Don't change canceled orders

We have a PR already that removes the UI for this when the order is
canceled. Implementing it on controller-side makes sure that it doesn't
happen accidentally if the user has multiple tabs open.

app/controllers/spree/admin/adjustments_controller.rb

@@ -5,6 +5,7 @@ module Spree
 
       prepend_before_action :set_included_tax, only: [:create, :update]
       before_action :set_order_id, only: [:create, :update]
+      before_action :skip_changing_canceled_orders, only: [:create, :update]
       after_action :update_order, only: [:create, :update, :destroy]
       before_action :set_default_tax_rate, only: :edit
       before_action :enable_updates, only: :update
@@ -28,6 +29,10 @@ module Spree
         @adjustment.order_id = parent.id
       end
 
+      def skip_changing_canceled_orders
+        redirect_to admin_order_adjustments_path(@order) if @order.canceled?
+      end
+
       # Choose a default tax rate to show on the edit form. The adjustment stores its included
       # tax in dollars, but doesn't store the source of the tax (ie. TaxRate that generated it).
       # We guess which tax rate here, choosing:

spec/controllers/spree/admin/adjustments_controller_spec.rb

@@ -90,5 +90,33 @@ module Spree
         end
       end
     end
+
+    describe "with a cancelled order" do
+      let(:order) { create(:completed_order_with_totals) }
+      let(:tax_rate) { create(:tax_rate, amount: 0.1, calculator: ::Calculator::DefaultTax.new) }
+      let(:adjustment) {
+        create(:adjustment, adjustable: order, order: order, amount: 1100, included_tax: 100)
+      }
+
+      before do
+        expect(order.cancel).to eq true
+      end
+
+      it "doesn't create adjustments" do
+        expect {
+          spree_post :create, order_id: order.number, adjustment: { label: "Testing", amount: "110" }, tax_rate_id: ""
+        }.to_not change { [Adjustment.count, order.reload.total] }
+
+        expect(response).to redirect_to spree.admin_order_adjustments_path(order)
+      end
+
+      it "doesn't change adjustments" do
+        expect {
+          spree_put :update, order_id: order.number, id: adjustment.id, adjustment: { label: "Testing", amount: "110" }, tax_rate_id: ""
+        }.to_not change { [adjustment.reload.amount, order.reload.total] }
+
+        expect(response).to redirect_to spree.admin_order_adjustments_path(order)
+      end
+    end
   end
 end