connection.py

from __future__ import with_statement
from distutils.version import StrictVersion
from itertools import chain
import os
import socket
import sys
import threading
import warnings

try:
    import ssl
    ssl_available = True
except ImportError:
    ssl_available = False

from redis._compat import (b, xrange, imap, byte_to_chr, unicode, bytes, long,
                           BytesIO, nativestr, basestring, iteritems,
                           LifoQueue, Empty, Full, urlparse, parse_qs,
                           recv, recv_into, select, unquote)
from redis.exceptions import (
    RedisError,
    ConnectionError,
    TimeoutError,
    BusyLoadingError,
    ResponseError,
    InvalidResponse,
    AuthenticationError,
    NoScriptError,
    ExecAbortError,
    ReadOnlyError
)
from redis.utils import HIREDIS_AVAILABLE
if HIREDIS_AVAILABLE:
    import hiredis

    hiredis_version = StrictVersion(hiredis.__version__)
    HIREDIS_SUPPORTS_CALLABLE_ERRORS = \
        hiredis_version >= StrictVersion('0.1.3')
    HIREDIS_SUPPORTS_BYTE_BUFFER = \
        hiredis_version >= StrictVersion('0.1.4')

    if not HIREDIS_SUPPORTS_BYTE_BUFFER:
        msg = ("redis-py works best with hiredis >= 0.1.4. You're running "
               "hiredis %s. Please consider upgrading." % hiredis.__version__)
        warnings.warn(msg)

    HIREDIS_USE_BYTE_BUFFER = True
    # only use byte buffer if hiredis supports it and the Python version
    # is >= 2.7
    if not HIREDIS_SUPPORTS_BYTE_BUFFER or (
            sys.version_info[0] == 2 and sys.version_info[1] < 7):
        HIREDIS_USE_BYTE_BUFFER = False

SYM_STAR = b('*')
SYM_DOLLAR = b('$')
SYM_CRLF = b('\r\n')
SYM_EMPTY = b('')

SERVER_CLOSED_CONNECTION_ERROR = "Connection closed by server."


class Token(object):
    """
    Literal strings in Redis commands, such as the command names and any
    hard-coded arguments are wrapped in this class so we know not to apply
    and encoding rules on them.
    """

    _cache = {}

    @classmethod
    def get_token(cls, value):
        "Gets a cached token object or creates a new one if not already cached"

        # Use try/except because after running for a short time most tokens
        # should already be cached
        try:
            return cls._cache[value]
        except KeyError:
            token = Token(value)
            cls._cache[value] = token
            return token

    def __init__(self, value):
        if isinstance(value, Token):
            value = value.value
        self.value = value
        self.encoded_value = b(value)

    def __repr__(self):
        return self.value

    def __str__(self):
        return self.value


class Encoder(object):
    "Encode strings to bytes and decode bytes to strings"

    def __init__(self, encoding, encoding_errors, decode_responses):
        self.encoding = encoding
        self.encoding_errors = encoding_errors
        self.decode_responses = decode_responses

    def encode(self, value):
        "Return a bytestring representation of the value"
        if isinstance(value, Token):
            return value.encoded_value
        elif isinstance(value, bytes):
            return value
        elif isinstance(value, (int, long)):
            value = b(str(value))
        elif isinstance(value, float):
            value = b(repr(value))
        elif not isinstance(value, basestring):
            # an object we don't know how to deal with. default to unicode()
            value = unicode(value)
        if isinstance(value, unicode):
            value = value.encode(self.encoding, self.encoding_errors)
        return value

    def decode(self, value, force=False):
        "Return a unicode string from the byte representation"
        if (self.decode_responses or force) and isinstance(value, bytes):
            value = value.decode(self.encoding, self.encoding_errors)
        return value


class BaseParser(object):
    EXCEPTION_CLASSES = {
        'ERR': {
            'max number of clients reached': ConnectionError
        },
        'EXECABORT': ExecAbortError,
        'LOADING': BusyLoadingError,
        'NOSCRIPT': NoScriptError,
        'READONLY': ReadOnlyError,
    }

    def parse_error(self, response):
        "Parse an error response"
        error_code = response.split(' ')[0]
        if error_code in self.EXCEPTION_CLASSES:
            response = response[len(error_code) + 1:]
            exception_class = self.EXCEPTION_CLASSES[error_code]
            if isinstance(exception_class, dict):
                exception_class = exception_class.get(response, ResponseError)
            return exception_class(response)
        return ResponseError(response)


class SocketBuffer(object):
    def __init__(self, socket, socket_read_size):
        self._sock = socket
        self.socket_read_size = socket_read_size
        self._buffer = BytesIO()
        # number of bytes written to the buffer from the socket
        self.bytes_written = 0
        # number of bytes read from the buffer
        self.bytes_read = 0

