Inventory sales backorder functionality

app/code/Magento/InventoryApi/Test/_files/products.php

@@ -14,6 +14,7 @@
 $productFactory = $objectManager->get(ProductInterfaceFactory::class);
 /** @var ProductRepositoryInterface $productRepository */
 $productRepository = $objectManager->get(ProductRepositoryInterface::class);
+$productRepository->cleanCache();
 
 for ($i = 1; $i <= 3; $i++) {
     $product = $productFactory->create();

app/code/Magento/InventorySales/Plugin/InventoryApi/BackorderStockStatusPlugin.php

@@ -0,0 +1,82 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+declare(strict_types=1);
+
+namespace Magento\InventorySales\Plugin\InventoryApi;
+
+use Magento\Catalog\Api\ProductRepositoryInterface;
+use Magento\CatalogInventory\Api\Data\StockItemInterface;
+use Magento\CatalogInventory\Api\StockItemCriteriaInterfaceFactory;
+use Magento\CatalogInventory\Api\StockItemRepositoryInterface;
+use Magento\InventoryApi\Api\IsProductInStockInterface;
+
+/**
+ * Adapt backorders to IsProductInStockInterface
+ */
+class BackorderStockStatusPlugin
+{
+    /**
+     * @var StockItemRepositoryInterface
+     */
+    private $stockItemRepository;
+
+    /**
+     * @var StockItemCriteriaInterfaceFactory
+     */
+    private $stockItemCriteriaFactory;
+
+    /**
+     * @var ProductRepositoryInterface
+     */
+    private $productRepository;
+
+    /**
+     * @param StockItemRepositoryInterface $stockItemRepository
+     * @param StockItemCriteriaInterfaceFactory $stockItemCriteriaFactory
+     * @param ProductRepositoryInterface $productRepository
+     */
+    public function __construct(
+        StockItemRepositoryInterface $stockItemRepository,
+        StockItemCriteriaInterfaceFactory $stockItemCriteriaFactory,
+        ProductRepositoryInterface $productRepository
+    ) {
+        $this->stockItemRepository = $stockItemRepository;
+        $this->stockItemCriteriaFactory = $stockItemCriteriaFactory;
+        $this->productRepository = $productRepository;
+    }
+
+    /**
+     * Return true status if backorders is enabled for the item
+     *
+     * @param IsProductInStockInterface $subject
+     * @param callable $proceed
+     * @param string $sku
+     * @param int $stockId
+     * @return bool
+     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
+     */
+    public function aroundExecute(
+        IsProductInStockInterface $subject,
+        callable $proceed,
+        string $sku,
+        int $stockId
+    ): bool {
+        $productData = $this->productRepository->get($sku);
+        $productId = $productData->getId();
+
+        $stockItemCriteria = $this->stockItemCriteriaFactory->create();
+        $stockItemCriteria->setProductsFilter($productId);
+        $stockItemsCollection = $this->stockItemRepository->getList($stockItemCriteria);
+
+        /** @var StockItemInterface $legacyStockItem */
+        $legacyStockItem = current($stockItemsCollection->getItems());
+
+        if ($legacyStockItem->getBackorders() > 0) {
+            return true;
+        }
+        return $proceed($sku, $stockId);
+    }
+}

app/code/Magento/InventorySales/Test/Integration/IsBackorderedProductInStockTest.php

