Guard if DB is read only

Ensure when UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker
is running the DB is not read only.

ee/app/workers/update_max_seats_used_for_gitlab_com_subscriptions_worker.rb

@@ -7,6 +7,7 @@ class UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker
   # rubocop: disable CodeReuse/ActiveRecord
   def perform
     return unless ::Gitlab::Database.postgresql?
+    return if ::Gitlab::Database.read_only?
     return unless ::Gitlab::CurrentSettings.should_check_namespace_plan?
 
     GitlabSubscription.with_a_paid_hosted_plan.find_in_batches(batch_size: 100) do |subscriptions|

ee/spec/workers/update_max_seats_used_for_gitlab_com_subscriptions_worker_spec.rb

@@ -9,22 +9,41 @@ describe UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker, :postgresql do
   let!(:early_adopter_plan)  { create(:early_adopter_plan) }
   let!(:gitlab_subscription) { create(:gitlab_subscription, namespace: group) }
 
+  let(:db_is_postgres) { true }
+  let(:db_is_read_only) { false }
+  let(:subscription_attrs) { nil }
+
   before do
+    allow(Gitlab::Database).to receive(:postgresql?) { db_is_postgres }
+    allow(Gitlab::Database).to receive(:read_only?) { db_is_read_only }
     allow(Gitlab::CurrentSettings).to receive(:should_check_namespace_plan?) { true }
 
     group.add_developer(user)
   end
 
   shared_examples 'keeps original max_seats_used value' do
-    before do
-      gitlab_subscription.update!(subscription_attrs)
-    end
-
     it 'does not update max_seats_used' do
       expect { subject.perform }.not_to change { gitlab_subscription.reload.max_seats_used }
     end
   end
 
+  context 'when the DB is not PostgreSQL' do
+    let(:db_is_postgres) { false }
+
+    include_examples 'keeps original max_seats_used value'
+  end
+
+  context 'where the DB is read only' do
+    let(:db_is_read_only) { true }
+
+    include_examples 'keeps original max_seats_used value'
+  end
+
+  context 'when the DB PostgreSQK AND is not read only' do
+    before do
+      gitlab_subscription.update!(subscription_attrs) if subscription_attrs
+    end
+
     context 'with a free plan' do
       let(:subscription_attrs) { { hosted_plan: nil } }
 
@@ -58,4 +77,5 @@ describe UpdateMaxSeatsUsedForGitlabComSubscriptionsWorker, :postgresql do
         expect { subject.perform }.not_to change { gitlab_subscription.reload.max_seats_used }
       end
     end
+  end
 end