    @property
    def length(self):
        return self.bytes_written - self.bytes_read

    def _read_from_socket(self, length=None):
        socket_read_size = self.socket_read_size
        buf = self._buffer
        buf.seek(self.bytes_written)
        marker = 0

        try:
            while True:
                data = recv(self._sock, socket_read_size)
                # an empty string indicates the server shutdown the socket
                if isinstance(data, bytes) and len(data) == 0:
                    raise socket.error(SERVER_CLOSED_CONNECTION_ERROR)
                buf.write(data)
                data_length = len(data)
                self.bytes_written += data_length
                marker += data_length

                if length is not None and length > marker:
                    continue
                break
        except socket.timeout:
            raise TimeoutError("Timeout reading from socket")
        except socket.error:
            e = sys.exc_info()[1]
            raise ConnectionError("Error while reading from socket: %s" %
                                  (e.args,))

    def read(self, length):
        length = length + 2  # make sure to read the \r\n terminator
        # make sure we've read enough data from the socket
        if length > self.length:
            self._read_from_socket(length - self.length)

        self._buffer.seek(self.bytes_read)
        data = self._buffer.read(length)
        self.bytes_read += len(data)

        # purge the buffer when we've consumed it all so it doesn't
        # grow forever
        if self.bytes_read == self.bytes_written:
            self.purge()

        return data[:-2]

    def readline(self):
        buf = self._buffer
        buf.seek(self.bytes_read)
        data = buf.readline()
        while not data.endswith(SYM_CRLF):
            # there's more data in the socket that we need
            self._read_from_socket()
            buf.seek(self.bytes_read)
            data = buf.readline()

        self.bytes_read += len(data)

        # purge the buffer when we've consumed it all so it doesn't
        # grow forever
        if self.bytes_read == self.bytes_written:
            self.purge()

        return data[:-2]

    def purge(self):
        self._buffer.seek(0)
        self._buffer.truncate()
        self.bytes_written = 0
        self.bytes_read = 0

    def close(self):
        try:
            self.purge()
            self._buffer.close()
        except:
            # issue #633 suggests the purge/close somehow raised a
            # BadFileDescriptor error. Perhaps the client ran out of
            # memory or something else? It's probably OK to ignore
            # any error being raised from purge/close since we're
            # removing the reference to the instance below.
            pass
        self._buffer = None
        self._sock = None


class PythonParser(BaseParser):
    "Plain Python parsing class"
    def __init__(self, socket_read_size):
        self.socket_read_size = socket_read_size
        self.encoder = None
        self._sock = None
        self._buffer = None

    def __del__(self):
        try:
            self.on_disconnect()
        except Exception:
            pass

    def on_connect(self, connection):
        "Called when the socket connects"
        self._sock = connection._sock
        self._buffer = SocketBuffer(self._sock, self.socket_read_size)
        self.encoder = connection.encoder

    def on_disconnect(self):
        "Called when the socket disconnects"
        if self._sock is not None:
            self._sock.close()
            self._sock = None
        if self._buffer is not None:
            self._buffer.close()
            self._buffer = None
        self.encoder = None

    def can_read(self):
        return self._buffer and bool(self._buffer.length)

    def read_response(self):
        response = self._buffer.readline()
        if not response:
            raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)

        byte, response = byte_to_chr(response[0]), response[1:]

        if byte not in ('-', '+', ':', '$', '*'):
            raise InvalidResponse("Protocol Error: %s, %s" %
                                  (str(byte), str(response)))

