clarify first ws message names / messages

IPython/frontend/html/notebook/handlers.py

@@ -1,924 +1,924
 """Tornado handlers for the notebook.
 
 Authors:
 
 * Brian Granger
 """
 
 #-----------------------------------------------------------------------------
 #  Copyright (C) 2008-2011  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 Cookie
 import datetime
 import email.utils
 import hashlib
 import logging
 import mimetypes
 import os
 import stat
 import threading
 import time
 import uuid
 
 from tornado.escape import url_escape
 from tornado import web
 from tornado import websocket
 
 try:
     from tornado.log import app_log
 except ImportError:
     app_log = logging.getLogger()
 
 from zmq.eventloop import ioloop
 from zmq.utils import jsonapi
 
 from IPython.config import Application
 from IPython.external.decorator import decorator
 from IPython.kernel.zmq.session import Session
 from IPython.lib.security import passwd_check
 from IPython.utils.jsonutil import date_default
 from IPython.utils.path import filefind
 from IPython.utils.py3compat import PY3
 
 try:
     from docutils.core import publish_string
 except ImportError:
     publish_string = None
 
 #-----------------------------------------------------------------------------
 # Monkeypatch for Tornado <= 2.1.1 - Remove when no longer necessary!
 #-----------------------------------------------------------------------------
 
 # Google Chrome, as of release 16, changed its websocket protocol number.  The
 # parts tornado cares about haven't really changed, so it's OK to continue
 # accepting Chrome connections, but as of Tornado 2.1.1 (the currently released
 # version as of Oct 30/2011) the version check fails, see the issue report:
 
 # https://github.com/facebook/tornado/issues/385
 
 # This issue has been fixed in Tornado post 2.1.1:
 
 # https://github.com/facebook/tornado/commit/84d7b458f956727c3b0d6710
 
 # Here we manually apply the same patch as above so that users of IPython can
 # continue to work with an officially released Tornado.  We make the
 # monkeypatch version check as narrow as possible to limit its effects; once
 # Tornado 2.1.1 is no longer found in the wild we'll delete this code.
 
 import tornado
 
 if tornado.version_info <= (2,1,1):
 
     def _execute(self, transforms, *args, **kwargs):
         from tornado.websocket import WebSocketProtocol8, WebSocketProtocol76
         
         self.open_args = args
         self.open_kwargs = kwargs
 
         # The difference between version 8 and 13 is that in 8 the
         # client sends a "Sec-Websocket-Origin" header and in 13 it's
         # simply "Origin".
         if self.request.headers.get("Sec-WebSocket-Version") in ("7", "8", "13"):
             self.ws_connection = WebSocketProtocol8(self)
             self.ws_connection.accept_connection()
             
         elif self.request.headers.get("Sec-WebSocket-Version"):
             self.stream.write(tornado.escape.utf8(
                 "HTTP/1.1 426 Upgrade Required\r\n"
                 "Sec-WebSocket-Version: 8\r\n\r\n"))
             self.stream.close()
             
         else:
             self.ws_connection = WebSocketProtocol76(self)
             self.ws_connection.accept_connection()
 
     websocket.WebSocketHandler._execute = _execute
     del _execute
 
 #-----------------------------------------------------------------------------
 # Decorator for disabling read-only handlers
 #-----------------------------------------------------------------------------
 
 @decorator
 def not_if_readonly(f, self, *args, **kwargs):
     if self.settings.get('read_only', False):
         raise web.HTTPError(403, "Notebook server is read-only")
     else:
         return f(self, *args, **kwargs)
 
 @decorator
 def authenticate_unless_readonly(f, self, *args, **kwargs):
     """authenticate this page *unless* readonly view is active.
     
