Reworked session handling.

 * All requests have a session ID, except for the initial create_session system messages.
 * User session can be in an authenticated or anonymous state.
 * In anonymous state it is not possible to perform user actions.
 * Logout has been implemented.
 * Sessions timeout and are cleared after a period of inactivity (1 hour).
 * Bugfixed the lang setting handling.
 * Renamed get_problem -> get_current_solution, return only the user's current solution, not the whole problem data.

5 files changed, 321 insertions, 206 deletions

diff --git a/errors/session.py b/errors/session.py
index 06916c3..a09c4ec 100644
--- a/errors/session.py
+++ b/errors/session.py
@@ -16,4 +16,7 @@ class PasswordChangeFailed(Exception):
     pass
 
 class RequestProcessingError(Exception):
-    pass
\ No newline at end of file
+    pass
+
+class NotLoggedIn(Exception):
+    pass
diff --git a/server/handlers.py b/server/handlers.py
index a1bfbd7..1ab09f3 100644
--- a/server/handlers.py
+++ b/server/handlers.py
@@ -16,25 +16,20 @@ class CodeqService(object):
         pass
 
 
-class ProblemList(CodeqService):
-    """List all available problems to the client.
+class CreateSession(CodeqService):
+    """Creates a new anonymous session.
+    To promote the session to the authenticated status, a subsequent
+    login or signup must follow.
     """
     session_is_optional = True
 
     def process(self, request):
-        js = request.data
-        language = js.get('language')
-        if language is None:
-            request.reply({'code': 1, 'message': 'Language was not provided'})
-        else:
-            request.reply({'code': 0, 'message': 'ok', 'problems': server.problems.list_problems(language)})
+        request.reply({'code': 0, 'message': 'OK', 'sid': server.user_session.UserSession().get_sid()})
 
 
 class Login(CodeqService):
-    """Logs in a client, creating a new session.
+    """Logs in a client, authenticating the session.
     """
-    session_is_optional = True
-
     def process(self, request):
         js = request.data
         username = js.get('username')
@@ -44,35 +39,39 @@ class Login(CodeqService):
         elif password is None:
             request.reply({'code': 2, 'message': 'Password was not provided'})
         else:
+            session = request.session
             try:
-                session = server.user_session.authenticate_and_create_session(username, password)
+                session.login(username, password)
             except AuthenticationFailed:
                 request.reply({'code': 3, 'message': 'Username or password do not match'})
             else:
-                if request.session:
-                    request.session.destroy()
                 settings = session.get_settings()
-                request.reply({'code': 0, 'message': 'OK', 'sid':session.get_sid(), 'settings':settings})
+                request.reply({'code': 0, 'message': 'OK', 'settings': settings})
 
 
-class Signup(CodeqService):
-    session_is_optional = True
+class Logout(CodeqService):
+    def process(self, request):
+        request.session.destroy()
+        request.reply({'code': 0, 'message': 'OK'})
+
 
+class Signup(CodeqService):
     def process(self, request):
         js = request.data
         username = js.get('username')
         password = js.get('password')
+        lang = js.get('lang') or 'en'
         if username is None:
             request.reply({'code': 1, 'message': 'Username was not provided'})
         elif password is None:
             request.reply({'code': 2, 'message': 'Password was not provided'})
         else:
             try:
-                server.user_session.signup(username, password)
+                request.session.signup(username, password, lang)
             except UserExists:
                 request.reply({'code': 3, 'message': 'Username already exists'})
             except SignupFailed:
-                request.reply({'code': 4, 'message': 'Signn up failed'})
+                request.reply({'code': 4, 'message': 'Sign-up failed'})
             else:
                 request.reply({'code': 0, 'message': 'OK'})
 
@@ -92,7 +91,7 @@ class ChangePassword(CodeqService):
                 request.reply({'code': 0, 'message': 'OK'})
 
 
-class Settings(CodeqService):
+class UpdateSettings(CodeqService):
     def process(self, request):
         js = request.data
         settings = js.get('settings')
@@ -101,7 +100,6 @@ class Settings(CodeqService):
         else:
             try:
                 request.session.update_settings(settings)
-                request.session.write_settings_to_db()
             except NoSuchSession:
                 request.reply({'code': 2, 'message': 'No such session'})
             else:
@@ -246,20 +244,15 @@ class Test(CodeqService):
                 request.reply({'code': 0, 'message': 'ok', 'hints': hints})
 