        # server returned an error
        if byte == '-':
            response = nativestr(response)
            error = self.parse_error(response)
            # if the error is a ConnectionError, raise immediately so the user
            # is notified
            if isinstance(error, ConnectionError):
                raise error
            # otherwise, we're dealing with a ResponseError that might belong
            # inside a pipeline response. the connection's read_response()
            # and/or the pipeline's execute() will raise this error if
            # necessary, so just return the exception instance here.
            return error
        # single value
        elif byte == '+':
            pass
        # int value
        elif byte == ':':
            response = long(response)
        # bulk response
        elif byte == '$':
            length = int(response)
            if length == -1:
                return None
            response = self._buffer.read(length)
        # multi-bulk response
        elif byte == '*':
            length = int(response)
            if length == -1:
                return None
            response = [self.read_response() for i in xrange(length)]
        if isinstance(response, bytes):
            response = self.encoder.decode(response)
        return response


class HiredisParser(BaseParser):
    "Parser class for connections using Hiredis"
    def __init__(self, socket_read_size):
        if not HIREDIS_AVAILABLE:
            raise RedisError("Hiredis is not installed")
        self.socket_read_size = socket_read_size

        if HIREDIS_USE_BYTE_BUFFER:
            self._buffer = bytearray(socket_read_size)

    def __del__(self):
        try:
            self.on_disconnect()
        except Exception:
            pass

    def on_connect(self, connection):
        self._sock = connection._sock
        kwargs = {
            'protocolError': InvalidResponse,
            'replyError': self.parse_error,
        }

        # hiredis < 0.1.3 doesn't support functions that create exceptions
        if not HIREDIS_SUPPORTS_CALLABLE_ERRORS:
            kwargs['replyError'] = ResponseError

        if connection.encoder.decode_responses:
            kwargs['encoding'] = connection.encoder.encoding
        self._reader = hiredis.Reader(**kwargs)
        self._next_response = False

    def on_disconnect(self):
        self._sock = None
        self._reader = None
        self._next_response = False

    def can_read(self):
        if not self._reader:
            raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)

        if self._next_response is False:
            self._next_response = self._reader.gets()
        return self._next_response is not False

    def read_response(self):
        if not self._reader:
            raise ConnectionError(SERVER_CLOSED_CONNECTION_ERROR)

        # _next_response might be cached from a can_read() call
        if self._next_response is not False:
            response = self._next_response
            self._next_response = False
            return response

        response = self._reader.gets()
        socket_read_size = self.socket_read_size
        while response is False:
            try:
                if HIREDIS_USE_BYTE_BUFFER:
                    bufflen = recv_into(self._sock, self._buffer)
                    if bufflen == 0:
                        raise socket.error(SERVER_CLOSED_CONNECTION_ERROR)
                else:
                    buffer = recv(self._sock, socket_read_size)
                    # an empty string indicates the server shutdown the socket
                    if not isinstance(buffer, bytes) or len(buffer) == 0:
                        raise socket.error(SERVER_CLOSED_CONNECTION_ERROR)
            except socket.timeout:
                raise TimeoutError("Timeout reading from socket")
            except socket.error:
                e = sys.exc_info()[1]
                raise ConnectionError("Error while reading from socket: %s" %
                                      (e.args,))
            if HIREDIS_USE_BYTE_BUFFER:
                self._reader.feed(self._buffer, 0, bufflen)
            else:
                self._reader.feed(buffer)
            response = self._reader.gets()
        # if an older version of hiredis is installed, we need to attempt
        # to convert ResponseErrors to their appropriate types.
        if not HIREDIS_SUPPORTS_CALLABLE_ERRORS:
            if isinstance(response, ResponseError):
                response = self.parse_error(response.args[0])
            elif isinstance(response, list) and response and \
                    isinstance(response[0], ResponseError):
                response[0] = self.parse_error(response[0].args[0])
        # if the response is a ConnectionError or the response is a list and
        # the first item is a ConnectionError, raise it as something bad
        # happened
        if isinstance(response, ConnectionError):
            raise response
        elif isinstance(response, list) and response and \
                isinstance(response[0], ConnectionError):
            raise response[0]
        return response


if HIREDIS_AVAILABLE:
    DefaultParser = HiredisParser
else:
    DefaultParser = PythonParser


class Connection(object):
    "Manages TCP communication to and from a Redis server"
    description_format = "Connection<host=%(host)s,port=%(port)s,db=%(db)s>"

