0.4 - D7 support: set $conf['cache'] properly

While trying to test #233091: Move code from update.php into includes/update.inc, I was told that my patch yielded a WSOD (white screen of death: no output in the browser window). After removing the patch and testing in a clean environment, I was able to reproduce the issue (so my patch is not the issue).

Here's how to reproduce the issue:

1. install a clean/simple 6.13 Drupal
2. untar a D7 devel snapshot (I am using current CVS, but that should be fairly similar)
3. copy over the sites directory
4. point your domain to the d7 snapshot directory
5. visit update.php

Expected result: the nice little drupal update system

Actual result: blank page.

While debugging this, I was able to find out the error is actually in cache.in:

Fatal error: Class name must be a valid object or a string in /var/hostmaster/drupal-7.x-git/includes/cache.inc on line 25

I was able to find this problem with the following trivial patch:

--- a/update.php
+++ b/update.php
@@ -653,7 +653,7 @@ function update_check_requirements() {
 
 // Some unavoidable errors happen because the database is not yet up-to-date.
 // Our custom error handler is not yet installed, so we just suppress them.
-ini_set('display_errors', FALSE);
+//ini_set('display_errors', FALSE);
 
 // We prepare a minimal bootstrap for the update requirements check to avoid
 // reaching the PHP memory limit.

(I find that instruction rather silly too...)

Here's a backtrace:

Array
(
    [0] => Array
        (
            [file] => /var/hostmaster/drupal-7.x-git/includes/cache.inc
            [line] => 46
            [function] => _cache_get_object
            [args] => Array
                (
                    [0] => cache
                )

        )

    [1] => Array
        (
            [file] => /var/hostmaster/drupal-7.x-git/includes/bootstrap.inc
            [line] => 670
            [function] => cache_get
            [args] => Array
                (
                    [0] => variables
                    [1] => cache
                )

        )

    [2] => Array
        (
            [file] => /var/hostmaster/drupal-7.x-git/includes/bootstrap.inc
            [line] => 1437
            [function] => variable_initialize
            [args] => Array
                (
                    [0] => Array
                        (
                            [file_directory_path] => sites/testd7-upg.hostmaster.koumbit.net/files
                            [file_directory_temp] => sites/testd7-upg.hostmaster.koumbit.net/files/tmp
                            [file_downloads] => 1
                            [cache] => 1
                            [clean_url] => 1
                        )

                )

        )

    [3] => Array
        (
            [file] => /var/hostmaster/drupal-7.x-git/includes/bootstrap.inc
            [line] => 1361
            [function] => _drupal_bootstrap
            [args] => Array
                (
                    [0] => 5
                )

        )

    [4] => Array
        (
            [file] => /var/hostmaster/drupal-7.x-git/update.php
            [line] => 697
            [function] => drupal_bootstrap
            [args] => Array
                (
                    [0] => 9
                )

        )

)

Interestingly enough, variable_init is called with [cache] => 1 although when I look at the code, I feel it shouldn't be that:

function _cache_get_object($bin) {
  // We do not use drupal_static() here because we do not want to change the
  // storage of a cache bin mid-request.
  static $cache_objects, $default_class;
  if (!isset($cache_objects[$bin])) {
    $class = variable_get($bin, FALSE);
    if (empty($class)) {
      $class = variable_get('cache_default_class', 'DrupalDatabaseCache');
    }
    $cache_objects[$bin] = new $class($bin); ##### <= this is what provokes the error, $class = 1 here, because $bin = 'cache'
  }
  return $cache_objects[$bin];
}

Maybe i'm just sitting in the tree in a wrong turn or something...