check stock before doing anything related to admin payments

app/controllers/spree/admin/payments_controller.rb

@@ -9,6 +9,7 @@ module Spree
       before_action :load_payment, only: [:fire, :show]
       before_action :load_data
       before_action :can_transition_to_payment
+      before_action :ensure_sufficient_stock_lines
 
       respond_to :html
 
@@ -142,6 +143,17 @@ module Spree
         redirect_to spree.edit_admin_order_customer_url(@order)
       end
 
+      def ensure_sufficient_stock_lines
+        return if @order.insufficient_stock_lines.blank? || !@order.payment?
+
+        out_of_stock_items = @order.insufficient_stock_lines.map do |line_item|
+          line_item.variant.name
+        end.join(", ")
+        flash[:error] = I18n.t("spree.orders.line_item.insufficient_stock",
+          on_hand: "0 #{out_of_stock_items}")
+        redirect_to spree.edit_admin_order_url(@order)
+      end
+
       def load_order
         @order = Order.find_by!(number: params[:order_id])
         authorize! action, @order

spec/controllers/spree/admin/orders/payments/payments_controller_spec.rb

@@ -309,5 +309,19 @@ describe Spree::Admin::PaymentsController, type: :controller do
         end
       end
     end
+
+    context "the order contains an item that is out of stock" do
+      let!(:order) { create(:order, distributor: shop, state: 'payment') }
+
+      before do
+        order.line_items.first.variant.update_attribute(:on_hand, 0)
+      end
+
+      it "redirects to the order details page" do
+        spree_get :index, order_id: order.number
+        expect(response.status).to eq 302
+        expect(response.location).to eq spree.edit_admin_order_url(order)
+      end
+    end
   end
 end