    def __init__(self, host='localhost', port=6379, db=0, password=None,
                 socket_timeout=None, socket_connect_timeout=None,
                 socket_keepalive=False, socket_keepalive_options=None,
                 socket_type=0, retry_on_timeout=False, encoding='utf-8',
                 encoding_errors='strict', decode_responses=False,
                 parser_class=DefaultParser, socket_read_size=65536):
        self.pid = os.getpid()
        self.host = host
        self.port = int(port)
        self.db = db
        self.password = password
        self.socket_timeout = socket_timeout
        self.socket_connect_timeout = socket_connect_timeout or socket_timeout
        self.socket_keepalive = socket_keepalive
        self.socket_keepalive_options = socket_keepalive_options or {}
        self.socket_type = socket_type
        self.retry_on_timeout = retry_on_timeout
        self.encoder = Encoder(encoding, encoding_errors, decode_responses)
        self._sock = None
        self._parser = parser_class(socket_read_size=socket_read_size)
        self._description_args = {
            'host': self.host,
            'port': self.port,
            'db': self.db,
        }
        self._connect_callbacks = []

    def __repr__(self):
        return self.description_format % self._description_args

    def __del__(self):
        try:
            self.disconnect()
        except Exception:
            pass

    def register_connect_callback(self, callback):
        self._connect_callbacks.append(callback)

    def clear_connect_callbacks(self):
        self._connect_callbacks = []

    def connect(self):
        "Connects to the Redis server if not already connected"
        if self._sock:
            return
        try:
            sock = self._connect()
        except socket.timeout:
            raise TimeoutError("Timeout connecting to server")
        except socket.error:
            e = sys.exc_info()[1]
            raise ConnectionError(self._error_message(e))

        self._sock = sock
        try:
            self.on_connect()
        except RedisError:
            # clean up after any error in on_connect
            self.disconnect()
            raise

        # run any user callbacks. right now the only internal callback
        # is for pubsub channel/pattern resubscription
        for callback in self._connect_callbacks:
            callback(self)

    def _connect(self):
        "Create a TCP socket connection"
        # we want to mimic what socket.create_connection does to support
        # ipv4/ipv6, but we want to set options prior to calling
        # socket.connect()
        err = None
        for res in socket.getaddrinfo(self.host, self.port, self.socket_type,
                                      socket.SOCK_STREAM):
            family, socktype, proto, canonname, socket_address = res
            sock = None
            try:
                sock = socket.socket(family, socktype, proto)
                # TCP_NODELAY
                sock.setsockopt(socket.IPPROTO_TCP, socket.TCP_NODELAY, 1)

                # TCP_KEEPALIVE
                if self.socket_keepalive:
                    sock.setsockopt(socket.SOL_SOCKET, socket.SO_KEEPALIVE, 1)
                    for k, v in iteritems(self.socket_keepalive_options):
                        sock.setsockopt(socket.SOL_TCP, k, v)

                # set the socket_connect_timeout before we connect
                sock.settimeout(self.socket_connect_timeout)

                # connect
                sock.connect(socket_address)

                # set the socket_timeout now that we're connected
                sock.settimeout(self.socket_timeout)
                return sock

            except socket.error as _:
                err = _
                if sock is not None:
                    sock.close()

        if err is not None:
            raise err
        raise socket.error("socket.getaddrinfo returned an empty list")

    def _error_message(self, exception):
        # args for socket.error can either be (errno, "message")
        # or just "message"
        if len(exception.args) == 1:
            return "Error connecting to %s:%s. %s." % \
                (self.host, self.port, exception.args[0])
        else:
            return "Error %s connecting to %s:%s. %s." % \
                (exception.args[0], self.host, self.port, exception.args[1])

    def on_connect(self):
        "Initialize the connection, authenticate and select a database"
        self._parser.on_connect(self)

        # if a password is specified, authenticate
        if self.password:
            self.send_command('AUTH', self.password)
            if nativestr(self.read_response()) != 'OK':
                raise AuthenticationError('Invalid Password')

