Selective merge train concurrency

This commits allow to select merge train concurrency via
feature flag. The new parallel factor is disabled by default.

ee/app/services/merge_trains/refresh_merge_requests_service.rb

@@ -4,8 +4,10 @@ module MergeTrains
     include ::Gitlab::ExclusiveLeaseHelpers
     include ::Gitlab::Utils::StrongMemoize
 
-    DEFAULT_MAX_CONCURRENCY = 4
-    ONE_AT_A_TIME_STRATEGY = 1
+    HIGH_CONCURRENCY = 20
+    MEDIUM_CONCURRENCY = 10
+    LOW_CONCURRENCY = 4
+    NO_CONCURRENCY = 1
 
     ##
     # merge_request ... A merge request pointer in a merge train.
@@ -68,9 +70,15 @@ module MergeTrains
     def max_concurrency
       strong_memoize(:max_concurrency) do
         if Feature.enabled?(:merge_trains_parallel_pipelines, project, default_enabled: true)
-          DEFAULT_MAX_CONCURRENCY
+          if Feature.enabled?(:merge_trains_high_concurrency, project)
+            HIGH_CONCURRENCY
+          elsif Feature.enabled?(:merge_trains_medium_concurrency, project)
+            MEDIUM_CONCURRENCY
           else
-          ONE_AT_A_TIME_STRATEGY
+            LOW_CONCURRENCY
+          end
+        else
+          NO_CONCURRENCY
         end
       end
     end

ee/spec/services/merge_trains/refresh_merge_requests_service_spec.rb

@@ -180,4 +180,60 @@ describe MergeTrains::RefreshMergeRequestsService do
       end
     end
   end
+
+  describe '#max_concurrency' do
+    subject { service.send(:max_concurrency) }
+
+    context 'when `merge_trains_parallel_pipelines` feature flag is enabled' do
+      before do
+        stub_feature_flags(merge_trains_parallel_pipelines: true)
+      end
+
+      context 'when `merge_trains_high_concurrency` feature flag is enabled' do
+        before do
+          stub_feature_flags(merge_trains_high_concurrency: true)
+        end
+
+        it 'returns high concurrency' do
+          is_expected.to eq(described_class::HIGH_CONCURRENCY)
+        end
+      end
+
+      context 'when `merge_trains_high_concurrency` feature flag is disabled' do
+        before do
+          stub_feature_flags(merge_trains_high_concurrency: false)
+        end
+
+        context 'when `merge_trains_medium_concurrency` feature flag is enabled' do
+          before do
+            stub_feature_flags(merge_trains_medium_concurrency: true)
+          end
+
+          it 'returns medium concurrency' do
+            is_expected.to eq(described_class::MEDIUM_CONCURRENCY)
+          end
+        end
+
+        context 'when `merge_trains_medium_concurrency` feature flag is disabled' do
+          before do
+            stub_feature_flags(merge_trains_medium_concurrency: false)
+          end
+
+          it 'returns low concurrency' do
+            is_expected.to eq(described_class::LOW_CONCURRENCY)
+          end
+        end
+      end
+    end
+
+    context 'when `merge_trains_parallel_pipelines` feature flag is disabled' do
+      before do
+        stub_feature_flags(merge_trains_parallel_pipelines: false)
+      end
+
+      it 'returns no concurrency' do
+        is_expected.to eq(described_class::NO_CONCURRENCY)
+      end
+    end
+  end
 end