     In read-only mode, the notebook list and print view should
     be accessible without authentication.
     """
     
     @web.authenticated
     def auth_f(self, *args, **kwargs):
         return f(self, *args, **kwargs)
 
     if self.settings.get('read_only', False):
         return f(self, *args, **kwargs)
     else:
         return auth_f(self, *args, **kwargs)
 
 def urljoin(*pieces):
     """Join components of url into a relative url
 
     Use to prevent double slash when joining subpath
     """
     striped = [s.strip('/') for s in pieces]
     return '/'.join(s for s in striped if s)
 
 #-----------------------------------------------------------------------------
 # Top-level handlers
 #-----------------------------------------------------------------------------
 
 class RequestHandler(web.RequestHandler):
     """RequestHandler with default variable setting."""
 
     def render(*args, **kwargs):
         kwargs.setdefault('message', '')
         return web.RequestHandler.render(*args, **kwargs)
 
 class AuthenticatedHandler(RequestHandler):
     """A RequestHandler with an authenticated user."""
 
     def clear_login_cookie(self):
         self.clear_cookie(self.cookie_name)
     
     def get_current_user(self):
         user_id = self.get_secure_cookie(self.cookie_name)
         # For now the user_id should not return empty, but it could eventually
         if user_id == '':
             user_id = 'anonymous'
         if user_id is None:
             # prevent extra Invalid cookie sig warnings:
             self.clear_login_cookie()
             if not self.read_only and not self.login_available:
                 user_id = 'anonymous'
         return user_id
 
     @property
     def cookie_name(self):
         return self.settings.get('cookie_name', '')
     
     @property
     def password(self):
         """our password"""
         return self.settings.get('password', '')
     
     @property
     def logged_in(self):
         """Is a user currently logged in?
 
         """
         user = self.get_current_user()
         return (user and not user == 'anonymous')
 
     @property
     def login_available(self):
         """May a user proceed to log in?
 
         This returns True if login capability is available, irrespective of
         whether the user is already logged in or not.
 
         """
         return bool(self.settings.get('password', ''))
 
     @property
     def read_only(self):
         """Is the notebook read-only?
 
         """
         return self.settings.get('read_only', False)
 
 
 class IPythonHandler(AuthenticatedHandler):
     """IPython-specific extensions to authenticated handling
     
     Mostly property shortcuts to IPython-specific settings.
     """
     
     @property
     def config(self):
         return self.settings.get('config', None)
     
     @property
     def log(self):
         """use the IPython log by default, falling back on tornado's logger"""
         if Application.initialized():
             return Application.instance().log
         else:
             return app_log
     
     @property
     def use_less(self):
         """Use less instead of css in templates"""
         return self.settings.get('use_less', False)
     
     #---------------------------------------------------------------
     # URLs
     #---------------------------------------------------------------
     
     @property
     def ws_url(self):
         """websocket url matching the current request
 
         turns http[s]://host[:port] into
                 ws[s]://host[:port]
         """
         proto = self.request.protocol.replace('http', 'ws')
         host = self.settings.get('websocket_host', '')
         # default to config value
         if host == '':
             host = self.request.host # get from request
         return "%s://%s" % (proto, host)
     
     @property
     def mathjax_url(self):
         return self.settings.get('mathjax_url', '')
     
     @property
     def base_project_url(self):
         return self.settings.get('base_project_url', '/')
     
     @property
     def base_kernel_url(self):
         return self.settings.get('base_kernel_url', '/')
     
     #---------------------------------------------------------------
     # Manager objects
     #---------------------------------------------------------------
     
     @property
     def kernel_manager(self):
         return self.settings['kernel_manager']
 
     @property
     def notebook_manager(self):
         return self.settings['notebook_manager']
     
     @property
     def cluster_manager(self):
         return self.settings['cluster_manager']
     
     @property
     def project(self):
         return self.notebook_manager.notebook_dir
     
     #---------------------------------------------------------------
     # template rendering
     #---------------------------------------------------------------
     
     def get_template(self, name):
         """Return the jinja template object for a given name"""
         return self.settings['jinja2_env'].get_template(name)
     
     def render_template(self, name, **ns):
         ns.update(self.template_namespace)
         template = self.get_template(name)
         return template.render(**ns)
     