 
-class GetProblem(CodeqService):
+class GetCurrentSolution(CodeqService):
     def process(self, request):
         js = request.data
-        language = js.get('language')
-        problem_group = js.get('problem_group')
-        problem = js.get('problem')
-        if language is None:
-            request.reply({'code': 1, 'message': 'Language identifier not given'})
-        elif problem_group is None:
-            request.reply({'code': 2, 'message': 'Problem group identifier not given'})
-        elif problem is None:
-            request.reply({'code': 3, 'message': 'Problem identifier not given'})
+        problem_id = js.get('problem_id')
+        if problem_id is None:
+            request.reply({'code': 1, 'message': 'Problem ID not specified'})
         else:
-            request.reply({'code': 0, 'message': 'ok', 'data': request.session.get_problem_data(language, problem_group, problem)})
+            request.reply({'code': 0, 'message': 'ok', 'data': request.session.current_solution(problem_id)})
+
 
 class LoadProblem(CodeqService):
     def process(self, request):
@@ -273,26 +266,28 @@ class LoadProblem(CodeqService):
             else:
                 request.reply({'code': 0, 'message': 'OK'})
 
+
 class EndProblem(CodeqService):
     def process(self, request):
         request.session.end_language_session()
         request.end()
 
+
 # maps actions to their handlers
 incoming_handlers = {
-    'list_problems': ProblemList(),
+    'create_session': CreateSession(),
     'login': Login(),
     'signup': Signup(),
     'change_password': ChangePassword(),
-    'get_problem': GetProblem(),
-    'logout': None,
+    'get_current_solution': GetCurrentSolution(),
+    'logout': Logout(),
     'activity': Activity(),
     'query': Query(),
     'python_exec': PythonExec(),
     'python_push': PythonPush(),
     'python_stop': PythonStop(),
     'hint': Hint(),
-    'settings': Settings(),
+    'update_settings': UpdateSettings(),
     'test': Test(),
     'load_problem': LoadProblem(),
     'end_problem': EndProblem()
@@ -323,6 +318,7 @@ class Request(object):
         """
         if data is None:
             self.end()
+            return
         if self._original_sid is not None:
             sid = data.get('sid')
             if sid is None:
@@ -356,10 +352,18 @@ def _invoke_handler(handler, request):
             logging.error('ERROR: the request was not concluded!')
             request.reply({'code': -1, 'message': 'Request processing did not provide a reply'})
         logging.debug('Processing finished')
+    except NotLoggedIn:
+        if request.is_finished:
+            logging.error('Caught the NotLoggedIn exception, but the request has already been replied to!')
+        else:
+            request.reply({'code': -1, 'message': 'Cannot execute the request: not logged in'})
     except Exception as e:
         logging.critical('ERROR: data processing failed: ' + str(e))
         logging.critical(traceback.format_exc())
-        request.reply({'code': -1, 'message': 'Internal error: ' + str(e)})
+        if request.is_finished:
+            logging.critical('The request has already been replied to, the client will not be aware of the error!')
+        else:
+            request.reply({'code': -1, 'message': 'Internal error: ' + str(e)})
 
 def serve_request(json_obj):
     if not isinstance(json_obj, dict):
diff --git a/server/socket.py b/server/socket.py
index ae2068f..561ab29 100644
--- a/server/socket.py
+++ b/server/socket.py
@@ -206,13 +206,12 @@ class JsonClientSocket(SocketHandler):
             # TODO: unregister from any internal mechanisms
 
     def send(self, json_obj):
-        js = json.dumps(json_obj)
-        m = str(len(js)) + u':' + js
-        bytes = m.encode('utf-8')
+        b = json.dumps(json_obj).encode('utf-8')
+        b = bytes(str(len(b)), 'utf-8') + b':' + b
         lock = self._write_lock
         lock.acquire()
         try:
-            self.socket.sendall(bytes)
+            self.socket.sendall(b)
         finally:
             lock.release()
 
