Fixing bug with non-existent customer group breaking quote

Author: kodubovik

app/code/Magento/Quote/Model/Quote.php

@@ -10,6 +10,7 @@
 use Magento\Directory\Model\AllowedCountries;
 use Magento\Framework\Api\AttributeValueFactory;
 use Magento\Framework\Api\ExtensionAttribute\JoinProcessorInterface;
+use Magento\Framework\Exception\NoSuchEntityException;
 use Magento\Framework\Model\AbstractExtensibleModel;
 use Magento\Quote\Api\Data\PaymentInterface;
 use Magento\Quote\Model\Quote\Address;
@@ -1104,7 +1105,22 @@ public function getCustomerTaxClassId()
         //if (!$this->getData('customer_group_id') && !$this->getData('customer_tax_class_id')) {
         $groupId = $this->getCustomerGroupId();
         if ($groupId !== null) {
-            $taxClassId = $this->groupRepository->getById($this->getCustomerGroupId())->getTaxClassId();
+            $taxClassId = null;
+            try {
+                $taxClassId = $this->groupRepository->getById($this->getCustomerGroupId())->getTaxClassId();
+            } catch (NoSuchEntityException $e) {
+                /**
+                 * A customer MAY create a quote and AFTER that customer group MAY be deleted.
+                 * That breaks a quote because it still refers no a non-existent customer group.
+                 * In such a case we should load a new customer group id from the current customer
+                 * object and use it to retrieve tax class and update quote.
+                 */
+                $groupId = $this->getCustomer()->getGroupId();
+                $this->setCustomerGroupId($groupId);
+                if ($groupId !== null) {
+                    $taxClassId = $this->groupRepository->getById($groupId)->getTaxClassId();
+                }
+            }
             $this->setCustomerTaxClassId($taxClassId);
         }
 

app/code/Magento/Quote/Test/Unit/Model/QuoteTest.php

@@ -30,7 +30,9 @@
 use Magento\Framework\DataObject\Copy;
 use Magento\Framework\DataObject\Factory;
 use Magento\Framework\Event\Manager;
+use Magento\Framework\Exception\NoSuchEntityException;
 use Magento\Framework\Model\Context;
+use Magento\Framework\Phrase;
 use Magento\Framework\TestFramework\Unit\Helper\ObjectManager;
 use Magento\Quote\Api\Data\CartInterface;
 use Magento\Quote\Model\Quote;
@@ -640,6 +642,48 @@ public function testGetCustomerTaxClassId()
         $this->assertEquals($taxClassId, $result);
     }
 
+    /**
+     * Test case when non-existent customer group is stored into the quote.
+     * In such a case we should get a NoSuchEntityException exception and try
+     * to get a valid customer group from the current customer object.
+     */
+    public function testGetCustomerTaxClassIdForNonExistentCustomerGroup()
+    {
+        $customerId = 1;
+        $nonExistentGroupId = 100;
+        $groupId = 1;
+        $taxClassId = 1;
+        $groupMock = $this->getMockForAbstractClass(GroupInterface::class, [], '', false);
+        $this->groupRepositoryMock->expects($this->at(0))
+            ->method('getById')
+            ->with($nonExistentGroupId)
+            ->willThrowException(new NoSuchEntityException(new Phrase('Entity Id does not exist')));
+        $customerMock = $this->getMockForAbstractClass(
+            CustomerInterface::class,
+            [],
+            '',
+            false
+        );
+        $customerMock->expects($this->once())
+            ->method('getGroupId')
+            ->willReturn($groupId);
+        $this->customerRepositoryMock->expects($this->once())
+            ->method('getById')
+            ->with($customerId)
+            ->willReturn($customerMock);
+        $this->groupRepositoryMock->expects($this->at(1))
+            ->method('getById')
+            ->with($groupId)
+            ->willReturn($groupMock);
+        $groupMock->expects($this->once())
+            ->method('getTaxClassId')
+            ->willReturn($taxClassId);
+        $this->quote->setData('customer_id', $customerId);
+        $this->quote->setData('customer_group_id', $nonExistentGroupId);
+        $result = $this->quote->getCustomerTaxClassId();
+        $this->assertEquals($taxClassId, $result);
+    }
+
     public function testGetAllAddresses()
     {
         $id = 1;