update comment

@ivanov is making the commit, but Kyle helped compose it, so he gets
points! Everything is awesome when you're part of a team!

IPython/kernel/zmq/session.py

@@ -1,868 +1,865 b''
 """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.
 """
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 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
 
 try:
+    # We are using compare_digest to limit the surface of timing attacks
     from hmac import compare_digest
 except ImportError:
-    # Python < 2.7.7
-    import warnings
-    warnings.warn("You are using Python older than 2.7.7, please consider "
-            "updating to the latest version as it reduces a possible security"
-            " vulnerability.")
-    def compare_digest(a,b):
-        return a == b
+    # Python < 2.7.7: When digests don't match no feedback is provided,
+    # limiting the surface of attack
+    def compare_digest(a,b): return a == b
 
 import zmq
 from zmq.utils import jsonapi
 from zmq.eventloop.ioloop import IOLoop
 from zmq.eventloop.zmqstream import ZMQStream
 
 from IPython.core.release import kernel_protocol_version
 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.utils.pickleutil import PICKLE_PROTOCOL
 from IPython.kernel.adapter import adapt
 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
 # allow unicode
 # disallow nan, because it's not actually valid JSON
 json_packer = lambda obj: jsonapi.dumps(obj, default=date_default,
     ensure_ascii=False, allow_nan=False,
 )
 json_unpacker = lambda s: jsonapi.loads(s)
 
 pickle_packer = lambda o: pickle.dumps(squash_dates(o), PICKLE_PROTOCOL)
 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()
     version = kernel_protocol_version
     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 serialization 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.""")
     
     # if 0, no adapting to do.
     adapt_version = Integer(0)
 
     # message signature related traits:
     
     key = CBytes(b'', config=True,
         help="""execution key, for extra authentication.""")
     def _key_changed(self):
         self._new_auth()
     
     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)
         self._new_auth()
     
     digest_mod = Any()
     def _digest_mod_default(self):
         return hashlib.sha256
     
     auth = Instance(hmac.HMAC)
     
     def _new_auth(self):
         if self.key:
             self.auth = hmac.HMAC(self.key, digestmod=self.digest_mod)
         else:
             self.auth = None
     
     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 next 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
         if self.adapt_version:
             msg = adapt(msg, self.adapt_version)
         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(to_send, 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 compare_digest(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:]
         # print("received: %s: %s\n    %s" % (message['msg_type'], message['header'], message['content']))
         # adapt to the current version
         return adapt(message)
         # print("adapted: %s: %s\n     %s" % (adapted['msg_type'], adapted['header'], adapted['content']))
 
 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']