diff --git a/server/user_session.py b/server/user_session.py
index 5618c90..cf5e120 100644
--- a/server/user_session.py
+++ b/server/user_session.py
@@ -1,4 +1,5 @@
 # coding=utf-8
+import traceback
 
 import uuid
 import threading  # multiprocessing.managers.BaseManager uses threading to serve incoming requests
@@ -7,64 +8,145 @@ import base64
 import random
 import db
 import server
-from errors.session import NoSuchSession, AuthenticationFailed, PasswordChangeFailed, UserExists, SignupFailed
+from errors.session import NoSuchSession, AuthenticationFailed, PasswordChangeFailed, UserExists, SignupFailed, NotLoggedIn
 import psycopg2.extras
 import datetime
+import logging
+import time
 
-__all__ = ['get_session_by_id', 'get_or_create_session', 'UserSession']
+__all__ = ['get_session_by_id', 'UserSession']
 
 sessions = {}  # maps session IDs to session objects
 
 module_access_lock = threading.Lock()  # use this lock to access the sessions dictionary
 
+_user_session_access_update_keys = {
+    'login', 'get_sid', 'get_uid', 'get_settings', 'update_settings',
+    'load_language_session', 'end_language_session', 'current_language_session',
+    'current_solution', 'update_solution', 'change_password'
+}
+
+_user_session_restricted_keys = {
+    'get_uid', 'get_settings', 'update_settings',
+    'load_language_session', 'end_language_session', 'current_language_session',
+    'current_solution', 'update_solution', 'change_password'
+}
+
+
 class UserSession(object):
     """Abstracts a user session.
-    Only public methods are available to the outside world due to the use of multiprocessing managers.
-    Therefore prefix any private methods with an underscore (_).
-    No properties are accessible; use getters and setters instead.
-    Values are passed by value instead of by reference (deep copy!).
+    When first created, the session represents an unauthenticated user.
+    As such only get_sid() and last_access are permitted, any other
+    action is refused.
+    To authenticate the session, call the login() or signup() method.
     """
-    def __init__(self, uid, username, settings):
+    def __init__(self):
         self._access_lock = threading.Lock()
         self.sid = uuid.uuid4().hex
-        self.uid = uid
-        self.username = username
+        self.uid = None
+        self.username = None
         self._lang_session = None
-        self.settings = settings
+        self.settings = {}
+        self.last_access = int(time.time())  # resolution: 1 second
+        with module_access_lock:
+            sessions[self.sid] = self
+
+    def __getattribute__(self, item):
+        """Update the last access time on every access to select methods/properties."""
+        if item in _user_session_access_update_keys:
+            self.last_access = int(time.time())
+        if (item in _user_session_restricted_keys) and (self.uid is None):  # be very careful! this works because 'uid' is not in _user_session_access_update_keys, otherwise we'd have an endless loop
+            raise NotLoggedIn('The user in session ' + self.sid + ' is not logged in')
+        return super(UserSession, self).__getattribute__(item)
+
+    def login(self, username, password):
+        with self._access_lock:
+            now = datetime.datetime.now()
+            conn = db.get_connection()
+            try:
+                cur = conn.cursor()
+                try:
+                    cur.execute('select id, password, gui_lang from codeq_user where username = %s', (username,))
+                    row = cur.fetchone()
+                    if row is None:
+                        raise AuthenticationFailed('No such user: {}'.format(username))
+                    if verify_password(password, row[1]):
+                        cur.execute('update codeq_user set last_login = %s  where id = %s', (str(now), row[0],))
+                        self.uid = row[0]
+                        self.username = username
+                        self.settings = {'lang': row[2]}
+                    else:
+                        raise AuthenticationFailed('Password mismatch')
+                finally:
+                    cur.close()
+            finally:
+                conn.commit()
+                db.return_connection(conn)
+
+    def signup(self, username, password, lang):
+        with self._access_lock:
+            now = datetime.datetime.now()
+            conn = db.get_connection()
+            try:
+                cur = conn.cursor()
+                try:
+                    cur.execute('select id from codeq_user where username = %s', (username,))
+                    row = cur.fetchone()
+                    if row:
+                        raise UserExists('User exists')
+                    else:
+                        cur.execute('insert into codeq_user (username, password, name, email, is_admin, is_active, date_joined, last_login, gui_lang) values (%s, %s, %s, %s, %s, %s, %s, %s, %s) returning id', (username, encrypt_password(password), None ,'', False, True, str(now), str(now), lang))
+                        row = cur.fetchone()
+                        if row is None:
+                            raise SignupFailed('Sign-up failed')
+                        self.uid = row[0]
+                        self.username = username
+                        self.settings = {'lang': lang}
+                finally:
+                    cur.close()
+            finally:
+                conn.commit()
+                db.return_connection(conn)
 
