⏫ Forwardport of #11550 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11550.patch (created by @ajpevers) based on commit(s):
  1. 9380c95
  2. 8a6c3f2
  3. 480f8ae

Author: magento-engcom-team

app/code/Magento/Sales/Model/Service/CreditmemoService.php

@@ -195,7 +195,7 @@ public function refund(
      */
     protected function validateForRefund(\Magento\Sales\Api\Data\CreditmemoInterface $creditmemo)
     {
-        if ($creditmemo->getId()) {
+        if ($creditmemo->getId() && $creditmemo->getState() != \Magento\Sales\Model\Order\Creditmemo::STATE_OPEN) {
             throw new \Magento\Framework\Exception\LocalizedException(
                 __('We cannot register an existing credit memo.')
             );

app/code/Magento/Sales/Test/Unit/Model/Service/CreditmemoServiceTest.php

@@ -243,6 +243,78 @@ public function testRefund()
         $this->assertSame($creditMemoMock, $this->creditmemoService->refund($creditMemoMock, true));
     }
 
+    public function testRefundPendingCreditMemo()
+    {
+        $creditMemoMock = $this->getMockBuilder(\Magento\Sales\Api\Data\CreditmemoInterface::class)
+            ->setMethods(['getId', 'getOrder', 'getState', 'getInvoice'])
+            ->disableOriginalConstructor()
+            ->getMockForAbstractClass();
+        $creditMemoMock->expects($this->once())->method('getId')->willReturn(444);
+        $creditMemoMock->expects($this->once())->method('getState')
+            ->willReturn(\Magento\Sales\Model\Order\Creditmemo::STATE_OPEN);
+        $orderMock = $this->getMockBuilder(Order::class)->disableOriginalConstructor()->getMock();
+
+        $creditMemoMock->expects($this->atLeastOnce())->method('getOrder')->willReturn($orderMock);
+        $orderMock->expects($this->once())->method('getBaseTotalRefunded')->willReturn(0);
+        $orderMock->expects($this->once())->method('getBaseTotalPaid')->willReturn(10);
+        $creditMemoMock->expects($this->once())->method('getBaseGrandTotal')->willReturn(10);
+
+        $this->priceCurrencyMock->expects($this->any())
+            ->method('round')
+            ->willReturnArgument(0);
+
+        // Set payment adapter dependency
+        $refundAdapterMock = $this->getMockBuilder(\Magento\Sales\Model\Order\RefundAdapterInterface::class)
+            ->disableOriginalConstructor()
+            ->getMockForAbstractClass();
+        $this->objectManagerHelper->setBackwardCompatibleProperty(
+            $this->creditmemoService,
+            'refundAdapter',
+            $refundAdapterMock
+        );
+
+        // Set resource dependency
+        $resourceMock = $this->getMockBuilder(\Magento\Framework\App\ResourceConnection::class)
+            ->disableOriginalConstructor()
+            ->getMock();
+        $this->objectManagerHelper->setBackwardCompatibleProperty(
+            $this->creditmemoService,
+            'resource',
+            $resourceMock
+        );
+
+        // Set order repository dependency
+        $orderRepositoryMock = $this->getMockBuilder(\Magento\Sales\Api\OrderRepositoryInterface::class)
+            ->disableOriginalConstructor()
+            ->getMockForAbstractClass();
+        $this->objectManagerHelper->setBackwardCompatibleProperty(
+            $this->creditmemoService,
+            'orderRepository',
+            $orderRepositoryMock
+        );
+
+        $adapterMock = $this->getMockBuilder(\Magento\Framework\DB\Adapter\AdapterInterface::class)
+            ->disableOriginalConstructor()
+            ->getMockForAbstractClass();
+        $resourceMock->expects($this->once())->method('getConnection')->with('sales')->willReturn($adapterMock);
+        $adapterMock->expects($this->once())->method('beginTransaction');
+        $refundAdapterMock->expects($this->once())
+            ->method('refund')
+            ->with($creditMemoMock, $orderMock, false)
+            ->willReturn($orderMock);
+        $orderRepositoryMock->expects($this->once())
+            ->method('save')
+            ->with($orderMock);
+        $creditMemoMock->expects($this->once())
+            ->method('getInvoice')
+            ->willReturn(null);
+        $adapterMock->expects($this->once())->method('commit');
+        $this->creditmemoRepositoryMock->expects($this->once())
+            ->method('save');
+
+        $this->assertSame($creditMemoMock, $this->creditmemoService->refund($creditMemoMock, true));
+    }
+
     /**
      * @expectedExceptionMessage The most money available to refund is 1.
      * @expectedException \Magento\Framework\Exception\LocalizedException