     @property
     def template_namespace(self):
         return dict(
             base_project_url=self.base_project_url,
             base_kernel_url=self.base_kernel_url,
             read_only=self.read_only,
             logged_in=self.logged_in,
             login_available=self.login_available,
             use_less=self.use_less,
         )
 
 class AuthenticatedFileHandler(IPythonHandler, web.StaticFileHandler):
     """static files should only be accessible when logged in"""
 
     @authenticate_unless_readonly
     def get(self, path):
         return web.StaticFileHandler.get(self, path)
 
 
 class ProjectDashboardHandler(IPythonHandler):
 
     @authenticate_unless_readonly
     def get(self):
         self.write(self.render_template('projectdashboard.html',
             project=self.project,
             project_component=self.project.split('/'),
         ))
 
 
 class LoginHandler(IPythonHandler):
 
     def _render(self, message=None):
         self.write(self.render_template('login.html',
                 next=url_escape(self.get_argument('next', default=self.base_project_url)),
                 message=message,
         ))
 
     def get(self):
         if self.current_user:
             self.redirect(self.get_argument('next', default=self.base_project_url))
         else:
             self._render()
 
     def post(self):
         pwd = self.get_argument('password', default=u'')
         if self.login_available:
             if passwd_check(self.password, pwd):
                 self.set_secure_cookie(self.cookie_name, str(uuid.uuid4()))
             else:
                 self._render(message={'error': 'Invalid password'})
                 return
 
         self.redirect(self.get_argument('next', default=self.base_project_url))
 
 
 class LogoutHandler(IPythonHandler):
 
     def get(self):
         self.clear_login_cookie()
         if self.login_available:
             message = {'info': 'Successfully logged out.'}
         else:
             message = {'warning': 'Cannot log out.  Notebook authentication '
                        'is disabled.'}
         self.write(self.render_template('logout.html',
                     message=message))
 
 
 class NewHandler(IPythonHandler):
 
     @web.authenticated
     def get(self):
         notebook_id = self.notebook_manager.new_notebook()
         self.redirect('/' + urljoin(self.base_project_url, notebook_id))
 
 class NamedNotebookHandler(IPythonHandler):
 
     @authenticate_unless_readonly
     def get(self, notebook_id):
         nbm = self.notebook_manager
         if not nbm.notebook_exists(notebook_id):
             raise web.HTTPError(404, u'Notebook does not exist: %s' % notebook_id)       
         self.write(self.render_template('notebook.html',
             project=self.project,
             notebook_id=notebook_id,
             kill_kernel=False,
             mathjax_url=self.mathjax_url,
             )
         )
 
 
 class PrintNotebookHandler(IPythonHandler):
 
     @authenticate_unless_readonly
     def get(self, notebook_id):
         if not self.notebook_manager.notebook_exists(notebook_id):
             raise web.HTTPError(404, u'Notebook does not exist: %s' % notebook_id)        
         self.write( self.render_template('printnotebook.html',
             project=self.project,
             notebook_id=notebook_id,
             kill_kernel=False,
             mathjax_url=self.mathjax_url,
         ))
 
 #-----------------------------------------------------------------------------
 # Kernel handlers
 #-----------------------------------------------------------------------------
 
 
 class MainKernelHandler(IPythonHandler):
 
     @web.authenticated
     def get(self):
         km = self.kernel_manager
         self.finish(jsonapi.dumps(km.list_kernel_ids()))
 
     @web.authenticated
     def post(self):
         km = self.kernel_manager
         nbm = self.notebook_manager
         notebook_id = self.get_argument('notebook', default=None)
         kernel_id = km.start_kernel(notebook_id, cwd=nbm.notebook_dir)
         data = {'ws_url':self.ws_url,'kernel_id':kernel_id}
         self.set_header('Location', '/'+kernel_id)
         self.finish(jsonapi.dumps(data))
 
 
 class KernelHandler(IPythonHandler):
 
     SUPPORTED_METHODS = ('DELETE')
 
     @web.authenticated
     def delete(self, kernel_id):
         km = self.kernel_manager
         km.shutdown_kernel(kernel_id)
         self.set_status(204)
         self.finish()
 
 
 class KernelActionHandler(IPythonHandler):
 