@@ -0,0 +1,144 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\InventorySales\Test\Integration;
+
+use Magento\Catalog\Api\ProductRepositoryInterface;
+use Magento\CatalogInventory\Api\Data\StockItemInterface;
+use Magento\CatalogInventory\Api\StockItemCriteriaInterfaceFactory;
+use Magento\CatalogInventory\Api\StockItemRepositoryInterface;
+use Magento\Framework\Api\SearchCriteriaBuilder;
+use Magento\Framework\Indexer\IndexerInterface;
+use Magento\Inventory\Indexer\StockItemIndexerInterface;
+use Magento\InventoryApi\Api\Data\SourceItemInterface;
+use Magento\InventoryApi\Api\IsProductInStockInterface;
+use Magento\InventoryApi\Api\SourceItemRepositoryInterface;
+use Magento\InventoryApi\Api\SourceItemsSaveInterface;
+use Magento\TestFramework\Helper\Bootstrap;
+use PHPUnit\Framework\TestCase;
+
+class IsBackorderedProductInStockTest extends TestCase
+{
+    /**
+     * @var SourceItemRepositoryInterface
+     */
+    private $sourceItemRepository;
+
+    /**
+     * @var SearchCriteriaBuilder
+     */
+    private $searchCriteriaBuilder;
+
+    /**
+     * @var SourceItemsSaveInterface
+     */
+    private $sourceItemsSave;
+
+    /**
+     * @var IndexerInterface
+     */
+    private $indexer;
+
+    /**
+     * @var ProductRepositoryInterface
+     */
+    private $productRepository;
+
+    /**
+     * @var IsProductInStockInterface
+     */
+    private $isProductInStock;
+
+    /**
+     * @var StockItemRepositoryInterface
+     */
+    private $stockItemRepository;
+
+    /**
+     * @var StockItemCriteriaInterfaceFactory
+     */
+    private $stockItemCriteriaFactory;
+
+    protected function setUp()
+    {
+        $this->productRepository = Bootstrap::getObjectManager()->create(ProductRepositoryInterface::class);
+        $this->stockItemRepository =  Bootstrap::getObjectManager()->create(StockItemRepositoryInterface::class);
+        $this->stockItemCriteriaFactory =  Bootstrap::getObjectManager()->create(
+            StockItemCriteriaInterfaceFactory::class
+        );
+        $this->sourceItemRepository = Bootstrap::getObjectManager()->create(SourceItemRepositoryInterface::class);
+        $this->searchCriteriaBuilder = Bootstrap::getObjectManager()->create(SearchCriteriaBuilder::class);
+        $this->sourceItemsSave = Bootstrap::getObjectManager()->create(SourceItemsSaveInterface::class);
+        $this->indexer = Bootstrap::getObjectManager()->create(IndexerInterface::class);
+        $this->indexer->load(StockItemIndexerInterface::INDEXER_ID);
+        $this->isProductInStock = Bootstrap::getObjectManager()->create(IsProductInStockInterface::class);
+    }
+
+    /**
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/products.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/sources.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/stocks.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/source_items.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/stock_source_link.php
+     */
+    public function testBackorderedZeroQtyProductIsInStock()
+    {
+        $product = $this->productRepository->get('SKU-2');
+        $stockItemSearchCriteria = $this->stockItemCriteriaFactory->create();
+        $stockItemSearchCriteria->setProductsFilter($product->getId());
+        $stockItemsCollection = $this->stockItemRepository->getList($stockItemSearchCriteria);
+
+        /** @var StockItemInterface $legacyStockItem */
+        $legacyStockItem = current($stockItemsCollection->getItems());
+        $legacyStockItem->setBackorders(1);
+        $legacyStockItem->setUseConfigBackorders(0);
+        $this->stockItemRepository->save($legacyStockItem);
+
+        $sourceItem = $this->getSourceItemBySku('SKU-2');
+        $this->changeSourceItemQty($sourceItem, -15);
+
+        $this->assertTrue($this->isProductInStock->execute('SKU-2', 1));
+    }
+
+    /**
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/products.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/sources.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/stocks.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/source_items.php
+     * @magentoDataFixture ../../../../app/code/Magento/InventoryApi/Test/_files/stock_source_link.php
+     */
+    public function testZeroQtyProductIsOutOfStock()
+    {
+        $sourceItem = $this->getSourceItemBySku('SKU-2');
+        $this->changeSourceItemQty($sourceItem, 0);
+
+        $this->assertFalse($this->isProductInStock->execute('SKU-2', 1));
+    }
+
+    /**
+     * @param string $sku
+     * @return SourceItemInterface
+     */
+    private function getSourceItemBySku(string $sku): SourceItemInterface
+    {
+        $sourceItemSearchCriteria = $this->searchCriteriaBuilder
+            ->addFilter('sku', $sku)
+            ->create();
+        $sourceItemSearchResult = $this->sourceItemRepository->getList($sourceItemSearchCriteria);
+
+        return current($sourceItemSearchResult->getItems());
+    }
+
+    /**
+     * @param SourceItemInterface $sourceItem
+     * @param float $qty
+     */
+    private function changeSourceItemQty(SourceItemInterface $sourceItem, float $qty)
+    {
+        $sourceItem->setQuantity($qty);
+        $this->sourceItemsSave->execute([$sourceItem]);
+        $this->indexer->reindexRow(5);
+    }
+}

app/code/Magento/InventorySales/etc/di.xml

@@ -9,4 +9,7 @@
     <type name="Magento\Inventory\Ui\DataProvider\StockDataProvider">
         <plugin name="sales_channel_data" type="Magento\InventorySales\Plugin\Inventory\Ui\StockDataProvider\SalesChannels" />
     </type>
+    <type name="Magento\InventoryApi\Api\IsProductInStockInterface">
+        <plugin name="backorder_inventory_stock_status" type="Magento\InventorySales\Plugin\InventoryApi\BackorderStockStatusPlugin"/>
+    </type>
 </config>