IPython/kernel/zmq/session.py

"""Session object for building, serializing, sending, and receiving messages in
IPython. The Session object supports serialization, HMAC signatures, and
metadata on messages.

Also defined here are utilities for working with Sessions:
* A SessionFactory to be used as a base class for configurables that work with
Sessions.
* A Message object for convenience that allows attribute-access to the msg dict.

Authors:

* Min RK
* Brian Granger
* Fernando Perez
"""
#-----------------------------------------------------------------------------
#  Copyright (C) 2010-2011  The IPython Development Team
#
#  Distributed under the terms of the BSD License.  The full license is in
#  the file COPYING, distributed as part of this software.
#-----------------------------------------------------------------------------

#-----------------------------------------------------------------------------
# Imports
#-----------------------------------------------------------------------------

import hashlib
import hmac
import logging
import os
import pprint
import random
import uuid
from datetime import datetime

try:
    import cPickle
    pickle = cPickle
except:
    cPickle = None
    import pickle

import zmq
from zmq.utils import jsonapi
from zmq.eventloop.ioloop import IOLoop
from zmq.eventloop.zmqstream import ZMQStream

from IPython.config.configurable import Configurable, LoggingConfigurable
from IPython.utils import io
from IPython.utils.importstring import import_item
from IPython.utils.jsonutil import extract_dates, squash_dates, date_default
from IPython.utils.py3compat import (str_to_bytes, str_to_unicode, unicode_type,
                                     iteritems)
from IPython.utils.traitlets import (CBytes, Unicode, Bool, Any, Instance, Set,
                                        DottedObjectName, CUnicode, Dict, Integer,
                                        TraitError,
)
from IPython.kernel.zmq.serialize import MAX_ITEMS, MAX_BYTES

#-----------------------------------------------------------------------------
# utility functions
#-----------------------------------------------------------------------------

def squash_unicode(obj):
    """coerce unicode back to bytestrings."""
    if isinstance(obj,dict):
        for key in obj.keys():
            obj[key] = squash_unicode(obj[key])
            if isinstance(key, unicode_type):
                obj[squash_unicode(key)] = obj.pop(key)
    elif isinstance(obj, list):
        for i,v in enumerate(obj):
            obj[i] = squash_unicode(v)
    elif isinstance(obj, unicode_type):
        obj = obj.encode('utf8')
    return obj

#-----------------------------------------------------------------------------
# globals and defaults
#-----------------------------------------------------------------------------

# ISO8601-ify datetime objects
json_packer = lambda obj: jsonapi.dumps(obj, default=date_default)
json_unpacker = lambda s: jsonapi.loads(s)

pickle_packer = lambda o: pickle.dumps(squash_dates(o),-1)
pickle_unpacker = pickle.loads

default_packer = json_packer
default_unpacker = json_unpacker

DELIM = b"<IDS|MSG>"
# singleton dummy tracker, which will always report as done
DONE = zmq.MessageTracker()

#-----------------------------------------------------------------------------
# Mixin tools for apps that use Sessions
#-----------------------------------------------------------------------------

session_aliases = dict(
    ident = 'Session.session',
    user = 'Session.username',
    keyfile = 'Session.keyfile',
)

session_flags  = {
    'secure' : ({'Session' : { 'key' : str_to_bytes(str(uuid.uuid4())),
                            'keyfile' : '' }},
        """Use HMAC digests for authentication of messages.
        Setting this flag will generate a new UUID to use as the HMAC key.
        """),
    'no-secure' : ({'Session' : { 'key' : b'', 'keyfile' : '' }},
        """Don't authenticate messages."""),
}

def default_secure(cfg):
    """Set the default behavior for a config environment to be secure.
    
    If Session.key/keyfile have not been set, set Session.key to
    a new random UUID.
    """
    
    if 'Session' in cfg:
        if 'key' in cfg.Session or 'keyfile' in cfg.Session:
            return
    # key/keyfile not specified, generate new UUID:
    cfg.Session.key = str_to_bytes(str(uuid.uuid4()))


#-----------------------------------------------------------------------------
# Classes
#-----------------------------------------------------------------------------

class SessionFactory(LoggingConfigurable):
    """The Base class for configurables that have a Session, Context, logger,
    and IOLoop.
    """

    logname = Unicode('')
    def _logname_changed(self, name, old, new):
        self.log = logging.getLogger(new)

    # not configurable:
    context = Instance('zmq.Context')
    def _context_default(self):
        return zmq.Context.instance()

    session = Instance('IPython.kernel.zmq.session.Session')

    loop = Instance('zmq.eventloop.ioloop.IOLoop', allow_none=False)
    def _loop_default(self):
        return IOLoop.instance()

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

        if self.session is None:
            # construct the session
            self.session = Session(**kwargs)


class Message(object):
    """A simple message object that maps dict keys to attributes.

