bugfix: Resolve issue with incorrect self URL when using symlinks.

Recent commits have introduced a new way to obtain the self URL, honouring whatever is specified in 'baseurlpath'. However, this new code breaks when accessing SimpleSAMLphp through a path containing symbolic links in the file system, since the base directory refers always to the real path while the $_SERVER contents reflect what the web server sees (symlinks included). We use realpath() to convert a path with symlinks to a canonical path that we can compare.

lib/SimpleSAML/Utils/HTTP.php

@@ -722,7 +722,7 @@ class HTTP
         $url = self::getBaseURL();
         $cfg = \SimpleSAML_Configuration::getInstance();
         $baseDir = $cfg->getBaseDir();
-        $rel_path = str_replace($baseDir.'www/', '', $_SERVER['SCRIPT_FILENAME']);
+        $rel_path = str_replace($baseDir.'www/', '', realpath($_SERVER['SCRIPT_FILENAME']));
         $pos = strpos($_SERVER['REQUEST_URI'], $rel_path) + strlen($rel_path);
         return $url.$rel_path.substr($_SERVER['REQUEST_URI'], $pos);
     }