Fix array to string conversion error when saving row system config with defaults

This PR solves issue #30314. The backend model that is designed to handle complex values serializes non-scalar values before they are persisted to database. But when the old value is loaded from config defaults (defined in config.xml) for comparison, the default value is fetched as array. When the array is casted to string it results in an array to string conversion error.

This issue is fixed by serializing the default value coming from the config.

Author: naitsirch

app/code/Magento/Config/Model/Config/Backend/Serialized.php

@@ -5,6 +5,7 @@
  */
 namespace Magento\Config\Model\Config\Backend;
 
+use Magento\Framework\App\Config\ScopeConfigInterface;
 use Magento\Framework\App\ObjectManager;
 use Magento\Framework\Serialize\Serializer\Json;
 
@@ -84,4 +85,26 @@ public function beforeSave()
         parent::beforeSave();
         return $this;
     }
+
+    /**
+     * Get old value from existing config
+     *
+     * @return string
+     */
+    public function getOldValue()
+    {
+        // If the value is retrieved from defaults defined in config.xml
+        // it may be returned as an array.
+        $value = $this->_config->getValue(
+            $this->getPath(),
+            $this->getScope() ?: ScopeConfigInterface::SCOPE_TYPE_DEFAULT,
+            $this->getScopeCode()
+        );
+
+        if (is_array($value)) {
+            return $this->serializer->serialize($value);
+        }
+
+        return (string)$value;
+    }
 }

app/code/Magento/Config/Test/Unit/Model/Config/Backend/SerializedTest.php

@@ -8,6 +8,7 @@
 namespace Magento\Config\Test\Unit\Model\Config\Backend;
 
 use Magento\Config\Model\Config\Backend\Serialized;
+use Magento\Framework\App\Config\ScopeConfigInterface;
 use Magento\Framework\Event\ManagerInterface;
 use Magento\Framework\Model\Context;
 use Magento\Framework\Serialize\Serializer\Json;
@@ -27,11 +28,14 @@ class SerializedTest extends TestCase
     /** @var LoggerInterface|MockObject */
     private $loggerMock;
 
+    private $scopeConfigMock;
+
     protected function setUp(): void
     {
         $objectManager = new ObjectManager($this);
         $this->serializerMock = $this->createMock(Json::class);
         $this->loggerMock = $this->getMockForAbstractClass(LoggerInterface::class);
+        $this->scopeConfigMock = $this->createMock(ScopeConfigInterface::class);
         $contextMock = $this->createMock(Context::class);
         $eventManagerMock = $this->getMockForAbstractClass(ManagerInterface::class);
         $contextMock->method('getEventDispatcher')
@@ -43,6 +47,7 @@ protected function setUp(): void
             [
                 'serializer' => $this->serializerMock,
                 'context' => $contextMock,
+                'config' => $this->scopeConfigMock,
             ]
         );
     }
@@ -135,4 +140,29 @@ public function beforeSaveDataProvider()
             ]
         ];
     }
+
+    /**
+     * If a config value is not available in core_confid_data the defaults are
+     * loaded from the config.xml file. Those defaults may be arrays.
+     * The Serialized backend model has to override its parent
+     * getOldValue function, to prevent an array to string conversion error
+     * and serialize those values.
+     */
+    public function testGetOldValueWithNonScalarDefaultValue(): void
+    {
+        $value = [
+            ['foo' => '1', 'bar' => '2'],
+        ];
+        $serializedValue = \json_encode($value);
+
+        $this->scopeConfigMock->method('getValue')->willReturn($value);
+        $this->serializerMock->method('serialize')->willReturn($serializedValue);
+
+        $this->serializedConfig->setData('value', $serializedValue);
+
+        $oldValue = $this->serializedConfig->getOldValue();
+
+        $this->assertIsString($oldValue, 'Default value from the config is not serialized.');
+        $this->assertSame($serializedValue, $oldValue);
+    }
 }