     @web.authenticated
     def post(self, kernel_id, action):
         km = self.kernel_manager
         if action == 'interrupt':
             km.interrupt_kernel(kernel_id)
             self.set_status(204)
         if action == 'restart':
             km.restart_kernel(kernel_id)
             data = {'ws_url':self.ws_url, 'kernel_id':kernel_id}
             self.set_header('Location', '/'+kernel_id)
             self.write(jsonapi.dumps(data))
         self.finish()
 
 
 class ZMQStreamHandler(websocket.WebSocketHandler):
     
     def clear_cookie(self, *args, **kwargs):
         """meaningless for websockets"""
         pass
 
     def _reserialize_reply(self, msg_list):
         """Reserialize a reply message using JSON.
 
         This takes the msg list from the ZMQ socket, unserializes it using
         self.session and then serializes the result using JSON. This method
         should be used by self._on_zmq_reply to build messages that can
         be sent back to the browser.
         """
         idents, msg_list = self.session.feed_identities(msg_list)
         msg = self.session.unserialize(msg_list)
         try:
             msg['header'].pop('date')
         except KeyError:
             pass
         try:
             msg['parent_header'].pop('date')
         except KeyError:
             pass
         msg.pop('buffers')
         return jsonapi.dumps(msg, default=date_default)
 
     def _on_zmq_reply(self, msg_list):
         # Sometimes this gets triggered when the on_close method is scheduled in the
         # eventloop but hasn't been called.
         if self.stream.closed(): return
         try:
             msg = self._reserialize_reply(msg_list)
         except Exception:
             self.log.critical("Malformed message: %r" % msg_list, exc_info=True)
         else:
             self.write_message(msg)
 
     def allow_draft76(self):
         """Allow draft 76, until browsers such as Safari update to RFC 6455.
         
         This has been disabled by default in tornado in release 2.2.0, and
         support will be removed in later versions.
         """
         return True
 
 
 class AuthenticatedZMQStreamHandler(ZMQStreamHandler, IPythonHandler):
 
     def open(self, kernel_id):
         self.kernel_id = kernel_id.decode('ascii')
         self.session = Session(config=self.config)
         self.save_on_message = self.on_message
         self.on_message = self.on_first_message
 
     def _inject_cookie_message(self, msg):
         """Inject the first message, which is the document cookie,
         for authentication."""
         if not PY3 and isinstance(msg, unicode):
             # Cookie constructor doesn't accept unicode strings
             # under Python 2.x for some reason
             msg = msg.encode('utf8', 'replace')
         try:
-            bsession, msg = msg.split(':', 1)
-            self.session.session = bsession.decode('ascii')
+            identity, msg = msg.split(':', 1)
+            self.session.session = identity.decode('ascii')
         except Exception:
-            logging.error("No bsession!", exc_info=True)
-            pass
+            logging.error("First ws message didn't have the form 'identity:[cookie]' - %r", msg)
+        
         try:
             self.request._cookies = Cookie.SimpleCookie(msg)
         except:
             self.log.warn("couldn't parse cookie string: %s",msg, exc_info=True)
 
     def on_first_message(self, msg):
         self._inject_cookie_message(msg)
         if self.get_current_user() is None:
             self.log.warn("Couldn't authenticate WebSocket connection")
             raise web.HTTPError(403)
         self.on_message = self.save_on_message
 
 
 class ZMQChannelHandler(AuthenticatedZMQStreamHandler):
     
