Notify when auto merge is enabled

There are bunch of auto merge related notifications, and
when auto merge is enabled, it should be evaluated immediately.

app/services/auto_merge/base_service.rb

@@ -14,6 +14,9 @@ module AutoMerge
 
       yield if block_given?
 
+      # Notify the event that auto merge is enabled or merge param is updated
+      AutoMergeProcessWorker.perform_async(merge_request.id)
+
       strategy.to_sym
     end
 

spec/services/auto_merge/base_service_spec.rb

@@ -12,6 +12,10 @@ describe AutoMerge::BaseService do
   describe '#execute' do
     subject { service.execute(merge_request) }
 
+    before do
+      allow(AutoMergeProcessWorker).to receive(:perform_async) {}
+    end
+
     it 'sets properies to the merge request' do
       subject
 
@@ -65,6 +69,12 @@ describe AutoMerge::BaseService do
       it 'returns activated strategy name' do
         is_expected.to eq(AutoMergeService::STRATEGY_MERGE_WHEN_PIPELINE_SUCCEEDS.to_sym)
       end
+
+      it 'calls AutoMergeProcessWorker' do
+        expect(AutoMergeProcessWorker).to receive(:perform_async).with(merge_request.id).once
+
+        subject
+      end
     end
 
     context 'when failed to save' do