Make code-dependent cache entries more volatile

If a cache entry is dependent on the code that's running, upgrading
(or enabling/disabling plugins) can generate hard-to-track
inconsistencies.

This change adds a close-to-unique fingerprint of the running code to
some cache keys, so that if the fingerprint changes, the old values
are ignored and new values are used.

If the automated uniqueness fails, an administrator can add an extra
config value, $config['site']['build'], that's thrown into the key also.

lib/cache.php

@@ -86,6 +86,55 @@ class Cache
         return 'statusnet:' . $base_key . ':' . $extra;
     }
 
+    /**
+     * Create a cache key for data dependent on code
+     *
+     * For cache elements that are dependent on changes in code, this creates
+     * a more-or-less fingerprint of the current running code and adds it to
+     * the cache key. In the case of an upgrade of core, or addition or
+     * removal of plugins, a new unique fingerprint is generated and used.
+     * 
+     * There can still be problems with a) differences in versions of the 
+     * plugins and b) people running code between official versions. This is
+     * usually a problem only for experienced users like developers, who know
+     * how to clear their cache.
+     *
+     * For sites that run code between versions (like the status.net cloud),
+     * there's an additional build number configuration setting.
+     * 
+     * @param string $extra the real part of the key
+     *
+     * @return string full key
+     */
+    
+    static function codeKey($extra)
+    {
+        static $prefix = null;
+	
+        if (empty($prefix)) {
+	    
+            $plugins     = StatusNet::getActivePlugins();
+            $names       = array();
+	    
+            foreach ($plugins as $plugin) {
+                $names[] = $plugin[0];
+            }
+	    
+            $names = array_unique($names);
+            asort($names);
+	    
+            // Unique enough.
+	
+            $uniq = crc32(implode(',', $names));
+
+            $build = common_config('site', 'build');
+
+            $prefix = STATUSNET_VERSION.':'.$build.':'.$uniq;
+        }
+	
+        return Cache::key($prefix.':'.$extra);
+    }
+    
     /**
      * Make a string suitable for use as a key
      *

lib/default.php

@@ -59,7 +59,8 @@ $default =
               'textlimit' => 140,
               'indent' => true,
               'use_x_sendfile' => false,
-              'notice' => null // site wide notice text
+              'notice' => null, // site wide notice text
+              'build' => 1, // build number, for code-dependent cache
               ),
         'db' =>
         array('database' => 'YOU HAVE TO SET THIS IN config.php',