     @property
     def max_msg_size(self):
         return self.settings.get('max_msg_size', 65535)
     
     def create_stream(self):
         km = self.kernel_manager
         meth = getattr(km, 'connect_%s' % self.channel)
         self.zmq_stream = meth(self.kernel_id, identity=self.session.bsession)
     
     def initialize(self, *args, **kwargs):
         self.zmq_stream = None
     
     def on_first_message(self, msg):
         try:
             super(ZMQChannelHandler, self).on_first_message(msg)
         except web.HTTPError:
             self.close()
             return
         try:
             self.create_stream()
         except web.HTTPError:
             # WebSockets don't response to traditional error codes so we
             # close the connection.
             if not self.stream.closed():
                 self.stream.close()
             self.close()
         else:
             self.zmq_stream.on_recv(self._on_zmq_reply)
 
     def on_message(self, msg):
         if len(msg) < self.max_msg_size:
             msg = jsonapi.loads(msg)
             self.session.send(self.zmq_stream, msg)
 
     def on_close(self):
         # This method can be called twice, once by self.kernel_died and once
         # from the WebSocket close event. If the WebSocket connection is
         # closed before the ZMQ streams are setup, they could be None.
         if self.zmq_stream is not None and not self.zmq_stream.closed():
             self.zmq_stream.on_recv(None)
             self.zmq_stream.close()
 
 
 class IOPubHandler(ZMQChannelHandler):
     channel = 'iopub'
     
     def create_stream(self):
         super(IOPubHandler, self).create_stream()
         km = self.kernel_manager
         km.add_restart_callback(self.kernel_id, self.on_kernel_restarted)
         km.add_restart_callback(self.kernel_id, self.on_restart_failed, 'dead')
     
     def on_close(self):
         km = self.kernel_manager
         if self.kernel_id in km:
             km.remove_restart_callback(
                 self.kernel_id, self.on_kernel_restarted,
             )
             km.remove_restart_callback(
                 self.kernel_id, self.on_restart_failed, 'dead',
             )
         super(IOPubHandler, self).on_close()
     
     def _send_status_message(self, status):
         msg = self.session.msg("status",
             {'execution_state': status}
         )
         self.write_message(jsonapi.dumps(msg, default=date_default))
 
     def on_kernel_restarted(self):
         logging.warn("kernel %s restarted", self.kernel_id)
         self._send_status_message('restarting')
 
     def on_restart_failed(self):
         logging.error("kernel %s restarted failed!", self.kernel_id)
         self._send_status_message('dead')
     
     def on_message(self, msg):
         """IOPub messages make no sense"""
         pass
 
 class ShellHandler(ZMQChannelHandler):
     channel = 'shell'
 
 class StdinHandler(ZMQChannelHandler):
     channel = 'stdin'
 
 
 #-----------------------------------------------------------------------------
 # Notebook web service handlers
 #-----------------------------------------------------------------------------
 
 class NotebookRedirectHandler(IPythonHandler):
     
     @authenticate_unless_readonly
     def get(self, notebook_name):
         # strip trailing .ipynb:
         notebook_name = os.path.splitext(notebook_name)[0]
         notebook_id = self.notebook_manager.rev_mapping.get(notebook_name, '')
         if notebook_id:
             url = self.settings.get('base_project_url', '/') + notebook_id
             return self.redirect(url)
         else:
             raise HTTPError(404)
 
 
 class NotebookRootHandler(IPythonHandler):
 
     @authenticate_unless_readonly
     def get(self):
         nbm = self.notebook_manager
         km = self.kernel_manager
         files = nbm.list_notebooks()
         for f in files :
             f['kernel_id'] = km.kernel_for_notebook(f['notebook_id'])
         self.finish(jsonapi.dumps(files))
 
     @web.authenticated
     def post(self):
         nbm = self.notebook_manager
         body = self.request.body.strip()
         format = self.get_argument('format', default='json')
         name = self.get_argument('name', default=None)
         if body:
             notebook_id = nbm.save_new_notebook(body, name=name, format=format)
         else:
             notebook_id = nbm.new_notebook()
         self.set_header('Location', '/'+notebook_id)
         self.finish(jsonapi.dumps(notebook_id))
 
 
 class NotebookHandler(IPythonHandler):
 
     SUPPORTED_METHODS = ('GET', 'PUT', 'DELETE')
 
