# Copyright 2014 Mirantis 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.

"""Thread-safe connection pool for python-memcached."""

# NOTE(yorik-sar): this file is copied between keystone and keystonemiddleware
# and should be kept in sync until we can use external library for this.

import collections
import contextlib
import itertools
import time

from oslo_log import log as logging
from six.moves import queue

_PoolItem = collections.namedtuple('_PoolItem', ['ttl', 'connection'])


class ConnectionGetTimeoutException(Exception):
    pass


class ConnectionPool(queue.Queue):
    """Base connection pool class.

    This class implements the basic connection pool logic as an abstract base
    class.
    """

    def __init__(self, maxsize, unused_timeout, conn_get_timeout=None):
        """Initialize the connection pool.

        :param maxsize: maximum number of client connections for the pool
        :type maxsize: int
        :param unused_timeout: idle time to live for unused clients (in
                               seconds). If a client connection object has been
                               in the pool and idle for longer than the
                               unused_timeout, it will be reaped. This is to
                               ensure resources are released as utilization
                               goes down.
        :type unused_timeout: int
        :param conn_get_timeout: maximum time in seconds to wait for a
                                 connection. If set to `None` timeout is
                                 indefinite.
        :type conn_get_timeout: int
        """
        queue.Queue.__init__(self, maxsize)
        self._unused_timeout = unused_timeout
        self._connection_get_timeout = conn_get_timeout
        self._acquired = 0
        self._LOG = logging.getLogger(__name__)

    def _create_connection(self):
        raise NotImplementedError

    def _destroy_connection(self, conn):
        raise NotImplementedError

    @contextlib.contextmanager
    def acquire(self):
        try:
            conn = self.get(timeout=self._connection_get_timeout)
        except queue.Empty:
            self._LOG.critical('Unable to get a connection from pool id '
                               '%(id)s after %(seconds)s seconds.',
                               {'id': id(self),
                                'seconds': self._connection_get_timeout})
            raise ConnectionGetTimeoutException()
        try:
            yield conn
        finally:
            self.put(conn)

    def _qsize(self):
        return self.maxsize - self._acquired

    if not hasattr(queue.Queue, '_qsize'):
        qsize = _qsize

    def _get(self):
        if self.queue:
            conn = self.queue.pop().connection
        else:
            conn = self._create_connection()
        self._acquired += 1
        return conn

    def _put(self, conn):
        self.queue.append(_PoolItem(
            ttl=time.time() + self._unused_timeout,
            connection=conn,
        ))
        self._acquired -= 1
        # Drop all expired connections from the right end of the queue
        now = time.time()
        while self.queue and self.queue[0].ttl < now:
            conn = self.queue.popleft().connection
            self._destroy_connection(conn)


class MemcacheClientPool(ConnectionPool):
    def __init__(self, urls, dead_retry=None, socket_timeout=None, **kwargs):
        ConnectionPool.__init__(self, **kwargs)
        self._urls = urls
        self._dead_retry = dead_retry
        self._socket_timeout = socket_timeout

        # NOTE(morganfainberg): The host objects expect an int for the
        # deaduntil value. Initialize this at 0 for each host with 0 indicating
        # the host is not dead.
        self._hosts_deaduntil = [0] * len(urls)

        # NOTE(morganfainberg): Lazy import to allow middleware to work with
        # python 3k even if memcache will not due to python 3k
        # incompatibilities within the python-memcache library.
        global memcache
        import memcache

        # This 'class' is taken from http://stackoverflow.com/a/22520633/238308
        # Don't inherit client from threading.local so that we can reuse
        # clients in different threads
        MemcacheClient = type('_MemcacheClient', (object,),
                              dict(memcache.Client.__dict__))

        self._memcache_client_class = MemcacheClient

    def _create_connection(self):
        return self._memcache_client_class(self._urls,
                                           dead_retry=self._dead_retry,
                                           socket_timeout=self._socket_timeout)

    def _destroy_connection(self, conn):
        conn.disconnect_all()

    def _get(self):
        conn = ConnectionPool._get(self)
        try:
            # Propagate host state known to us to this client's list
            now = time.time()
            for deaduntil, host in zip(self._hosts_deaduntil, conn.servers):
                if deaduntil > now and host.deaduntil <= now:
                    host.mark_dead('propagating death mark from the pool')
                host.deaduntil = deaduntil
        except Exception:
            # We need to be sure that connection doesn't leak from the pool.
            # This code runs before we enter context manager's try-finally
            # block, so we need to explicitly release it here
            ConnectionPool._put(self, conn)
            raise
        return conn

    def _put(self, conn):
        try:
            # If this client found that one of the hosts is dead, mark it as
            # such in our internal list
            now = time.time()
            for i, deaduntil, host in zip(itertools.count(),
                                          self._hosts_deaduntil,
                                          conn.servers):
                # Do nothing if we already know this host is dead
                if deaduntil <= now:
                    if host.deaduntil > now:
                        self._hosts_deaduntil[i] = host.deaduntil
                    else:
                        self._hosts_deaduntil[i] = 0
            # If all hosts are dead we should forget that they're dead. This
            # way we won't get completely shut off until dead_retry seconds
            # pass, but will be checking servers as frequent as we can (over
            # way smaller socket_timeout)
            if all(deaduntil > now for deaduntil in self._hosts_deaduntil):
                self._hosts_deaduntil[:] = [0] * len(self._hosts_deaduntil)
        finally:
            ConnectionPool._put(self, conn)