Fix cluster reindexing preflight check

EE: true
Changelog: fixed

ee/app/services/elastic/cluster_reindexing_service.rb

@@ -61,7 +61,13 @@ def initial!
         return false
       end
 
-      expected_free_size = alias_names.sum { |name| elastic_helper.index_size_bytes(index_name: name) } * 2
+      target_classes = current_task.target_classes
+      current_size = target_classes.sum do |klass|
+        name = elastic_helper.klass_to_alias_name(klass: klass)
+        elastic_helper.index_size_bytes(index_name: name)
+      end
+
+      expected_free_size = current_size * 2
       if elastic_helper.cluster_free_size_bytes < expected_free_size
         abort_reindexing!("You should have at least #{expected_free_size} bytes of storage available to perform reindexing. Please increase the storage in your Elasticsearch cluster before reindexing.")
         return false

ee/spec/services/elastic/cluster_reindexing_service_spec.rb

@@ -43,6 +43,18 @@
 
       expect(Gitlab::CurrentSettings.elasticsearch_pause_indexing).to eq(true)
     end
+
+    context 'when partial reindexing' do
+      let(:task) { create(:elastic_reindexing_task, state: :initial, targets: %w[Project User]) }
+
+      it 'errors when there is not enough space' do
+        allow(helper).to receive(:index_size_bytes).twice.and_return(10.megabytes)
+        allow(helper).to receive(:cluster_free_size_bytes).and_return(30.megabytes)
+
+        expect { cluster_reindexing_service.execute }.to change { task.reload.state }.from('initial').to('failure')
+        expect(task.reload.error_message).to match(/storage available/)
+      end
+    end
   end
 
   context 'state: indexing_paused' do