Specs to expose stock validation issues on Solidus checkout confirm page

checkout_insufficient_stock_spec.rb

require 'spec_helper'

describe "Checkout confirm page", type: :feature do
  include_context 'checkout setup'

  context 'when there is not enough stock at the default stock location' do
    context "when the product is not backorderable" do
      let(:user) { create(:user) }

      let(:order) { Spree::TestingSupport::OrderWalkthrough.up_to(:payment) }
      let(:order_product) { order.products.first }
      let(:order_stock_item) { order_product.stock_items.first }

      before do
        order_stock_item.update! backorderable: false

        allow_any_instance_of(Spree::CheckoutController).to receive_messages(current_order: order)
        allow_any_instance_of(Spree::CheckoutController).to receive_messages(try_spree_current_user: user)
        allow_any_instance_of(Spree::OrdersController).to receive_messages(try_spree_current_user: user)
      end

      context 'when there are not other backorderable stock locations' do
        before { visit spree.checkout_state_path(:confirm) }

        it 'redirects to cart page and shows an unavailable product message' do
          expect(page).to have_content "#{order_product.name} became unavailable"
          expect(page).to have_current_path spree.cart_path
        end
      end

      context 'when there is another backorderable stock location' do
        before do
          create :stock_location, backorderable_default: true, default: false
          visit spree.checkout_state_path(:confirm)
        end

        it "redirects to the delivery checkout page and shows an availability changed message" do
          click_button "Place Order"
          expect(page).to have_content "#{order_product.name} availability changed. Please restart the checkout process"
          expect(page).to have_current_path spree.checkout_state_path(:delivery)
        end
      end
    end
  end
end

When two users try to purchase the last item remaining from a non-backordeable stock location at the same time then the last one will experience an unhandled error Spree::Order::InsufficientStock. This happens only if there is a second backorderable stock location for the product.

The error is generated by the method Spree::Order#validate_line_item_availability triggered by this state machine guard:

before_transition to: :complete, do: :validate_line_item_availability

So, controller validations (provided here via before_action :ensure_sufficient_stock_lines) are following a different path, calling different methods, and this causes that error to be raised later in the order finalization process. Unifying the process seems to me the most reasonable way to fix the issue.

On Solidus < 2.7 the issue is a bit more involuted, as `Spree::Stock::AvailabilityValidator does not exist yet and its equivalent code is spread among many different models.