_service ||= ::MergeRequests::UpdateAssigneesService
+        .new(project, current_user, params)
+    end
   end
 end
 

changelogs/unreleased/kerrizor-use-specialized-service-for-assignee-updates.yml

+---
+title: Add framework for using specialized services to improve performance of MergeRequests::UpdateService
+merge_request: 58836
+author:
+type: performance

ee/app/services/ee/merge_requests/update_service.rb

@@ -7,6 +7,10 @@ module EE
 
       override :execute
       def execute(merge_request)
+        updated_merge_request = update_merge_request_with_specialized_service(merge_request)
+
+        return updated_merge_request unless updated_merge_request.nil?
+
         unless update_task_event?
           old_approvers = merge_request.overall_approvers(exclude_code_owners: true)
         end

spec/services/merge_requests/update_service_spec.rb

@@ -943,6 +943,18 @@ RSpec.describe MergeRequests::UpdateService, :mailer do
     end
 
     context 'updating asssignee_ids' do
+      context 'when :use_specialized_service is true' do
+        it 'passes the update action to ::MergeRequests::UpdateAssigneesService' do
+          expect(::MergeRequests::UpdateAssigneesService)
+            .to receive(:new).and_call_original
+
+          update_merge_request({
+            assignee_ids: [user2.id],
+            use_specialized_service: true
+          })
+        end
+      end
+
       it 'does not update assignee when assignee_id is invalid' do
         merge_request.update!(assignee_ids: [user.id])