Revert "Remove unreachable order recovery code"

This reverts commit 355c5f5c55.

This code is necessary to preserver cart contents across logins on
different browser sessions.

lib/spree/core/controller_helpers/order.rb

@@ -68,9 +68,18 @@ module Spree
           session[:guest_token] = nil
         end
 
-        # Load current order and create a new one if necessary.
+        # Recover incomplete orders from other sessions after logging in.
         def set_current_order
-          current_order(true) if spree_current_user
+          return unless (user = spree_current_user)
+
+          last_incomplete_order = user.last_incomplete_spree_order
+
+          if session[:order_id].nil? && last_incomplete_order
+            session[:order_id] = last_incomplete_order.id
+          end
+
+          # Load current order and create a new one if necessary.
+          current_order(true)
         end
 
         def current_currency

spec/controllers/base_controller_spec.rb

@@ -28,6 +28,17 @@ describe BaseController, type: :controller do
       expect(user.orders.count).to eq 1
     end
 
+    it "uses the last incomplete order" do
+      last_cart = create(:order, user: user, created_by: user, state: "cart", completed_at: nil)
+      allow(controller).to receive(:spree_current_user).and_return(user)
+
+      expect {
+        get :index
+      }.to_not change { Spree::Order.count }
+
+      expect(session[:order_id]).to eq last_cart.id
+    end
+
     it "ignores the last incomplete order" do
       # Spree used to merge the last order with the current one.
       # And we used to override that logic to delete old incomplete orders.