Fix shared runner minutes reset on self-managed

When namespace count is lower than 100,000,
the :ci_parallel_minutes_reset feature fails
with a zero division error due to integer division

This affects most self-managed instances but not
GitLab.com which has well over 100,000 namespaces

Closes: https://gitlab.com/gitlab-org/gitlab/-/issues/330068

Changelog: fixed
EE: true

ee/app/workers/clear_shared_runners_minutes_worker.rb

@@ -20,7 +20,11 @@ class ClearSharedRunnersMinutesWorker # rubocop:disable Scalability/IdempotentWo
       start_id = Namespace.minimum(:id)
       last_id = Namespace.maximum(:id)
 
-      execution_offset = TIME_SPREAD / ((last_id - start_id) / BATCH_SIZE)
+      # Uses float division to avoid a ZeroDivisionError
+      # when namespace ID range is lower than BATCH_SIZE
+      # See: https://gitlab.com/gitlab-org/gitlab/-/issues/330068
+      batches = (last_id - start_id).fdiv(BATCH_SIZE)
+      execution_offset = (TIME_SPREAD / batches).to_i
 
       (start_id..last_id).step(BATCH_SIZE).with_index do |batch_start_id, batch_index|
         batch_end_id = batch_start_id + BATCH_SIZE - 1

ee/spec/workers/clear_shared_runners_minutes_worker_spec.rb

@@ -133,7 +133,7 @@ RSpec.describe ClearSharedRunnersMinutesWorker do
       end
     end
 
-    context 'when ci_parallel_minutes_reset feature flag is enabled' do
+    context 'when ci_parallel_minutes_reset feature flag is enabled, with batch size lower than namespace count' do
       subject { worker.perform }
 
       before do
@@ -143,6 +143,7 @@ RSpec.describe ClearSharedRunnersMinutesWorker do
           create(:namespace, id: id)
         end
 
+        # Test with a batch size lower than count of namespaces
         stub_const("#{described_class}::BATCH_SIZE", 3)
       end
 
@@ -157,5 +158,25 @@ RSpec.describe ClearSharedRunnersMinutesWorker do
         subject
       end
     end
+
+    context 'when ci_parallel_minutes_reset feature flag is enabled, with batch size higher than namespace count' do
+      subject { worker.perform }
+
+      before do
+        stub_feature_flags(ci_parallel_minutes_reset: true)
+        [2, 3, 4, 5, 7, 8, 10, 14].each do |id|
+          create(:namespace, id: id)
+        end
+
+        # Use the default BATCH_SIZE (100_000)
+      end
+
+      it 'runs the worker in a single batch', :aggregate_failures do
+        # Runs a single batch, immediately
+        expect(Ci::BatchResetMinutesWorker).to receive(:perform_in).with(0.seconds, 2, 100_001)
+
+        subject
+      end
+    end
   end
 end