MDL-25290 cache: Added optional CFG setting to control location of cache config file

cache/classes/config.php

@@ -105,6 +105,17 @@ class cache_config {
      */
     protected static function get_config_file_path() {
         global $CFG;
+        if (!empty($CFG->altcacheconfigpath)) {
+            $path = $CFG->altcacheconfigpath;
+            if (is_dir($path) && is_writable($path)) {
+                // Its a writable directory, thats fine.
+                return $path.'/cacheconfig.php';
+            } else if (is_writable(dirname($path)) && (!file_exists($path) || is_writable($path))) {
+                // Its a file, either it doesn't exist and the directory is writable or the file exists and is writable.
+                return $path;
+            }
+        }
+        // Return the default location within dataroot.
         return $CFG->dataroot.'/muc/config.php';
     }
 

config-dist.php

@@ -411,6 +411,17 @@ $CFG->admin = 'admin';
 //
 //     $CFG->extramemorylimit = '1G';
 //
+// Moodle 2.4 introduced a new cache API.
+// The cache API stores a configuration file within the Moodle data directory and
+// uses that rather than the database in order to function in a stand-alone manner.
+// Using altcacheconfigpath you can change the location where this config file is
+// looked for.
+// It can either be a directory in which to store the file, or the full path to the
+// file if you want to take full control. Either way it must be writable by the
+// webserver.
+//
+//     $CFG->altcacheconfigpath = '/var/www/shared/moodle.cache.config.php
+//
 // The CSS files the Moodle produces can be extremely large and complex, especially
 // if you are using a custom theme that builds upon several other themes.
 // In Moodle 2.3 a CSS optimiser was added as an experimental feature for advanced