Rework for PR #16222 .

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

@@ -5,8 +5,10 @@
  */
 namespace Magento\Config\Model;
 
+use Magento\Config\Model\Config\Reader\Source\Deployed\SettingChecker;
 use Magento\Config\Model\Config\Structure\Element\Group;
 use Magento\Config\Model\Config\Structure\Element\Field;
+use Magento\Framework\App\ObjectManager;
 
 /**
  * Backend config model
@@ -80,6 +82,11 @@ class Config extends \Magento\Framework\DataObject
      */
     protected $_storeManager;
 
+    /**
+     * @var Config\Reader\Source\Deployed\SettingChecker
+     */
+    private $settingChecker;
+
     /**
      * @param \Magento\Framework\App\Config\ReinitableConfigInterface $config
      * @param \Magento\Framework\Event\ManagerInterface $eventManager
@@ -88,6 +95,7 @@ class Config extends \Magento\Framework\DataObject
      * @param \Magento\Config\Model\Config\Loader $configLoader
      * @param \Magento\Framework\App\Config\ValueFactory $configValueFactory
      * @param \Magento\Store\Model\StoreManagerInterface $storeManager
+     * @param Config\Reader\Source\Deployed\SettingChecker|null $settingChecker
      * @param array $data
      */
     public function __construct(
@@ -98,6 +106,7 @@ public function __construct(
         \Magento\Config\Model\Config\Loader $configLoader,
         \Magento\Framework\App\Config\ValueFactory $configValueFactory,
         \Magento\Store\Model\StoreManagerInterface $storeManager,
+        SettingChecker $settingChecker = null,
         array $data = []
     ) {
         parent::__construct($data);
@@ -108,6 +117,7 @@ public function __construct(
         $this->_configLoader = $configLoader;
         $this->_configValueFactory = $configValueFactory;
         $this->_storeManager = $storeManager;
+        $this->settingChecker = $settingChecker ?: ObjectManager::getInstance()->get(SettingChecker::class);
     }
 
     /**
@@ -351,6 +361,16 @@ protected function _processGroup(
             // use extra memory
             $fieldsetData = [];
             foreach ($groupData['fields'] as $fieldId => $fieldData) {
+                $isReadOnly = $this->settingChecker->isReadOnly(
+                    $groupPath . '/' . $fieldId,
+                    $this->getScope(),
+                    $this->getScopeCode()
+                );
+
+                if ($isReadOnly) {
+                    continue;
+                }
+
                 $field = $this->getField($sectionPath, $groupId, $fieldId);
                 /** @var \Magento\Framework\App\Config\ValueInterface $backendModel */
                 $backendModel = $field->hasBackendModel()

app/code/Magento/Config/Test/Unit/Model/ConfigTest.php

@@ -60,6 +60,11 @@ class ConfigTest extends \PHPUnit\Framework\TestCase
      */
     protected $_configStructure;
 
+    /**
+     * @var \PHPUnit_Framework_MockObject_MockObject
+     */
+    private $_settingsChecker;
+
     protected function setUp()
     {
         $this->_eventManagerMock = $this->createMock(\Magento\Framework\Event\ManagerInterface::class);
@@ -79,7 +84,7 @@ protected function setUp()
 
         $this->_transFactoryMock = $this->createPartialMock(
             \Magento\Framework\DB\TransactionFactory::class,
-            ['create']
+            ['create', 'addObject']
         );
         $this->_appConfigMock = $this->createMock(\Magento\Framework\App\Config\ReinitableConfigInterface::class);
         $this->_configLoaderMock = $this->createPartialMock(
@@ -90,14 +95,18 @@ protected function setUp()
 
         $this->_storeManager = $this->getMockForAbstractClass(\Magento\Store\Model\StoreManagerInterface::class);
 
+        $this->_settingsChecker = $this
+            ->createMock(\Magento\Config\Model\Config\Reader\Source\Deployed\SettingChecker::class);
+
         $this->_model = new \Magento\Config\Model\Config(
             $this->_appConfigMock,
             $this->_eventManagerMock,
             $this->_configStructure,
             $this->_transFactoryMock,
             $this->_configLoaderMock,
             $this->_dataFactoryMock,
-            $this->_storeManager
+            $this->_storeManager,
+            $this->_settingsChecker
         );
     }
 
@@ -149,6 +158,49 @@ public function testSaveToCheckAdminSystemConfigChangedSectionEvent()
         $this->_model->save();
     }
 
+    public function testDoNotSaveReadOnlyFields()
+    {
+        $transactionMock = $this->createMock(\Magento\Framework\DB\Transaction::class);
+        $this->_transFactoryMock->expects($this->any())->method('create')->will($this->returnValue($transactionMock));
+
+        $this->_settingsChecker->expects($this->any())->method('isReadOnly')->will($this->returnValue(true));
+        $this->_configLoaderMock->expects($this->any())->method('getConfigByPath')->will($this->returnValue([]));
+
+        $this->_model->setGroups(['1' => ['fields' => ['key' => ['data']]]]);
+        $this->_model->setSection('section');
+
+        $group = $this->createMock(\Magento\Config\Model\Config\Structure\Element\Group::class);
+        $group->method('getPath')->willReturn('section/1');
+
+        $field = $this->createMock(\Magento\Config\Model\Config\Structure\Element\Field::class);
+        $field->method('getGroupPath')->willReturn('section/1');
+        $field->method('getId')->willReturn('key');
+
+        $this->_configStructure->expects($this->at(0))
+            ->method('getElement')
+            ->with('section/1')
+            ->will($this->returnValue($group));
+        $this->_configStructure->expects($this->at(1))
+            ->method('getElement')
+            ->with('section/1')
+            ->will($this->returnValue($group));
+        $this->_configStructure->expects($this->at(2))
+            ->method('getElement')
+            ->with('section/1/key')
+            ->will($this->returnValue($field));
+
+        $backendModel = $this->createPartialMock(
+            \Magento\Framework\App\Config\Value::class,
+            ['addData']
+        );
+        $this->_dataFactoryMock->expects($this->any())->method('create')->will($this->returnValue($backendModel));
+
+        $this->_transFactoryMock->expects($this->never())->method('addObject');
+        $backendModel->expects($this->never())->method('addData');
+
+        $this->_model->save();
+    }
+
     public function testSaveToCheckScopeDataSet()
     {
         $transactionMock = $this->createMock(\Magento\Framework\DB\Transaction::class);