     @authenticate_unless_readonly
     def get(self, notebook_id):
         nbm = self.notebook_manager
         format = self.get_argument('format', default='json')
         last_mod, name, data = nbm.get_notebook(notebook_id, format)
         
         if format == u'json':
             self.set_header('Content-Type', 'application/json')
             self.set_header('Content-Disposition','attachment; filename="%s.ipynb"' % name)
         elif format == u'py':
             self.set_header('Content-Type', 'application/x-python')
             self.set_header('Content-Disposition','attachment; filename="%s.py"' % name)
         self.set_header('Last-Modified', last_mod)
         self.finish(data)
 
     @web.authenticated
     def put(self, notebook_id):
         nbm = self.notebook_manager
         format = self.get_argument('format', default='json')
         name = self.get_argument('name', default=None)
         nbm.save_notebook(notebook_id, self.request.body, name=name, format=format)
         self.set_status(204)
         self.finish()
 
     @web.authenticated
     def delete(self, notebook_id):
         self.notebook_manager.delete_notebook(notebook_id)
         self.set_status(204)
         self.finish()
 
 
 class NotebookCopyHandler(IPythonHandler):
 
     @web.authenticated
     def get(self, notebook_id):
         notebook_id = self.notebook_manager.copy_notebook(notebook_id)
         self.redirect('/'+urljoin(self.base_project_url, notebook_id))
 
 
 #-----------------------------------------------------------------------------
 # Cluster handlers
 #-----------------------------------------------------------------------------
 
 
 class MainClusterHandler(IPythonHandler):
 
     @web.authenticated
     def get(self):
         self.finish(jsonapi.dumps(self.cluster_manager.list_profiles()))
 
 
 class ClusterProfileHandler(IPythonHandler):
 
     @web.authenticated
     def get(self, profile):
         self.finish(jsonapi.dumps(self.cluster_manager.profile_info(profile)))
 
 
 class ClusterActionHandler(IPythonHandler):
 
     @web.authenticated
     def post(self, profile, action):
         cm = self.cluster_manager
         if action == 'start':
             n = self.get_argument('n',default=None)
             if n is None:
                 data = cm.start_cluster(profile)
             else:
                 data = cm.start_cluster(profile, int(n))
         if action == 'stop':
             data = cm.stop_cluster(profile)
         self.finish(jsonapi.dumps(data))
 
 
 #-----------------------------------------------------------------------------
 # RST web service handlers
 #-----------------------------------------------------------------------------
 
 
 class RSTHandler(IPythonHandler):
 
     @web.authenticated
     def post(self):
         if publish_string is None:
             raise web.HTTPError(503, u'docutils not available')
         body = self.request.body.strip()
         source = body
         # template_path=os.path.join(os.path.dirname(__file__), u'templates', u'rst_template.html')
         defaults = {'file_insertion_enabled': 0,
                     'raw_enabled': 0,
                     '_disable_config': 1,
                     'stylesheet_path': 0
                     # 'template': template_path
         }
         try:
             html = publish_string(source, writer_name='html',
                                   settings_overrides=defaults
             )
         except:
             raise web.HTTPError(400, u'Invalid RST')
         print html
         self.set_header('Content-Type', 'text/html')
         self.finish(html)
 
 # to minimize subclass changes:
 HTTPError = web.HTTPError
 
 class FileFindHandler(web.StaticFileHandler):
     """subclass of StaticFileHandler for serving files from a search path"""
     
     _static_paths = {}
     # _lock is needed for tornado < 2.2.0 compat
     _lock = threading.Lock()  # protects _static_hashes
     
     def initialize(self, path, default_filename=None):
         if isinstance(path, basestring):
             path = [path]
         self.roots = tuple(
             os.path.abspath(os.path.expanduser(p)) + os.path.sep for p in path
         )
         self.default_filename = default_filename
     
     @classmethod
     def locate_file(cls, path, roots):
         """locate a file to serve on our static file search path"""
         with cls._lock:
             if path in cls._static_paths:
                 return cls._static_paths[path]
             try:
                 abspath = os.path.abspath(filefind(path, roots))
             except IOError:
                 # empty string should always give exists=False
                 return ''
         
