Issue #3421731 by larowlan, byrond: field_field_config_create should check if...

Issue #3421731 by larowlan, byrond: field_field_config_create should check if the config installer is syncing

core/modules/field/field.module

@@ -392,6 +392,9 @@ function field_field_storage_config_update(FieldStorageConfigInterface $field_st
  * Determine the selection handler plugin ID for an entity reference field.
  */
 function field_field_config_create(FieldConfigInterface $field) {
+  if ($field->isSyncing()) {
+    return;
+  }
   // Act on all sub-types of the entity_reference field type.
   /** @var \Drupal\Core\Field\FieldTypePluginManager $field_type_manager */
   $field_type_manager = \Drupal::service('plugin.manager.field.field_type');

core/modules/field/tests/src/Kernel/EntityReference/EntityReferenceItemTest.php

@@ -404,6 +404,17 @@ public function testSelectionHandlerSettings() {
     $field_storage->save();
     $field = FieldConfig::load($field->id());
     $this->assertEquals('views', $field->getSetting('handler'));
+
+    // Check that selection handlers aren't changed during sync.
+    $field = FieldConfig::create([
+      'field_storage' => $field_storage,
+      'bundle' => 'entity_test',
+      'settings' => [
+        'handler' => 'fake:thing',
+      ],
+      'isSyncing' => TRUE,
+    ]);
+    $this->assertEquals('fake:thing', $field->getSetting('handler'));
   }
 
   /**