Make the inventory unit builder configurable

This adds the inventory unit builder (Spree::Stock::InventoryUnitBuilder
by default) to the Spree::Config.stock configuration, so that it can be
overridden.

This is desirable for any store that doesn't have a one-to-one mapping
between purchased variants and the fulfilled inventory units. The
extension solidus_product_assembly heavily overrides this class to
accomplish this, so with this change it will be able to stop decorating
Spree::Stock::InventoryUnitBuilder and instead provide its own.

I'll admit that I'm not a huge fan of how I tested the functionality in
the order class, but there's a reason for stubbing the preference like
that. Spree::Config.stock isn't a normal config object, so it doesn't
support our test helpers for stubbing preferences within the context of
only a single test.

core/app/models/spree/order.rb

@@ -486,7 +486,8 @@ def create_proposed_shipments
     end
 
     def create_shipments_for_line_item(line_item)
-      units = Spree::Stock::InventoryUnitBuilder.new(self).missing_units_for_line_item(line_item)
+      units = Spree::Config.stock.inventory_unit_builder_class.new(self).missing_units_for_line_item(line_item)
+
       Spree::Config.stock.coordinator_class.new(self, units).shipments.each do |shipment|
         shipments << shipment
       end

core/app/models/spree/stock/simple_coordinator.rb

@@ -24,7 +24,8 @@ class SimpleCoordinator
 
       def initialize(order, inventory_units = nil)
         @order = order
-        @inventory_units = inventory_units || InventoryUnitBuilder.new(order).units
+        @inventory_units =
+          inventory_units || Spree::Config.stock.inventory_unit_builder_class.new(order).units
         @splitters = Spree::Config.environment.stock_splitters
 
         filtered_stock_locations = Spree::Config.stock.location_filter_class.new(Spree::StockLocation.all, @order).filter

core/lib/spree/core/stock_configuration.rb

@@ -8,6 +8,7 @@ class StockConfiguration
       attr_writer :location_filter_class
       attr_writer :location_sorter_class
       attr_writer :allocator_class
+      attr_writer :inventory_unit_builder_class
 
       def coordinator_class
         @coordinator_class ||= '::Spree::Stock::SimpleCoordinator'
@@ -33,6 +34,11 @@ def allocator_class
         @allocator_class ||= '::Spree::Stock::Allocator::OnHandFirst'
         @allocator_class.constantize
       end
+
+      def inventory_unit_builder_class
+        @inventory_unit_builder_class ||= '::Spree::Stock::InventoryUnitBuilder'
+        @inventory_unit_builder_class.constantize
+      end
     end
   end
 end

core/spec/lib/spree/core/stock_configuration_spec.rb

@@ -92,4 +92,26 @@
       end
     end
   end
+
+  describe '#inventory_unit_builder_class' do
+    subject { stock_configuration.inventory_unit_builder_class }
+
+    let(:stock_configuration) { described_class.new }
+
+    it "returns Spree::Stock::InventoryUnitBuilder" do
+      is_expected.to be ::Spree::Stock::InventoryUnitBuilder
+    end
+
+    context "with another constant name assigned" do
+      MyInventoryUnitBuilder = Class.new
+
+      before do
+        stock_configuration.inventory_unit_builder_class = MyInventoryUnitBuilder.to_s
+      end
+
+      it "returns the constant" do
+        expect(subject).to be MyInventoryUnitBuilder
+      end
+    end
+  end
 end

core/spec/models/spree/order_spec.rb

@@ -1600,6 +1600,42 @@ def generate
     it 'creates at least one new shipment for the order' do
       expect { subject }.to change { order.shipments.count }.by 1
     end
+
+    context "with a custom inventory unit builder" do
+      before do
+        class TestInventoryUnitBuilder
+          def initialize(order)
+          end
+
+          def missing_units_for_line_item(line_item)
+            []
+          end
+        end
+
+        # This return value is stubbed this way, so that it can access the
+        # arbitrary_inventory_unit let variable.
+        allow_any_instance_of(TestInventoryUnitBuilder)
+          .to receive(:missing_units_for_line_item)
+          .and_return([arbitrary_inventory_unit])
+
+        # This is stubbed out rather than set, so that it doesn't leak out into
+        # other tests.
+        allow(Spree::Config.stock)
+          .to receive(:inventory_unit_builder_class)
+          .and_return(TestInventoryUnitBuilder)
+
+        order.shipments.first.update!(created_at: 2.years.ago)
+      end
+
+      let(:arbitrary_inventory_unit) { build :inventory_unit, order: order }
+
+      it "relies on the custom builder" do
+        expect { subject }.to change { order.shipments.count }.by 1
+
+        expect(order.shipments.order(:created_at).last.inventory_units)
+          .to contain_exactly arbitrary_inventory_unit
+      end
+    end
   end
 
   describe '#shipping_discount' do

core/spec/models/spree/stock/simple_coordinator_spec.rb

@@ -38,6 +38,14 @@ module Stock
           subject.shipments.count == StockLocation.count
         end
 
+        it 'uses the pluggable inventory unit builder class' do
+          expect(Spree::Config.stock)
+            .to receive(:inventory_unit_builder_class)
+            .and_call_original
+
+          subject.shipments
+        end
+
         context "missing stock items in active stock location" do
           let!(:another_location) { create(:stock_location, propagate_all_variants: false) }