    A Message can be created from a dict and a dict from a Message instance
    simply by calling dict(msg_obj)."""

    def __init__(self, msg_dict):
        dct = self.__dict__
        for k, v in iteritems(dict(msg_dict)):
            if isinstance(v, dict):
                v = Message(v)
            dct[k] = v

    # Having this iterator lets dict(msg_obj) work out of the box.
    def __iter__(self):
        return iter(iteritems(self.__dict__))

    def __repr__(self):
        return repr(self.__dict__)

    def __str__(self):
        return pprint.pformat(self.__dict__)

    def __contains__(self, k):
        return k in self.__dict__

    def __getitem__(self, k):
        return self.__dict__[k]


def msg_header(msg_id, msg_type, username, session):
    date = datetime.now()
    return locals()

def extract_header(msg_or_header):
    """Given a message or header, return the header."""
    if not msg_or_header:
        return {}
    try:
        # See if msg_or_header is the entire message.
        h = msg_or_header['header']
    except KeyError:
        try:
            # See if msg_or_header is just the header
            h = msg_or_header['msg_id']
        except KeyError:
            raise
        else:
            h = msg_or_header
    if not isinstance(h, dict):
        h = dict(h)
    return h

class Session(Configurable):
    """Object for handling serialization and sending of messages.

    The Session object handles building messages and sending them
    with ZMQ sockets or ZMQStream objects.  Objects can communicate with each
    other over the network via Session objects, and only need to work with the
    dict-based IPython message spec. The Session will handle
    serialization/deserialization, security, and metadata.

    Sessions support configurable serialiization via packer/unpacker traits,
    and signing with HMAC digests via the key/keyfile traits.

    Parameters
    ----------

    debug : bool
        whether to trigger extra debugging statements
    packer/unpacker : str : 'json', 'pickle' or import_string
        importstrings for methods to serialize message parts.  If just
        'json' or 'pickle', predefined JSON and pickle packers will be used.
        Otherwise, the entire importstring must be used.

        The functions must accept at least valid JSON input, and output *bytes*.

        For example, to use msgpack:
        packer = 'msgpack.packb', unpacker='msgpack.unpackb'
    pack/unpack : callables
        You can also set the pack/unpack callables for serialization directly.
    session : bytes
        the ID of this Session object.  The default is to generate a new UUID.
    username : unicode
        username added to message headers.  The default is to ask the OS.
    key : bytes
        The key used to initialize an HMAC signature.  If unset, messages
        will not be signed or checked.
    keyfile : filepath
        The file containing a key.  If this is set, `key` will be initialized
        to the contents of the file.

    """

    debug=Bool(False, config=True, help="""Debug output in the Session""")

    packer = DottedObjectName('json',config=True,
            help="""The name of the packer for serializing messages.
            Should be one of 'json', 'pickle', or an import name
            for a custom callable serializer.""")
    def _packer_changed(self, name, old, new):
        if new.lower() == 'json':
            self.pack = json_packer
            self.unpack = json_unpacker
            self.unpacker = new
        elif new.lower() == 'pickle':
            self.pack = pickle_packer
            self.unpack = pickle_unpacker
            self.unpacker = new
        else:
            self.pack = import_item(str(new))

    unpacker = DottedObjectName('json', config=True,
        help="""The name of the unpacker for unserializing messages.
        Only used with custom functions for `packer`.""")
    def _unpacker_changed(self, name, old, new):
        if new.lower() == 'json':
            self.pack = json_packer
            self.unpack = json_unpacker
            self.packer = new
        elif new.lower() == 'pickle':
            self.pack = pickle_packer
            self.unpack = pickle_unpacker
            self.packer = new
        else:
            self.unpack = import_item(str(new))

    session = CUnicode(u'', config=True,
        help="""The UUID identifying this session.""")
    def _session_default(self):
        u = unicode_type(uuid.uuid4())
        self.bsession = u.encode('ascii')
        return u

    def _session_changed(self, name, old, new):
        self.bsession = self.session.encode('ascii')

    # bsession is the session as bytes
    bsession = CBytes(b'')

    username = Unicode(str_to_unicode(os.environ.get('USER', 'username')),
        help="""Username for the Session. Default is your system username.""",
        config=True)

    metadata = Dict({}, config=True,
        help="""Metadata dictionary, which serves as the default top-level metadata dict for each message.""")

