Returns (usually) writable directory for an application cache

def get_global_cache_dir(appname='default', ensure=False):
    """ Returns (usually) writable directory for an application cache """
    if appname is None or  appname == 'default':
        appname = get_default_appname()
    global_cache_dir = util_cplat.get_app_resource_dir(appname,
                                                       meta_util_constants.global_cache_dname)
    if ensure:
        util_path.ensuredir(global_cache_dir)
    return global_cache_dir