# Copyright (c) 2013 Red Hat, Inc.
#
# 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.

"""This module implements a common Pipeline object.

The pipeline can be used to enhance the storage layer with filtering, routing,
multiplexing and the like. For example:

    >>> stages = [MessageFilter(), EncryptionFilter(), QueueController()]
    >>> pipeline = Pipeline(stages)

Every stage has to implement the method it wants to hook into. This method
will be called when the pipeline consumption gets to that point - stage
ordering matters - and will continue unless the method call returns a value
that is not None.

At least one of the stages has to implement the calling method. If none of
them do, an AttributeError exception will be raised.
"""

import contextlib

from oslo_log import log as logging
import six

from zaqar.common import decorators
from zaqar.i18n import _

LOG = logging.getLogger(__name__)


class Pipeline(object):

    def __init__(self, pipeline=None):
        self._pipeline = pipeline and list(pipeline) or []

    @decorators.memoized_getattr
    def __getattr__(self, name):
        with self.consumer_for(name) as consumer:
            return consumer

    @contextlib.contextmanager
    def consumer_for(self, method):
        """Creates a closure for `method`

        This method creates a closure to consume the pipeline
        for `method`.

        :params method: The method name to call on each stage
        :type method: `six.text_type`

        :returns: A callable to consume the pipeline.
        """

        def consumer(*args, **kwargs):
            """Consumes the pipeline for `method`

            This function walks through the pipeline and calls
            `method` for each of the items in the pipeline. A
            warning will be logged for each stage not implementing
            `method` and an Attribute error will be raised if
            none of the stages do.

            :param args: Positional arguments to pass to the call.
            :param kwargs: Keyword arguments to pass to the call.

            :raises: AttributeError if none of the stages implement `method`
            """
            # NOTE(flaper87): Used as a way to verify
            # the requested method exists in at least
            # one of the stages, otherwise AttributeError
            # will be raised.
            target = None
            result = None

            for stage in self._pipeline:
                try:
                    target = getattr(stage, method)
                except AttributeError:
                    sstage = six.text_type(stage)
                    msgtmpl = _(u"Stage %(stage)s does not "
                                "implement %(method)s")
                    LOG.debug(msgtmpl, {'stage': sstage, 'method': method})
                    continue

                tmp = target(*args, **kwargs)

                # NOTE(flaper87): preserve the last, not None, result
                if tmp is not None:
                    result = tmp

                # NOTE(flaper87): Will keep going forward
                # through the stageline unless the call returns
                # something.
                if result is not None:
                    return result

            if target is None:
                msg = _(u'Method %s not found in any of '
                        'the registered stages') % method
                LOG.error(msg)
                raise AttributeError(msg)

        yield consumer