    # message signature related traits:
    
    key = CBytes(b'', config=True,
        help="""execution key, for extra authentication.""")
    def _key_changed(self, name, old, new):
        if new:
            self.auth = hmac.HMAC(new, digestmod=self.digest_mod)
        else:
            self.auth = None
    
    signature_scheme = Unicode('hmac-sha256', config=True,
        help="""The digest scheme used to construct the message signatures.
        Must have the form 'hmac-HASH'.""")
    def _signature_scheme_changed(self, name, old, new):
        if not new.startswith('hmac-'):
            raise TraitError("signature_scheme must start with 'hmac-', got %r" % new)
        hash_name = new.split('-', 1)[1]
        try:
            self.digest_mod = getattr(hashlib, hash_name)
        except AttributeError:
            raise TraitError("hashlib has no such attribute: %s" % hash_name)
    
    digest_mod = Any()
    def _digest_mod_default(self):
        return hashlib.sha256
    
    auth = Instance(hmac.HMAC)
    
    digest_history = Set()
    digest_history_size = Integer(2**16, config=True,
        help="""The maximum number of digests to remember.
        
        The digest history will be culled when it exceeds this value.
        """
    )

    keyfile = Unicode('', config=True,
        help="""path to file containing execution key.""")
    def _keyfile_changed(self, name, old, new):
        with open(new, 'rb') as f:
            self.key = f.read().strip()

    # for protecting against sends from forks
    pid = Integer()
    
    # serialization traits:
    
    pack = Any(default_packer) # the actual packer function
    def _pack_changed(self, name, old, new):
        if not callable(new):
            raise TypeError("packer must be callable, not %s"%type(new))

    unpack = Any(default_unpacker) # the actual packer function
    def _unpack_changed(self, name, old, new):
        # unpacker is not checked - it is assumed to be
        if not callable(new):
            raise TypeError("unpacker must be callable, not %s"%type(new))
    
    # thresholds:
    copy_threshold = Integer(2**16, config=True,
        help="Threshold (in bytes) beyond which a buffer should be sent without copying.")
    buffer_threshold = Integer(MAX_BYTES, config=True,
        help="Threshold (in bytes) beyond which an object's buffer should be extracted to avoid pickling.")
    item_threshold = Integer(MAX_ITEMS, config=True,
        help="""The maximum number of items for a container to be introspected for custom serialization.
        Containers larger than this are pickled outright.
        """
    )

    
    def __init__(self, **kwargs):
        """create a Session object

        Parameters
        ----------

        debug : bool
            whether to trigger extra debugging statements
        packer/unpacker : str : 'json', 'pickle' or import_string
            importstrings for methods to serialize message parts.  If just
            'json' or 'pickle', predefined JSON and pickle packers will be used.
            Otherwise, the entire importstring must be used.

            The functions must accept at least valid JSON input, and output
            *bytes*.

            For example, to use msgpack:
            packer = 'msgpack.packb', unpacker='msgpack.unpackb'
        pack/unpack : callables
            You can also set the pack/unpack callables for serialization
            directly.
        session : unicode (must be ascii)
            the ID of this Session object.  The default is to generate a new
            UUID.
        bsession : bytes
            The session as bytes
        username : unicode
            username added to message headers.  The default is to ask the OS.
        key : bytes
            The key used to initialize an HMAC signature.  If unset, messages
            will not be signed or checked.
        signature_scheme : str
            The message digest scheme. Currently must be of the form 'hmac-HASH',
            where 'HASH' is a hashing function available in Python's hashlib.
            The default is 'hmac-sha256'.
            This is ignored if 'key' is empty.
        keyfile : filepath
            The file containing a key.  If this is set, `key` will be
            initialized to the contents of the file.
        """
        super(Session, self).__init__(**kwargs)
        self._check_packers()
        self.none = self.pack({})
        # ensure self._session_default() if necessary, so bsession is defined:
        self.session
        self.pid = os.getpid()

