⏫ Forwardport of #11670 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11670.patch (created by @ajpevers) based on commit(s):
  1. cfd92c4
  2. 94ae23d
  3. 9145209
  4. ce20c0a
  5. 23f315c
  6. 72533ad
  7. 1d341ef
  8. f2ba426

Fixed GitHub Issues in 2.3-develop branch:
  - #11669: API salesRefundInvoiceV1 does no save invoice ID on credit memo (reported by @ajpevers)

Author: magento-engcom-team

app/code/Magento/Sales/Model/Order/Creditmemo.php

@@ -7,10 +7,12 @@
 namespace Magento\Sales\Model\Order;
 
 use Magento\Framework\Api\AttributeValueFactory;
+use Magento\Framework\App\ObjectManager;
 use Magento\Framework\Pricing\PriceCurrencyInterface;
 use Magento\Sales\Api\Data\CreditmemoInterface;
 use Magento\Sales\Model\AbstractModel;
 use Magento\Sales\Model\EntityInterface;
+use Magento\Sales\Model\Order\InvoiceFactory;
 
 /**
  * Order creditmemo model
@@ -112,6 +114,11 @@ class Creditmemo extends AbstractModel implements EntityInterface, CreditmemoInt
      */
     protected $priceCurrency;
 
+    /**
+     * @var InvoiceFactory
+     */
+    private $invoiceFactory;
+
     /**
      * @param \Magento\Framework\Model\Context $context
      * @param \Magento\Framework\Registry $registry
@@ -128,6 +135,7 @@ class Creditmemo extends AbstractModel implements EntityInterface, CreditmemoInt
      * @param \Magento\Framework\Model\ResourceModel\AbstractResource $resource
      * @param \Magento\Framework\Data\Collection\AbstractDb $resourceCollection
      * @param array $data
+     * @param InvoiceFactory $invoiceFactory
      * @SuppressWarnings(PHPMD.ExcessiveParameterList)
      */
     public function __construct(
@@ -145,7 +153,8 @@ public function __construct(
         PriceCurrencyInterface $priceCurrency,
         \Magento\Framework\Model\ResourceModel\AbstractResource $resource = null,
         \Magento\Framework\Data\Collection\AbstractDb $resourceCollection = null,
-        array $data = []
+        array $data = [],
+        InvoiceFactory $invoiceFactory = null
     ) {
         $this->_creditmemoConfig = $creditmemoConfig;
         $this->_orderFactory = $orderFactory;
@@ -155,6 +164,7 @@ public function __construct(
         $this->_commentFactory = $commentFactory;
         $this->_commentCollectionFactory = $commentCollectionFactory;
         $this->priceCurrency = $priceCurrency;
+        $this->invoiceFactory = $invoiceFactory ?: ObjectManager::getInstance()->get(InvoiceFactory::class);
         parent::__construct(
             $context,
             $registry,
@@ -377,6 +387,9 @@ public function canRefund()
      */
     public function getInvoice()
     {
+        if (!$this->getData('invoice') instanceof \Magento\Sales\Api\Data\InvoiceInterface && $this->getInvoiceId()) {
+            $this->setInvoice($this->invoiceFactory->create()->load($this->getInvoiceId()));
+        }
         return $this->getData('invoice');
     }
 
@@ -1505,6 +1518,5 @@ public function setExtensionAttributes(\Magento\Sales\Api\Data\CreditmemoExtensi
     {
         return $this->_setExtensionAttributes($extensionAttributes);
     }
-
     //@codeCoverageIgnoreEnd
 }

app/code/Magento/Sales/Model/ResourceModel/Order/Creditmemo.php

@@ -50,6 +50,10 @@ protected function _beforeSave(\Magento\Framework\Model\AbstractModel $object)
             $object->setBillingAddressId($object->getOrder()->getBillingAddress()->getId());
         }
 
+        if (!$object->getInvoiceId() && $object->getInvoice()) {
+            $object->setInvoiceId($object->getInvoice()->getId());
+        }
+
         return parent::_beforeSave($object);
     }
 }

dev/tests/integration/testsuite/Magento/Paypal/Model/IpnTest.php

@@ -123,7 +123,7 @@ public function testProcessIpnRequestPartialRefund()
     /**
      * Refund rest of order amount by Paypal Express IPN message service.
      *
-     * @magentoDataFixture Magento/Paypal/_files/order_express_with_invoice_and_creditmemo.php
+     * @magentoDataFixture Magento/Paypal/_files/order_express_with_invoice_and_shipping.php
      * @magentoConfigFixture current_store payment/paypal_express/active 1
      * @magentoConfigFixture current_store paypal/general/merchant_country US
      */
@@ -147,7 +147,7 @@ public function testProcessIpnRequestRestRefund()
         $creditmemoItems = $order->getCreditmemosCollection()->getItems();
 
         $this->assertEquals(Order::STATE_CLOSED, $order->getState()) ;
-        $this->assertEquals(2, count($creditmemoItems));
+        $this->assertEquals(1, count($creditmemoItems));
         $this->assertEquals(10, $order->getSubtotalRefunded());
         $this->assertEquals(10, $order->getBaseSubtotalRefunded());
         $this->assertEquals(20, $order->getShippingRefunded());