Description
After installing Drupal, revisiting the install page renders a blank page.
The Problem
It seems, the user_template_preprocess_default_variables_alter is tring to clone the $user variable without checking to make sure it has been initialized.
Reproducing the problem
This one is pretty easy to reproduce. Install a fresh copy of drupal and visit the newly installed site. Next, go back and visit the install page and you will get a blank page.
The Solution
Check for the $user variable first.
+ // Make sure the user object exists.
+ if (!isset($user)) { $user = drupal_anonymous_user(); }
+
$variables['user'] = clone $user;Conclusion
Whether or not this function should ever be called during the installation process is one issue but since cloning an uninitialized variable causes php to fault and exit (at least it does on my system), it's probably a good idea to make sure it is initialized first.