    @property
    def msg_id(self):
        """always return new uuid"""
        return str(uuid.uuid4())

    def _check_packers(self):
        """check packers for datetime support."""
        pack = self.pack
        unpack = self.unpack

        # check simple serialization
        msg = dict(a=[1,'hi'])
        try:
            packed = pack(msg)
        except Exception as e:
            msg = "packer '{packer}' could not serialize a simple message: {e}{jsonmsg}"
            if self.packer == 'json':
                jsonmsg = "\nzmq.utils.jsonapi.jsonmod = %s" % jsonapi.jsonmod
            else:
                jsonmsg = ""
            raise ValueError(
                msg.format(packer=self.packer, e=e, jsonmsg=jsonmsg)
            )

        # ensure packed message is bytes
        if not isinstance(packed, bytes):
            raise ValueError("message packed to %r, but bytes are required"%type(packed))

        # check that unpack is pack's inverse
        try:
            unpacked = unpack(packed)
            assert unpacked == msg
        except Exception as e:
            msg = "unpacker '{unpacker}' could not handle output from packer '{packer}': {e}{jsonmsg}"
            if self.packer == 'json':
                jsonmsg = "\nzmq.utils.jsonapi.jsonmod = %s" % jsonapi.jsonmod
            else:
                jsonmsg = ""
            raise ValueError(
                msg.format(packer=self.packer, unpacker=self.unpacker, e=e, jsonmsg=jsonmsg)
            )

        # check datetime support
        msg = dict(t=datetime.now())
        try:
            unpacked = unpack(pack(msg))
            if isinstance(unpacked['t'], datetime):
                raise ValueError("Shouldn't deserialize to datetime")
        except Exception:
            self.pack = lambda o: pack(squash_dates(o))
            self.unpack = lambda s: unpack(s)

    def msg_header(self, msg_type):
        return msg_header(self.msg_id, msg_type, self.username, self.session)

    def msg(self, msg_type, content=None, parent=None, header=None, metadata=None):
        """Return the nested message dict.

        This format is different from what is sent over the wire. The
        serialize/unserialize methods converts this nested message dict to the wire
        format, which is a list of message parts.
        """
        msg = {}
        header = self.msg_header(msg_type) if header is None else header
        msg['header'] = header
        msg['msg_id'] = header['msg_id']
        msg['msg_type'] = header['msg_type']
        msg['parent_header'] = {} if parent is None else extract_header(parent)
        msg['content'] = {} if content is None else content
        msg['metadata'] = self.metadata.copy()
        if metadata is not None:
            msg['metadata'].update(metadata)
        return msg

    def sign(self, msg_list):
        """Sign a message with HMAC digest. If no auth, return b''.

        Parameters
        ----------
        msg_list : list
            The [p_header,p_parent,p_content] part of the message list.
        """
        if self.auth is None:
            return b''
        h = self.auth.copy()
        for m in msg_list:
            h.update(m)
        return str_to_bytes(h.hexdigest())

    def serialize(self, msg, ident=None):
        """Serialize the message components to bytes.

        This is roughly the inverse of unserialize. The serialize/unserialize
        methods work with full message lists, whereas pack/unpack work with
        the individual message parts in the message list.

        Parameters
        ----------
        msg : dict or Message
            The nexted message dict as returned by the self.msg method.

        Returns
        -------
        msg_list : list
            The list of bytes objects to be sent with the format::

                [ident1, ident2, ..., DELIM, HMAC, p_header, p_parent,
                 p_metadata, p_content, buffer1, buffer2, ...]

            In this list, the ``p_*`` entities are the packed or serialized
            versions, so if JSON is used, these are utf8 encoded JSON strings.
        """
        content = msg.get('content', {})
        if content is None:
            content = self.none
        elif isinstance(content, dict):
            content = self.pack(content)
        elif isinstance(content, bytes):
            # content is already packed, as in a relayed message
            pass
        elif isinstance(content, unicode_type):
            # should be bytes, but JSON often spits out unicode
            content = content.encode('utf8')
        else:
            raise TypeError("Content incorrect type: %s"%type(content))

        real_message = [self.pack(msg['header']),
                        self.pack(msg['parent_header']),
                        self.pack(msg['metadata']),
                        content,
        ]

        to_send = []

        if isinstance(ident, list):
            # accept list of idents
            to_send.extend(ident)
        elif ident is not None:
            to_send.append(ident)
        to_send.append(DELIM)

        signature = self.sign(real_message)
        to_send.append(signature)

        to_send.extend(real_message)

        return to_send

    def send(self, stream, msg_or_type, content=None, parent=None, ident=None,
             buffers=None, track=False, header=None, metadata=None):
        """Build and send a message via stream or socket.