             # os.path.abspath strips a trailing /
             # it needs to be temporarily added back for requests to root/
             if not (abspath + os.path.sep).startswith(roots):
                 raise HTTPError(403, "%s is not in root static directory", path)
         
             cls._static_paths[path] = abspath
             return abspath
     
     def get(self, path, include_body=True):
         path = self.parse_url_path(path)
         
         # begin subclass override
         abspath = self.locate_file(path, self.roots)
         # end subclass override
         
         if os.path.isdir(abspath) and self.default_filename is not None:
             # need to look at the request.path here for when path is empty
             # but there is some prefix to the path that was already
             # trimmed by the routing
             if not self.request.path.endswith("/"):
                 self.redirect(self.request.path + "/")
                 return
             abspath = os.path.join(abspath, self.default_filename)
         if not os.path.exists(abspath):
             raise HTTPError(404)
         if not os.path.isfile(abspath):
             raise HTTPError(403, "%s is not a file", path)
 
         stat_result = os.stat(abspath)
         modified = datetime.datetime.utcfromtimestamp(stat_result[stat.ST_MTIME])
 
         self.set_header("Last-Modified", modified)
 
         mime_type, encoding = mimetypes.guess_type(abspath)
         if mime_type:
             self.set_header("Content-Type", mime_type)
 
         cache_time = self.get_cache_time(path, modified, mime_type)
 
         if cache_time > 0:
             self.set_header("Expires", datetime.datetime.utcnow() + \
                                        datetime.timedelta(seconds=cache_time))
             self.set_header("Cache-Control", "max-age=" + str(cache_time))
         else:
             self.set_header("Cache-Control", "public")
 
         self.set_extra_headers(path)
 
         # Check the If-Modified-Since, and don't send the result if the
         # content has not been modified
         ims_value = self.request.headers.get("If-Modified-Since")
         if ims_value is not None:
             date_tuple = email.utils.parsedate(ims_value)
             if_since = datetime.datetime(*date_tuple[:6])
             if if_since >= modified:
                 self.set_status(304)
                 return
 
         with open(abspath, "rb") as file:
             data = file.read()
             hasher = hashlib.sha1()
             hasher.update(data)
             self.set_header("Etag", '"%s"' % hasher.hexdigest())
             if include_body:
                 self.write(data)
             else:
                 assert self.request.method == "HEAD"
                 self.set_header("Content-Length", len(data))
 
     @classmethod
     def get_version(cls, settings, path):
         """Generate the version string to be used in static URLs.
 
         This method may be overridden in subclasses (but note that it
         is a class method rather than a static method).  The default
         implementation uses a hash of the file's contents.
 
         ``settings`` is the `Application.settings` dictionary and ``path``
         is the relative location of the requested asset on the filesystem.
         The returned value should be a string, or ``None`` if no version
         could be determined.
         """
         # begin subclass override:
         static_paths = settings['static_path']
         if isinstance(static_paths, basestring):
             static_paths = [static_paths]
         roots = tuple(
             os.path.abspath(os.path.expanduser(p)) + os.path.sep for p in static_paths
         )
 
         try:
             abs_path = filefind(path, roots)
         except IOError:
             app_log.error("Could not find static file %r", path)
             return None
         
         # end subclass override
         
         with cls._lock:
             hashes = cls._static_hashes
             if abs_path not in hashes:
                 try:
                     f = open(abs_path, "rb")
                     hashes[abs_path] = hashlib.md5(f.read()).hexdigest()
                     f.close()
                 except Exception:
                     app_log.error("Could not open static file %r", path)
                     hashes[abs_path] = None
             hsh = hashes.get(abs_path)
             if hsh:
                 return hsh[:5]
         return None
 
 
     def parse_url_path(self, url_path):
         """Converts a static URL path into a filesystem path.
 
         ``url_path`` is the path component of the URL with
         ``static_url_prefix`` removed.  The return value should be
         filesystem path relative to ``static_path``.
         """
         if os.path.sep != "/":
             url_path = url_path.replace("/", os.path.sep)
         return url_path