this is a better solution to the previous committed fix. This solution works...

this is a better solution to the previous committed fix. This solution works in all cases (even when not running tests) and makes it useable for other CacheFile classes as well. In addition it keeps the dependencies in the PersistenceCache class

core/Cache/PersistentCache.php

@@ -96,7 +96,7 @@ class PersistentCache
 
     public static function _reset()
     {
-        self::$content = null;
+        self::$content = array();
     }
 
     /**
@@ -106,6 +106,9 @@ class PersistentCache
     {
         if (is_null(self::$storage)) {
             self::$storage = new CacheFile('tracker', 43200);
+            self::$storage->addOnDeleteCallback(function () {
+                PersistentCache::_reset();
+            });
         }
 
         return self::$storage;

core/CacheFile.php

@@ -36,6 +36,11 @@ class CacheFile
      */
     const MINIMUM_TTL = 60;
 
+    /**
+     * @var \Callable[]
+     */
+    private static $onDeleteCallback = array();
+
     /**
      * @param string $directory directory to use
      * @param int $timeToLiveInSeconds TTL
@@ -182,6 +187,11 @@ class CacheFile
         return false;
     }
 
+    public function addOnDeleteCallback($onDeleteCallback)
+    {
+        self::$onDeleteCallback[] = $onDeleteCallback;
+    }
+
     /**
      * A function to delete all cache entries in the directory
      */
@@ -193,6 +203,12 @@ class CacheFile
         };
 
         Filesystem::unlinkRecursive($this->cachePath, $deleteRootToo = false, $beforeUnlink);
+
+        if (!empty(self::$onDeleteCallback)) {
+            foreach (self::$onDeleteCallback as $callback) {
+                $callback();
+            }
+        }
     }
 
     public function opCacheInvalidate($filepath)

tests/PHPUnit/Fixture.php

@@ -179,7 +179,6 @@ class Fixture extends PHPUnit_Framework_Assert
         Piwik::setUserHasSuperUserAccess();
 
         Cache::deleteTrackerCache();
-        \Piwik\Cache\PersistentCache::_reset();
 
         static::loadAllPlugins($this->getTestEnvironment(), $this->testCaseClass);