        The message format used by this function internally is as follows:

        [ident1,ident2,...,DELIM,HMAC,p_header,p_parent,p_content,
         buffer1,buffer2,...]

        The serialize/unserialize methods convert the nested message dict into this
        format.

        Parameters
        ----------

        stream : zmq.Socket or ZMQStream
            The socket-like object used to send the data.
        msg_or_type : str or Message/dict
            Normally, msg_or_type will be a msg_type unless a message is being
            sent more than once. If a header is supplied, this can be set to
            None and the msg_type will be pulled from the header.

        content : dict or None
            The content of the message (ignored if msg_or_type is a message).
        header : dict or None
            The header dict for the message (ignored if msg_to_type is a message).
        parent : Message or dict or None
            The parent or parent header describing the parent of this message
            (ignored if msg_or_type is a message).
        ident : bytes or list of bytes
            The zmq.IDENTITY routing path.
        metadata : dict or None
            The metadata describing the message
        buffers : list or None
            The already-serialized buffers to be appended to the message.
        track : bool
            Whether to track.  Only for use with Sockets, because ZMQStream
            objects cannot track messages.
            

        Returns
        -------
        msg : dict
            The constructed message.
        """
        if not isinstance(stream, zmq.Socket):
            # ZMQStreams and dummy sockets do not support tracking.
            track = False

        if isinstance(msg_or_type, (Message, dict)):
            # We got a Message or message dict, not a msg_type so don't
            # build a new Message.
            msg = msg_or_type
        else:
            msg = self.msg(msg_or_type, content=content, parent=parent,
                           header=header, metadata=metadata)
        if not os.getpid() == self.pid:
            io.rprint("WARNING: attempted to send message from fork")
            io.rprint(msg)
            return
        buffers = [] if buffers is None else buffers
        to_send = self.serialize(msg, ident)
        to_send.extend(buffers)
        longest = max([ len(s) for s in to_send ])
        copy = (longest < self.copy_threshold)
        
        if buffers and track and not copy:
            # only really track when we are doing zero-copy buffers
            tracker = stream.send_multipart(to_send, copy=False, track=True)
        else:
            # use dummy tracker, which will be done immediately
            tracker = DONE
            stream.send_multipart(to_send, copy=copy)

        if self.debug:
            pprint.pprint(msg)
            pprint.pprint(to_send)
            pprint.pprint(buffers)

        msg['tracker'] = tracker

        return msg

    def send_raw(self, stream, msg_list, flags=0, copy=True, ident=None):
        """Send a raw message via ident path.

        This method is used to send a already serialized message.

        Parameters
        ----------
        stream : ZMQStream or Socket
            The ZMQ stream or socket to use for sending the message.
        msg_list : list
            The serialized list of messages to send. This only includes the
            [p_header,p_parent,p_metadata,p_content,buffer1,buffer2,...] portion of
            the message.
        ident : ident or list
            A single ident or a list of idents to use in sending.
        """
        to_send = []
        if isinstance(ident, bytes):
            ident = [ident]
        if ident is not None:
            to_send.extend(ident)

        to_send.append(DELIM)
        to_send.append(self.sign(msg_list))
        to_send.extend(msg_list)
        stream.send_multipart(msg_list, flags, copy=copy)

    def recv(self, socket, mode=zmq.NOBLOCK, content=True, copy=True):
        """Receive and unpack a message.

        Parameters
        ----------
        socket : ZMQStream or Socket
            The socket or stream to use in receiving.

        Returns
        -------
        [idents], msg
            [idents] is a list of idents and msg is a nested message dict of
            same format as self.msg returns.
        """
        if isinstance(socket, ZMQStream):
            socket = socket.socket
        try:
            msg_list = socket.recv_multipart(mode, copy=copy)
        except zmq.ZMQError as e:
            if e.errno == zmq.EAGAIN:
                # We can convert EAGAIN to None as we know in this case
                # recv_multipart won't return None.
                return None,None
            else:
                raise
        # split multipart message into identity list and message dict
        # invalid large messages can cause very expensive string comparisons
        idents, msg_list = self.feed_identities(msg_list, copy)
        try:
            return idents, self.unserialize(msg_list, content=content, copy=copy)
        except Exception as e:
            # TODO: handle it
            raise e

    def feed_identities(self, msg_list, copy=True):
        """Split the identities from the rest of the message.

