def loadplugin(plugin):

    """

    Imports, instantiates, and returns the authentication plugin in the module named by plugin

    (e.g., plugin='kallithea.lib.auth_modules.auth_internal'). Returns an instance of the

    KallitheaAuthPluginBase subclass on success, raises exceptions on failure.

    raises:

        AttributeError -- no KallitheaAuthPlugin class in the module

        TypeError -- if the KallitheaAuthPlugin is not a subclass of ours KallitheaAuthPluginBase

        ImportError -- if we couldn't import the plugin at all

    """

    log.debug("Importing %s", plugin)

    if not plugin.startswith(u'kallithea.lib.auth_modules.auth_'):

        parts = plugin.split(u'.lib.auth_modules.auth_', 1)

        if len(parts) == 2:

            _module, pn = parts

            plugin = u'kallithea.lib.auth_modules.auth_' + pn

    PLUGIN_CLASS_NAME = "KallitheaAuthPlugin"

    try:

        module = importlib.import_module(plugin)

    except (ImportError, TypeError):

        log.error(traceback.format_exc())

        # TODO: make this more error prone, if by some accident we screw up

        # the plugin name, the crash is pretty bad and hard to recover

        raise

    log.debug("Loaded auth plugin from %s (module:%s, file:%s)",

              plugin, module.__name__, module.__file__)

    pluginclass = getattr(module, PLUGIN_CLASS_NAME)

    if not issubclass(pluginclass, KallitheaAuthPluginBase):

        raise TypeError("Authentication class %s.KallitheaAuthPlugin is not "

                        "a subclass of %s" % (plugin, KallitheaAuthPluginBase))

    plugin = pluginclass()

    if plugin.plugin_settings.im_func != KallitheaAuthPluginBase.plugin_settings.im_func:

        raise TypeError("Authentication class %s.KallitheaAuthPluginBase "

                        "has overridden the plugin_settings method, which is "

                        "forbidden." % plugin)

    return plugin

def get_auth_plugins():

    """Return a list of instances of plugins that are available and enabled"""

    auth_plugins = []

    for plugin_name in Setting.get_by_name("auth_plugins").app_settings_value:

        try:

            plugin = loadplugin(plugin_name)

            log.exception('Failed to load authentication module %s' % (plugin_name))

        else:

            auth_plugins.append(plugin)

    return auth_plugins

def authenticate(username, password, environ=None):

    """

    Authentication function used for access control,

    It tries to authenticate based on enabled authentication modules.

    :param username: username can be empty for container auth

    :param password: password can be empty for container auth

    :param environ: environ headers passed for container auth

    :returns: None if auth failed, user_data dict if auth is correct

    """

    auth_plugins = get_auth_plugins()

    log.debug('Authentication against %s plugins', auth_plugins)

    for plugin in auth_plugins:

        module = plugin.__class__.__module__

        log.debug('Trying authentication using ** %s **', module)

        # load plugin settings from Kallithea database

        plugin_name = plugin.name

        plugin_settings = {}

        for v in plugin.plugin_settings():

            conf_key = "auth_%s_%s" % (plugin_name, v["name"])

            setting = Setting.get_by_name(conf_key)

            plugin_settings[v["name"]] = setting.app_settings_value if setting else None

        log.debug('Plugin settings \n%s', formatted_json(plugin_settings))

        if not str2bool(plugin_settings["enabled"]):

            log.info("Authentication plugin %s is disabled, skipping for %s",

                     module, username)

            continue

        # use plugin's method of user extraction.

        user = plugin.get_user(username, environ=environ,

                               settings=plugin_settings)

        log.debug('Plugin %s extracted user is `%s`', module, user)

        if not plugin.accepts(user):

            log.debug('Plugin %s does not accept user `%s` for authentication',

                      module, user)

            continue

        else:

            log.debug('Plugin %s accepted user `%s` for authentication',

                      module, user)

            # The user might have tried to authenticate using their email address,