Fix CI mirror table sync race condition

After consuming sync events, we need to remove those.
Before this commit, we used to use first id and last id range of events.
However, this caused a race condition. If an event is on-the-fly
transaction, and before and after events are tired to be consumed,
that event gets lost.

app/services/ci/process_sync_events_service.rb

@@ -28,18 +28,16 @@ module Ci
 
       return if events.empty?
 
-      first = events.first
-      last_processed = nil
+      processed_events = []
 
       begin
         events.each do |event|
           @sync_class.sync!(event)
 
-          last_processed = event
+          processed_events << event
         end
       ensure
-        # remove events till the one that was last succesfully processed
-        @sync_event_class.id_in(first.id..last_processed.id).delete_all if last_processed
+        @sync_event_class.id_in(processed_events).delete_all
       end
     end
 

spec/services/ci/process_sync_events_service_spec.rb

@@ -71,6 +71,24 @@ RSpec.describe Ci::ProcessSyncEventsService do
           expect { execute }.not_to change(Projects::SyncEvent, :count)
         end
       end
+
+      it 'does not delete non-executed events' do
+        new_project = create(:project)
+        sync_event_class.delete_all
+
+        project1.update!(group: parent_group_2)
+        new_project.update!(group: parent_group_1)
+        project2.update!(group: parent_group_1)
+
+        new_project_sync_event = new_project.sync_events.last
+
+        allow(sync_event_class).to receive(:preload_synced_relation).and_return(
+          sync_event_class.where.not(id: new_project_sync_event)
+        )
+
+        expect { execute }.to change(Projects::SyncEvent, :count).from(3).to(1)
+        expect(new_project_sync_event.reload).to be_persisted
+      end
     end
 
     context 'for Namespaces::SyncEvent' do