        Feed until DELIM is reached, then return the prefix as idents and
        remainder as msg_list. This is easily broken by setting an IDENT to DELIM,
        but that would be silly.

        Parameters
        ----------
        msg_list : a list of Message or bytes objects
            The message to be split.
        copy : bool
            flag determining whether the arguments are bytes or Messages

        Returns
        -------
        (idents, msg_list) : two lists
            idents will always be a list of bytes, each of which is a ZMQ
            identity. msg_list will be a list of bytes or zmq.Messages of the
            form [HMAC,p_header,p_parent,p_content,buffer1,buffer2,...] and
            should be unpackable/unserializable via self.unserialize at this
            point.
        """
        if copy:
            idx = msg_list.index(DELIM)
            return msg_list[:idx], msg_list[idx+1:]
        else:
            failed = True
            for idx,m in enumerate(msg_list):
                if m.bytes == DELIM:
                    failed = False
                    break
            if failed:
                raise ValueError("DELIM not in msg_list")
            idents, msg_list = msg_list[:idx], msg_list[idx+1:]
            return [m.bytes for m in idents], msg_list

    def _add_digest(self, signature):
        """add a digest to history to protect against replay attacks"""
        if self.digest_history_size == 0:
            # no history, never add digests
            return
        
        self.digest_history.add(signature)
        if len(self.digest_history) > self.digest_history_size:
            # threshold reached, cull 10%
            self._cull_digest_history()
    
    def _cull_digest_history(self):
        """cull the digest history
        
        Removes a randomly selected 10% of the digest history
        """
        current = len(self.digest_history)
        n_to_cull = max(int(current // 10), current - self.digest_history_size)
        if n_to_cull >= current:
            self.digest_history = set()
            return
        to_cull = random.sample(self.digest_history, n_to_cull)
        self.digest_history.difference_update(to_cull)
    
    def unserialize(self, msg_list, content=True, copy=True):
        """Unserialize a msg_list to a nested message dict.

        This is roughly the inverse of serialize. The serialize/unserialize
        methods work with full message lists, whereas pack/unpack work with
        the individual message parts in the message list.

        Parameters
        ----------
        msg_list : list of bytes or Message objects
            The list of message parts of the form [HMAC,p_header,p_parent,
            p_metadata,p_content,buffer1,buffer2,...].
        content : bool (True)
            Whether to unpack the content dict (True), or leave it packed
            (False).
        copy : bool (True)
            Whether to return the bytes (True), or the non-copying Message
            object in each place (False).

        Returns
        -------
        msg : dict
            The nested message dict with top-level keys [header, parent_header,
            content, buffers].
        """
        minlen = 5
        message = {}
        if not copy:
            for i in range(minlen):
                msg_list[i] = msg_list[i].bytes
        if self.auth is not None:
            signature = msg_list[0]
            if not signature:
                raise ValueError("Unsigned Message")
            if signature in self.digest_history:
                raise ValueError("Duplicate Signature: %r" % signature)
            self._add_digest(signature)
            check = self.sign(msg_list[1:5])
            if not signature == check:
                raise ValueError("Invalid Signature: %r" % signature)
        if not len(msg_list) >= minlen:
            raise TypeError("malformed message, must have at least %i elements"%minlen)
        header = self.unpack(msg_list[1])
        message['header'] = extract_dates(header)
        message['msg_id'] = header['msg_id']
        message['msg_type'] = header['msg_type']
        message['parent_header'] = extract_dates(self.unpack(msg_list[2]))
        message['metadata'] = self.unpack(msg_list[3])
        if content:
            message['content'] = self.unpack(msg_list[4])
        else:
            message['content'] = msg_list[4]

        message['buffers'] = msg_list[5:]
        return message

def test_msg2obj():
    am = dict(x=1)
    ao = Message(am)
    assert ao.x == am['x']

    am['y'] = dict(z=1)
    ao = Message(am)
    assert ao.y.z == am['y']['z']

    k1, k2 = 'y', 'z'
    assert ao[k1][k2] == am[k1][k2]

    am2 = dict(ao)
    assert am['x'] == am2['x']
    assert am['y']['z'] == am2['y']['z']