iotronic/iotronic/common/driver_factory.py

# Copyright 2013 Red Hat, Inc.
# All Rights Reserved.
#
#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#
#         http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.

from oslo_concurrency import lockutils
from oslo_config import cfg
from oslo_log import log
from stevedore import dispatch

from iotronic.common import exception
from iotronic.common.i18n import _LI


LOG = log.getLogger(__name__)

driver_opts = [
    cfg.ListOpt('enabled_drivers',
                default=['pxe_ipmitool'],
                help='Specify the list of drivers to load during service '
                     'initialization. Missing drivers, or drivers which '
                     'fail to initialize, will prevent the conductor '
                     'service from starting. The option default is a '
                     'recommended set of production-oriented drivers. A '
                     'complete list of drivers present on your system may '
                     'be found by enumerating the "iotronic.drivers" '
                     'entrypoint. An example may be found in the '
                     'developer documentation online.'),
]

CONF = cfg.CONF
CONF.register_opts(driver_opts)

EM_SEMAPHORE = 'extension_manager'


def get_driver(driver_name):
    """Simple method to get a ref to an instance of a driver.

    Driver loading is handled by the DriverFactory class. This method
    conveniently wraps that class and returns the actual driver object.

    :param driver_name: the name of the driver class to load
    :returns: An instance of a class which implements
              iotronic.drivers.base.BaseDriver
    :raises: DriverNotFound if the requested driver_name could not be
             found in the "iotronic.drivers" namespace.

    """

    try:
        factory = DriverFactory()
        return factory[driver_name].obj
    except KeyError:
        raise exception.DriverNotFound(driver_name=driver_name)


def drivers():
    """Get all drivers as a dict name -> driver object."""
    factory = DriverFactory()
    return {name: factory[name].obj for name in factory.names}


class DriverFactory(object):
    """Discover, load and manage the drivers available."""

    # NOTE(deva): loading the _extension_manager as a class member will break
    #             stevedore when it loads a driver, because the driver will
    #             import this file (and thus instantiate another factory).
    #             Instead, we instantiate a NameDispatchExtensionManager only
    #             once, the first time DriverFactory.__init__ is called.
    _extension_manager = None

    def __init__(self):
        if not DriverFactory._extension_manager:
            DriverFactory._init_extension_manager()

    def __getitem__(self, name):
        return self._extension_manager[name]

    # NOTE(deva): Use lockutils to avoid a potential race in eventlet
    #             that might try to create two driver factories.
    @classmethod
    @lockutils.synchronized(EM_SEMAPHORE, 'iotronic-')
    def _init_extension_manager(cls):
        # NOTE(deva): In case multiple greenthreads queue up on this lock
        #             before _extension_manager is initialized, prevent
        #             creation of multiple NameDispatchExtensionManagers.
        if cls._extension_manager:
            return

        # NOTE(deva): Drivers raise "DriverLoadError" if they are unable to be
        #             loaded, eg. due to missing external dependencies.
        #             We capture that exception, and, only if it is for an
        #             enabled driver, raise it from here. If enabled driver
        #             raises other exception type, it is wrapped in
        #             "DriverLoadError", providing the name of the driver that
        #             caused it, and raised. If the exception is for a
        #             non-enabled driver, we suppress it.
        def _catch_driver_not_found(mgr, ep, exc):
            # NOTE(deva): stevedore loads plugins *before* evaluating
            #             _check_func, so we need to check here, too.
            if ep.name in CONF.enabled_drivers:
                if not isinstance(exc, exception.DriverLoadError):
                    raise exception.DriverLoadError(driver=ep.name, reason=exc)
                raise exc

        def _check_func(ext):
            return ext.name in CONF.enabled_drivers

        cls._extension_manager = (
            dispatch.NameDispatchExtensionManager(
                'iotronic.drivers',
                _check_func,
                invoke_on_load=True,
                on_load_failure_callback=_catch_driver_not_found))

        # NOTE(deva): if we were unable to load any configured driver, perhaps
        #             because it is not present on the system, raise an error.
        if (sorted(CONF.enabled_drivers) !=
                sorted(cls._extension_manager.names())):
            found = cls._extension_manager.names()
            names = [n for n in CONF.enabled_drivers if n not in found]
            # just in case more than one could not be found ...
            names = ', '.join(names)
            raise exception.DriverNotFound(driver_name=names)

        LOG.info(_LI("Loaded the following drivers: %s"),
                 cls._extension_manager.names())

    @property
    def names(self):
        """The list of driver names available."""
        return self._extension_manager.names()