        # if a database is specified, switch to it
        if self.db:
            self.send_command('SELECT', self.db)
            if nativestr(self.read_response()) != 'OK':
                raise ConnectionError('Invalid Database')

    def disconnect(self):
        "Disconnects from the Redis server"
        self._parser.on_disconnect()
        if self._sock is None:
            return
        try:
            self._sock.shutdown(socket.SHUT_RDWR)
            self._sock.close()
        except socket.error:
            pass
        self._sock = None

    def send_packed_command(self, command):
        "Send an already packed command to the Redis server"
        if not self._sock:
            self.connect()
        try:
            if isinstance(command, str):
                command = [command]
            for item in command:
                self._sock.sendall(item)
        except socket.timeout:
            self.disconnect()
            raise TimeoutError("Timeout writing to socket")
        except socket.error:
            e = sys.exc_info()[1]
            self.disconnect()
            if len(e.args) == 1:
                errno, errmsg = 'UNKNOWN', e.args[0]
            else:
                errno = e.args[0]
                errmsg = e.args[1]
            raise ConnectionError("Error %s while writing to socket. %s." %
                                  (errno, errmsg))
        except:
            self.disconnect()
            raise

    def send_command(self, *args):
        "Pack and send a command to the Redis server"
        self.send_packed_command(self.pack_command(*args))

    def can_read(self, timeout=0):
        "Poll the socket to see if there's data that can be read."
        sock = self._sock
        if not sock:
            self.connect()
            sock = self._sock
        return self._parser.can_read() or \
            bool(select([sock], [], [], timeout)[0])

    def read_response(self):
        "Read the response from a previously sent command"
        try:
            response = self._parser.read_response()
        except:
            self.disconnect()
            raise
        if isinstance(response, ResponseError):
            raise response
        return response

    def pack_command(self, *args):
        "Pack a series of arguments into the Redis protocol"
        output = []
        # the client might have included 1 or more literal arguments in
        # the command name, e.g., 'CONFIG GET'. The Redis server expects these
        # arguments to be sent separately, so split the first argument
        # manually. All of these arguements get wrapped in the Token class
        # to prevent them from being encoded.
        command = args[0]
        if ' ' in command:
            args = tuple([Token.get_token(s)
                          for s in command.split()]) + args[1:]
        else:
            args = (Token.get_token(command),) + args[1:]

        buff = SYM_EMPTY.join(
            (SYM_STAR, b(str(len(args))), SYM_CRLF))

        for arg in imap(self.encoder.encode, args):
            # to avoid large string mallocs, chunk the command into the
            # output list if we're sending large values
            if len(buff) > 6000 or len(arg) > 6000:
                buff = SYM_EMPTY.join(
                    (buff, SYM_DOLLAR, b(str(len(arg))), SYM_CRLF))
                output.append(buff)
                output.append(arg)
                buff = SYM_CRLF
            else:
                buff = SYM_EMPTY.join((buff, SYM_DOLLAR, b(str(len(arg))),
                                       SYM_CRLF, arg, SYM_CRLF))
        output.append(buff)
        return output

    def pack_commands(self, commands):
        "Pack multiple commands into the Redis protocol"
        output = []
        pieces = []
        buffer_length = 0

        for cmd in commands:
            for chunk in self.pack_command(*cmd):
                pieces.append(chunk)
                buffer_length += len(chunk)

            if buffer_length > 6000:
                output.append(SYM_EMPTY.join(pieces))
                buffer_length = 0
                pieces = []

        if pieces:
            output.append(SYM_EMPTY.join(pieces))
        return output


class SSLConnection(Connection):
    description_format = "SSLConnection<host=%(host)s,port=%(port)s,db=%(db)s>"

    def __init__(self, ssl_keyfile=None, ssl_certfile=None, ssl_cert_reqs=None,
                 ssl_ca_certs=None, **kwargs):
        if not ssl_available:
            raise RedisError("Python wasn't built with SSL support")

        super(SSLConnection, self).__init__(**kwargs)

        self.keyfile = ssl_keyfile
        self.certfile = ssl_certfile
        if ssl_cert_reqs is None:
            ssl_cert_reqs = ssl.CERT_NONE
        elif isinstance(ssl_cert_reqs, basestring):
            CERT_REQS = {
                'none': ssl.CERT_NONE,
                'optional': ssl.CERT_OPTIONAL,
                'required': ssl.CERT_REQUIRED
            }
            if ssl_cert_reqs not in CERT_REQS:
                raise RedisError(
                    "Invalid SSL Certificate Requirements Flag: %s" %
                    ssl_cert_reqs)
            ssl_cert_reqs = CERT_REQS[ssl_cert_reqs]
        self.cert_reqs = ssl_cert_reqs
        self.ca_certs = ssl_ca_certs

