# 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 logging
import threading
import time

import memcache
from six.moves import queue

from keystone import exception
from keystone.i18n import _
from keystone.openstack.common import log


LOG = log.getLogger(__name__)

# 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__))

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


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

    def _create_connection(self):
        """Returns a connection instance.

        This is called when the pool needs another instance created.

        :returns: a new connection instance

        """
        raise NotImplementedError

    def _destroy_connection(self, conn):
        """Destroy and cleanup a connection instance.

        This is called when the pool wishes to get rid of an existing
        connection. This is the opportunity for a subclass to free up
        resources and cleaup after itself.

        :param conn: the connection object to destroy

        """
        raise NotImplementedError

    def _debug_logger(self, msg, *args, **kwargs):
        if LOG.isEnabledFor(logging.DEBUG):
            thread_id = threading.current_thread().ident
            args = (id(self), thread_id) + args
            prefix = 'Memcached pool %s, thread %s: '
            LOG.debug(prefix + msg, *args, **kwargs)

    @contextlib.contextmanager
    def acquire(self):
        self._debug_logger('Acquiring connection')
        try:
            conn = self.get(timeout=self._connection_get_timeout)
        except queue.Empty:
            raise exception.UnexpectedError(
                _('Unable to get a connection from pool id %(id)s after '
                  '%(seconds)s seconds.') %
                {'id': id(self), 'seconds': self._connection_get_timeout})
        self._debug_logger('Acquired connection %s', id(conn))
        try:
            yield conn
        finally:
            self._debug_logger('Releasing connection %s', id(conn))
            self.put(conn)

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

    # NOTE(dstanek): stdlib and eventlet Queue implementations
    # have different names for the qsize method. This ensures
    # that we override both of them.
    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 _drop_expired_connections(self, conn):
        """Drop all expired connections from the right end of the queue.

        :param conn: connection object
        """
        now = time.time()
        while self.queue and self.queue[0].ttl < now:
            conn = self.queue.popleft().connection
            self._debug_logger('Reaping connection %s', id(conn))
            self._destroy_connection(conn)

    def _put(self, conn):
        self.queue.append(_PoolItem(
            ttl=time.time() + self._unused_timeout,
            connection=conn,
        ))
        self._acquired -= 1
        self._drop_expired_connections(conn)


class MemcacheClientPool(ConnectionPool):
    def __init__(self, urls, arguments, **kwargs):
        ConnectionPool.__init__(self, **kwargs)
        self.urls = urls
        self._arguments = arguments
        # 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)

    def _create_connection(self):
        return _MemcacheClient(self.urls, **self._arguments)

    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, host in zip(itertools.count(), conn.servers):
                deaduntil = self._hosts_deaduntil[i]
                # Do nothing if we already know this host is dead
                if deaduntil <= now:
                    if host.deaduntil > now:
                        self._hosts_deaduntil[i] = host.deaduntil
                        self._debug_logger(
                            'Marked host %s dead until %s',
                            self.urls[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._debug_logger('All hosts are dead. Marking them as live.')
                self._hosts_deaduntil[:] = [0] * len(self._hosts_deaduntil)
        finally:
            ConnectionPool._put(self, conn)
