"""Consul result store backend.

- :class:`ConsulBackend` implements KeyValueStoreBackend to store results
    in the key-value store of Consul.
"""
from kombu.utils.encoding import bytes_to_str
from kombu.utils.url import parse_url

from celery.backends.base import KeyValueStoreBackend
from celery.exceptions import ImproperlyConfigured
from celery.utils.log import get_logger

try:
    import consul
except ImportError:
    consul = None

logger = get_logger(__name__)

__all__ = ('ConsulBackend',)

CONSUL_MISSING = """\
You need to install the python-consul library in order to use \
the Consul result store backend."""


class ConsulBackend(KeyValueStoreBackend):
    """Consul.io K/V store backend for Celery."""

    consul = consul

    supports_autoexpire = True

    client = None
    consistency = 'consistent'
    path = None

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        if self.consul is None:
            raise ImproperlyConfigured(CONSUL_MISSING)

        self._init_from_params(**parse_url(self.url))

    def _init_from_params(self, hostname, port, virtual_host, **params):
        logger.debug('Setting on Consul client to connect to %s:%d',
                     hostname, port)
        self.path = virtual_host
        self.client = consul.Consul(host=hostname, port=port,
                                    consistency=self.consistency)

    def _key_to_consul_key(self, key):
        key = bytes_to_str(key)
        return key if self.path is None else f'{self.path}/{key}'

    def get(self, key):
        key = self._key_to_consul_key(key)
        logger.debug('Trying to fetch key %s from Consul', key)
        try:
            _, data = self.client.kv.get(key)
            return data['Value']
        except TypeError:
            pass

    def mget(self, keys):
        for key in keys:
            yield self.get(key)

    def set(self, key, value):
        """Set a key in Consul.

        Before creating the key it will create a session inside Consul
        where it creates a session with a TTL

        The key created afterwards will reference to the session's ID.

        If the session expires it will remove the key so that results
        can auto expire from the K/V store
        """
        session_name = bytes_to_str(key)

        key = self._key_to_consul_key(key)

        logger.debug('Trying to create Consul session %s with TTL %d',
                     session_name, self.expires)
        session_id = self.client.session.create(name=session_name,
                                                behavior='delete',
                                                ttl=self.expires)
        logger.debug('Created Consul session %s', session_id)

        logger.debug('Writing key %s to Consul', key)
        return self.client.kv.put(key=key,
                                  value=value,
                                  acquire=session_id)

    def delete(self, key):
        key = self._key_to_consul_key(key)
        logger.debug('Removing key %s from Consul', key)
        return self.client.kv.delete(key)