Update to Drupal 8.1.8. For more information, see https://www.drupal.org/project/drupal/releases/8.1.8

core/modules/config/config.install

@@ -0,0 +1,31 @@
+<?php
+
+/**
+ * @file
+ * Install, update and uninstall functions for the config module.
+ */
+
+/**
+* Implements hook_requirements().
+*/
+function config_requirements($phase) {
+  $requirements = [];
+  try {
+    $directory = config_get_config_directory(CONFIG_SYNC_DIRECTORY);
+  }
+  catch (\Exception $e) {
+    // system_requirements() guarantees that the CONFIG_SYNC_DIRECTORY exists
+    // as the config.storage.staging service relies on it.
+    $directory = FALSE;
+  }
+  // Ensure the configuration sync directory is writable. This is only a warning
+  // because only configuration import from a tarball requires the folder to be
+  // web writable.
+  if ($phase !== 'install' && !is_writable($directory)) {
+    $requirements['config directory ' . CONFIG_SYNC_DIRECTORY] = [
+      'title' => t('The directory %directory is not writable.', ['%directory' => $directory]),
+      'severity' => REQUIREMENT_WARNING,
+    ];
+  }
+  return $requirements;
+}

core/modules/config/src/Form/ConfigImportForm.php

@@ -50,6 +50,11 @@ class ConfigImportForm extends FormBase {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    $directory = config_get_config_directory(CONFIG_SYNC_DIRECTORY);
+    $directory_is_writable = is_writable($directory);
+    if (!$directory_is_writable) {
+      drupal_set_message($this->t('The directory %directory is not writable.', ['%directory' => $directory]), 'error');
+    }
     $form['import_tarball'] = array(
       '#type' => 'file',
       '#title' => $this->t('Configuration archive'),
@@ -59,6 +64,7 @@ class ConfigImportForm extends FormBase {
     $form['submit'] = array(
       '#type' => 'submit',
       '#value' => $this->t('Upload'),
+      '#disabled' => !$directory_is_writable,
     );
     return $form;
   }

core/modules/config/src/Tests/ConfigImportUploadTest.php

@@ -45,6 +45,16 @@ class ConfigImportUploadTest extends WebTestBase {
     $edit = array('files[import_tarball]' => drupal_realpath($text_file->uri));
     $this->drupalPostForm('admin/config/development/configuration/full/import', $edit, t('Upload'));
     $this->assertText(t('Could not extract the contents of the tar file'));
+
+    // Make the sync directory read-only.
+    $directory = config_get_config_directory(CONFIG_SYNC_DIRECTORY);
+    \Drupal::service('file_system')->chmod($directory, 0555);
+    $this->drupalGet('admin/config/development/configuration/full/import');
+    $this->assertRaw(t('The directory %directory is not writable.', ['%directory' => $directory]));
+    // Ensure submit button for \Drupal\config\Form\ConfigImportForm is
+    // disabled.
+    $submit_is_disabled = $this->cssSelect('form.config-import-form input[type="submit"]:disabled');
+    $this->assertTrue(count($submit_is_disabled) === 1, 'The submit button is disabled.');
   }
 
 }

core/modules/config/src/Tests/ConfigInstallWebTest.php

@@ -26,7 +26,7 @@ class ConfigInstallWebTest extends WebTestBase {
   protected function setUp() {
     parent::setUp();
 
-    $this->adminUser = $this->drupalCreateUser(array('administer modules', 'administer themes'));
+    $this->adminUser = $this->drupalCreateUser(array('administer modules', 'administer themes', 'administer site configuration'));
 
     // Ensure the global variable being asserted by this test does not exist;
     // a previous test executed in this request/process might have set it.
@@ -188,4 +188,22 @@ class ConfigInstallWebTest extends WebTestBase {
     $this->assertTrue(entity_load('config_test', 'other_module_test_with_dependency'), 'The config_test.dynamic.other_module_test_with_dependency configuration has been created during install.');
   }
 
+  /**
+   * Tests config_requirements().
+   */
+  public function testConfigModuleRequirements() {
+    $this->drupalLogin($this->adminUser);
+    $this->drupalPostForm('admin/modules', array('modules[Core][config][enable]' => TRUE), t('Install'));
+
+    $directory = config_get_config_directory(CONFIG_SYNC_DIRECTORY);
+    file_unmanaged_delete_recursive($directory);
+    $this->drupalGet('/admin/reports/status');
+    $this->assertRaw(t('The directory %directory does not exist.', array('%directory' => $directory)));
+
+    file_prepare_directory($directory, FILE_CREATE_DIRECTORY);
+    \Drupal::service('file_system')->chmod($directory, 0555);
+    $this->drupalGet('/admin/reports/status');
+    $this->assertRaw(t('The directory %directory is not writable.', ['%directory' => $directory]));
+  }
+
 }

core/modules/config/tests/config_test/config_test.routing.yml

@@ -21,7 +21,7 @@ entity.config_test.edit_form:
   requirements:
     _permission: 'administer site configuration'
 
-entity.config_test.edit_form_config_test_no_status:
+entity.config_test_no_status.edit_form:
   path: '/admin/structure/config_test/manage/{config_test_no_status}'
   defaults:
     _entity_form: config_test_no_status