diff -u b/core/includes/install.core.inc b/core/includes/install.core.inc
--- b/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -436,7 +436,7 @@
     $task = NULL;
 
     // Do not install over a configured settings.php.
-    if (!empty($GLOBALS['databases']) && !drupal_valid_test_ua()) {
+    if (!empty($GLOBALS['databases'])) {
       throw new Exception(install_already_done_error());
     }
   }
@@ -1079,9 +1079,8 @@
   $database['namespace'] = substr($install_namespace, 0, strrpos($install_namespace, '\\'));
   $database['driver'] = $driver;
 
-  // TODO: remove when PIFR will be updated to use 'db_prefix' instead of
-  // 'prefix' in the database settings form with the non-interactive
-  // installer.
+  // TODO: PIFR uses 'db_prefix' instead of 'prefix'. Remove this when it gets
+  // fixed.
   if (!$test_prefix) {
     $database['prefix'] = $database['db_prefix'];
   }
@@ -1146,8 +1145,14 @@
   // Update global settings array and save.
   $settings = array();
   $database = $form_state['storage']['database'];
+  // Ideally, there is no difference between the code executed by the
+  // automated test browser and an ordinary browser. However, the database
+  // settings need a different format and also need to skip the password
+  // when testing. The hash salt also needs to be skipped because the original
+  // salt is used to verify the validity of the automated test browser.
+  // Because of these, there's a little difference in the code following but
+  // it is small and self-contained.
   if ($test_prefix = drupal_valid_test_ua()) {
-    // This is test specific code, however it is very small and contained.
     foreach ($form_state['storage']['database'] as $k => $v) {
       if ($k != 'password') {
         $settings['databases']['default']['default'][$k] = (object) array(
@@ -1158,15 +1163,10 @@
     }
   }
   else {
-    // Because of the test specific code above, this assignment here must be
-    // kept simple and must not call any functions to avoid creating a tested
-    // and a non-tested code path.
     $settings['databases']['default']['default'] = (object) array(
       'value'    => $database,
       'required' => TRUE,
     );
-    // Tests must not override the hash salt so that the user agent can be
-    // verified.
     $settings['drupal_hash_salt'] = (object) array(
       'value'    => drupal_hash_base64(drupal_random_bytes(55)),
       'required' => TRUE,
diff -u b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
--- b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/InstallerTest.php
@@ -102,8 +102,8 @@
     config('system.mail')->set('interface.default', 'Drupal\Core\Mail\VariableLog')->save();
 
     drupal_set_time_limit($this->timeLimit);
-    // Temporary fix so that when running from run-tests.sh we don't get an
-    // empty current path which would indicate we're on the home page.
+    // When running from run-tests.sh we don't get an mpty current path which
+    // would indicate we're on the home page.
     $path = current_path();
     if (empty($path)) {
       _current_path('run-tests');
@@ -112,10 +112,11 @@
   }
 
   /**
-   * Overrides \Drupal\simpletest\WebTestBase::drupalGet().
+   * Overrides \Drupal\simpletest\WebTestBase::refreshVariables().
    *
-   * @todo
-   *   Why do we need to override this?
+   * During setup(), drupalPost costs refreshVariables which tries to read
+   * variables which are not yet there because the child Drupal is not yet
+   * installed.
    */
   protected function refreshVariables() {
     if (!empty($this->setup)) {
@@ -127,11 +128,10 @@
    * Overrides \Drupal\simpletest\WebTestBase::drupalGet().
    *
    * This override is necessary because the parent drupalGet() calls t(), which
-   * is not availble early during installation.
+   * is not available early during installation.
    */
   protected function drupalGet($path, array $options = array(), array $headers = array()) {
-
-    // We are  re-using a CURL connection here. If that connection still has
+    // We are re-using a CURL connection here. If that connection still has
     // certain options set, it might change the GET into a POST. Make sure we
     // clear out previous options.
     $out = $this->curlExec(array(CURLOPT_HTTPGET => TRUE, CURLOPT_URL => $this->getAbsoluteUrl($path), CURLOPT_NOBODY => FALSE, CURLOPT_HTTPHEADER => $headers));
only in patch2:
unchanged:
--- a/core/includes/errors.inc
+++ b/core/includes/errors.inc
@@ -136,7 +136,9 @@ function _drupal_decode_exception($exception) {
  *   An error message.
  */
 function _drupal_render_exception_safe($exception) {
-  return check_plain(strtr('%type: !message in %function (line %line of %file).', _drupal_decode_exception($exception)));
+  $decode = _drupal_decode_exception($exception);
+  unset($decode['backtrace']);
+  return check_plain(strtr('%type: !message in %function (line %line of %file).', $decode));
 }
 
 /**