    def _connect(self):
        "Wrap the socket with SSL support"
        sock = super(SSLConnection, self)._connect()
        sock = ssl.wrap_socket(sock,
                               cert_reqs=self.cert_reqs,
                               keyfile=self.keyfile,
                               certfile=self.certfile,
                               ca_certs=self.ca_certs)
        return sock


class UnixDomainSocketConnection(Connection):
    description_format = "UnixDomainSocketConnection<path=%(path)s,db=%(db)s>"

    def __init__(self, path='', db=0, password=None,
                 socket_timeout=None, encoding='utf-8',
                 encoding_errors='strict', decode_responses=False,
                 retry_on_timeout=False,
                 parser_class=DefaultParser, socket_read_size=65536):
        self.pid = os.getpid()
        self.path = path
        self.db = db
        self.password = password
        self.socket_timeout = socket_timeout
        self.retry_on_timeout = retry_on_timeout
        self.encoder = Encoder(encoding, encoding_errors, decode_responses)
        self._sock = None
        self._parser = parser_class(socket_read_size=socket_read_size)
        self._description_args = {
            'path': self.path,
            'db': self.db,
        }
        self._connect_callbacks = []

    def _connect(self):
        "Create a Unix domain socket connection"
        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
        sock.settimeout(self.socket_timeout)
        sock.connect(self.path)
        return sock

    def _error_message(self, exception):
        # args for socket.error can either be (errno, "message")
        # or just "message"
        if len(exception.args) == 1:
            return "Error connecting to unix socket: %s. %s." % \
                (self.path, exception.args[0])
        else:
            return "Error %s connecting to unix socket: %s. %s." % \
                (exception.args[0], self.path, exception.args[1])


FALSE_STRINGS = ('0', 'F', 'FALSE', 'N', 'NO')


def to_bool(value):
    if value is None or value == '':
        return None
    if isinstance(value, basestring) and value.upper() in FALSE_STRINGS:
        return False
    return bool(value)


URL_QUERY_ARGUMENT_PARSERS = {
    'socket_timeout': float,
    'socket_connect_timeout': float,
    'socket_keepalive': to_bool,
    'retry_on_timeout': to_bool
}


class ConnectionPool(object):
    "Generic connection pool"
    @classmethod
    def from_url(cls, url, db=None, decode_components=False, **kwargs):
        """
        Return a connection pool configured from the given URL.

        For example::

            redis://[:password]@localhost:6379/0
            rediss://[:password]@localhost:6379/0
            unix://[:password]@/path/to/socket.sock?db=0

        Three URL schemes are supported:

        - ```redis://``
          <http://www.iana.org/assignments/uri-schemes/prov/redis>`_ creates a
          normal TCP socket connection
        - ```rediss://``
          <http://www.iana.org/assignments/uri-schemes/prov/rediss>`_ creates a
          SSL wrapped TCP socket connection
        - ``unix://`` creates a Unix Domain Socket connection

        There are several ways to specify a database number. The parse function
        will return the first specified option:
            1. A ``db`` querystring option, e.g. redis://localhost?db=0
            2. If using the redis:// scheme, the path argument of the url, e.g.
               redis://localhost/0
            3. The ``db`` argument to this function.

        If none of these options are specified, db=0 is used.

        The ``decode_components`` argument allows this function to work with
        percent-encoded URLs. If this argument is set to ``True`` all ``%xx``
        escapes will be replaced by their single-character equivalents after
        the URL has been parsed. This only applies to the ``hostname``,
        ``path``, and ``password`` components.

        Any additional querystring arguments and keyword arguments will be
        passed along to the ConnectionPool class's initializer. The querystring
        arguments ``socket_connect_timeout`` and ``socket_timeout`` if supplied
        are parsed as float values. The arguments ``socket_keepalive`` and
        ``retry_on_timeout`` are parsed to boolean values that accept
        True/False, Yes/No values to indicate state. Invalid types cause a
        ``UserWarning`` to be raised. In the case of conflicting arguments,
        querystring arguments always win.
        """
        url_string = url
        url = urlparse(url)
        qs = ''

        # in python2.6, custom URL schemes don't recognize querystring values
        # they're left as part of the url.path.
        if '?' in url.path and not url.query:
            # chop the querystring including the ? off the end of the url
            # and reparse it.
            qs = url.path.split('?', 1)[1]
            url = urlparse(url_string[:-(len(qs) + 1)])
        else:
            qs = url.query

        url_options = {}

        for name, value in iteritems(parse_qs(qs)):
            if value and len(value) > 0:
                parser = URL_QUERY_ARGUMENT_PARSERS.get(name)
                if parser:
                    try:
                        url_options[name] = parser(value[0])
                    except (TypeError, ValueError):
                        warnings.warn(UserWarning(
                            "Invalid value for `%s` in connection URL." % name
                        ))
                else:
                    url_options[name] = value[0]

        if decode_components:
            password = unquote(url.password) if url.password else None
            path = unquote(url.path) if url.path else None
            hostname = unquote(url.hostname) if url.hostname else None
        else:
            password = url.password
            path = url.path
            hostname = url.hostname

        # We only support redis:// and unix:// schemes.
        if url.scheme == 'unix':
            url_options.update({
                'password': password,
                'path': path,
                'connection_class': UnixDomainSocketConnection,
            })

        else:
            url_options.update({
                'host': hostname,
                'port': int(url.port or 6379),
                'password': password,
            })

            # If there's a path argument, use it as the db argument if a
            # querystring value wasn't specified
            if 'db' not in url_options and path:
                try:
                    url_options['db'] = int(path.replace('/', ''))
                except (AttributeError, ValueError):
                    pass

            if url.scheme == 'rediss':
                url_options['connection_class'] = SSLConnection

        # last shot at the db value
        url_options['db'] = int(url_options.get('db', db or 0))

        # update the arguments from the URL values
        kwargs.update(url_options)

        # backwards compatability
        if 'charset' in kwargs:
            warnings.warn(DeprecationWarning(
                '"charset" is deprecated. Use "encoding" instead'))
            kwargs['encoding'] = kwargs.pop('charset')
        if 'errors' in kwargs:
            warnings.warn(DeprecationWarning(
                '"errors" is deprecated. Use "encoding_errors" instead'))
            kwargs['encoding_errors'] = kwargs.pop('errors')

        return cls(**kwargs)

    def __init__(self, connection_class=Connection, max_connections=None,
                 **connection_kwargs):
        """
        Create a connection pool. If max_connections is set, then this
        object raises redis.ConnectionError when the pool's limit is reached.

        By default, TCP connections are created unless connection_class is
        specified. Use redis.UnixDomainSocketConnection for unix sockets.

        Any additional keyword arguments are passed to the constructor of
        connection_class.
        """
        max_connections = max_connections or 2 ** 31
        if not isinstance(max_connections, (int, long)) or max_connections < 0:
            raise ValueError('"max_connections" must be a positive integer')

        self.connection_class = connection_class
        self.connection_kwargs = connection_kwargs
        self.max_connections = max_connections

        self.reset()

    def __repr__(self):
        return "%s<%s>" % (
            type(self).__name__,
            self.connection_class.description_format % self.connection_kwargs,
        )

    def reset(self):
        self.pid = os.getpid()
        self._created_connections = 0
        self._available_connections = []
        self._in_use_connections = set()
        self._check_lock = threading.Lock()

    def _checkpid(self):
        if self.pid != os.getpid():
            with self._check_lock:
                if self.pid == os.getpid():
                    # another thread already did the work while we waited
                    # on the lock.
                    return
                self.disconnect()
                self.reset()

    def get_connection(self, command_name, *keys, **options):
        "Get a connection from the pool"
        self._checkpid()
        try:
            connection = self._available_connections.pop()
        except IndexError:
            connection = self.make_connection()
        self._in_use_connections.add(connection)
        return connection

    def get_encoder(self):
        "Return an encoder based on encoding settings"
        kwargs = self.connection_kwargs
        return Encoder(
            encoding=kwargs.get('encoding', 'utf-8'),
            encoding_errors=kwargs.get('encoding_errors', 'strict'),
            decode_responses=kwargs.get('decode_responses', False)
        )

    def make_connection(self):
        "Create a new connection"
        if self._created_connections >= self.max_connections:
            raise ConnectionError("Too many connections")
        self._created_connections += 1
        return self.connection_class(**self.connection_kwargs)

    def release(self, connection):
        "Releases the connection back to the pool"
        self._checkpid()
        if connection.pid != self.pid:
            return
        self._in_use_connections.remove(connection)
        self._available_connections.append(connection)

    def disconnect(self):
        "Disconnects all connections in the pool"
        all_conns = chain(self._available_connections,
                          self._in_use_connections)
        for connection in all_conns:
            connection.disconnect()


class BlockingConnectionPool(ConnectionPool):
    """
    Thread-safe blocking connection pool::

        >>> from redis.client import Redis
        >>> client = Redis(connection_pool=BlockingConnectionPool())

    It performs the same function as the default
    ``:py:class: ~redis.connection.ConnectionPool`` implementation, in that,
    it maintains a pool of reusable connections that can be shared by
    multiple redis clients (safely across threads if required).

    The difference is that, in the event that a client tries to get a
    connection from the pool when all of connections are in use, rather than
    raising a ``:py:class: ~redis.exceptions.ConnectionError`` (as the default
    ``:py:class: ~redis.connection.ConnectionPool`` implementation does), it
    makes the client wait ("blocks") for a specified number of seconds until
    a connection becomes available.

    Use ``max_connections`` to increase / decrease the pool size::

        >>> pool = BlockingConnectionPool(max_connections=10)

    Use ``timeout`` to tell it either how many seconds to wait for a connection
    to become available, or to block forever:

        # Block forever.
        >>> pool = BlockingConnectionPool(timeout=None)

        # Raise a ``ConnectionError`` after five seconds if a connection is
        # not available.
        >>> pool = BlockingConnectionPool(timeout=5)
    """
    def __init__(self, max_connections=50, timeout=20,
                 connection_class=Connection, queue_class=LifoQueue,
                 **connection_kwargs):

        self.queue_class = queue_class
        self.timeout = timeout
        super(BlockingConnectionPool, self).__init__(
            connection_class=connection_class,
            max_connections=max_connections,
            **connection_kwargs)

    def reset(self):
        self.pid = os.getpid()
        self._check_lock = threading.Lock()

        # Create and fill up a thread safe queue with ``None`` values.
        self.pool = self.queue_class(self.max_connections)
        while True:
            try:
                self.pool.put_nowait(None)
            except Full:
                break

        # Keep a list of actual connection instances so that we can
        # disconnect them later.
        self._connections = []

    def make_connection(self):
        "Make a fresh connection."
        connection = self.connection_class(**self.connection_kwargs)
        self._connections.append(connection)
        return connection

    def get_connection(self, command_name, *keys, **options):
        """
        Get a connection, blocking for ``self.timeout`` until a connection
        is available from the pool.

        If the connection returned is ``None`` then creates a new connection.
        Because we use a last-in first-out queue, the existing connections
        (having been returned to the pool after the initial ``None`` values
        were added) will be returned before ``None`` values. This means we only
        create new connections when we need to, i.e.: the actual number of
        connections will only increase in response to demand.
        """
        # Make sure we haven't changed process.
        self._checkpid()

        # Try and get a connection from the pool. If one isn't available within
        # self.timeout then raise a ``ConnectionError``.
        connection = None
        try:
            connection = self.pool.get(block=True, timeout=self.timeout)
        except Empty:
            # Note that this is not caught by the redis client and will be
            # raised unless handled by application code. If you want never to
            raise ConnectionError("No connection available.")

        # If the ``connection`` is actually ``None`` then that's a cue to make
        # a new connection to add to the pool.
        if connection is None:
            connection = self.make_connection()

        return connection

    def release(self, connection):
        "Releases the connection back to the pool."
        # Make sure we haven't changed process.
        self._checkpid()
        if connection.pid != self.pid:
            return

        # Put the connection back into the pool.
        try:
            self.pool.put_nowait(connection)
        except Full:
            # perhaps the pool has been reset() after a fork? regardless,
            # we don't want this connection
            pass

    def disconnect(self):
        "Disconnects all connections in the pool."
        for connection in self._connections:
            connection.disconnect()