     def destroy(self):
         """Destroys the session."""
+        with module_access_lock:
+            del sessions[self.sid]
         with self._access_lock:
-            with module_access_lock:
-                del sessions[self.sid]
-            if self._lang_session is not None:
-                self._lang_session.destroy()
-                self._lang_session = None
-            # TODO: add any cleanups as features are added!
+            lang_session = self._lang_session
+            self._lang_session = None
+            uid = self.uid
+            sid = self.sid
+            username = self.username
+        if lang_session is not None:  # do not handle the language session holding the lock: we may deadlock if the callee calls the caller
+            lang_session.destroy()
+        logging.debug('User session destroyed: username={0}, uid={1}, sid={2}'.format(username, uid, sid))
+        # TODO: add any cleanups as features are added!
 
     def get_sid(self):
         return self.sid
 
     def get_uid(self):
-        return self.uid
+        with self._access_lock:
+            return self.uid
 
     def get_settings(self):
-        return self.settings
-
-    def update_settings(self, newSettings):
-        self.settings.update(newSettings)
+        with self._access_lock:  # settings are mutable, so we need a locked access
+            return self.settings
 
-    def write_settings_to_db(self):
-        conn = db.get_connection()
-        try:
-            cur = conn.cursor()
+    def update_settings(self, new_settings):
+        with self._access_lock:
+            self.settings.update(new_settings)
+            conn = db.get_connection()
             try:
-                cur.execute("UPDATE codeq_user SET gui_lang='" + self.settings['lang'] + "' WHERE id="+str(self.uid))
+                cur = conn.cursor()
+                try:
+                    cur.execute("update codeq_user set gui_lang = %s where id = %s", (self.settings['lang'], self.uid))
+                finally:
+                    cur.close()
             finally:
-                cur.close()
-        finally:
-            conn.commit()
-            db.return_connection(conn)
+                conn.commit()
+                db.return_connection(conn)
 
     def load_language_session(self, problem_id):
         with self._access_lock:
@@ -103,31 +185,18 @@ class UserSession(object):
         with self._access_lock:
             return self._lang_session
 
-    def get_problem_data(self, language, problem_group, problem):
-        mod = server.problems.load_problem(language, problem_group, problem, 'sl')
-        mod_language = server.problems.load_language(language, 'sl')
-
-        # Get generic and problem-specific hints.
-        hint = dict(mod_language.hint)
-        hint.update(mod.hint)
-        plan = mod.plan if hasattr(mod, 'plan') else []
-
+    def current_solution(self, problem_id):
+        with self._access_lock:
+            uid = self.uid
         conn = db.get_connection()
         try:
             cur = conn.cursor()
             try:
-                cur.execute("select l.id, l.name, g.id, g.name, p.id, p.name from problem p inner join language l on l.id = p.language_id inner join problem_group g on g.id = p.problem_group_id where l.identifier = %s and g.identifier = %s and p.identifier = %s", (language, problem_group, problem))
-                row = cur.fetchone()
-                problem_id = row[4]
-                result = {
-                    'language': {'id': row[0], 'identifier': language, 'name': row[1]},
-                    'problem_group': {'id': row[2], 'identifier': problem_group, 'name': row[3]},
-                    'problem': {'id': problem_id, 'identifier': problem, 'name': row[5], 'slug': mod.slug, 'description': mod.description, 'hint': hint, 'plan': plan}
-                }
-                cur.execute("select content from solution where problem_id = %s and codeq_user_id = %s", (problem_id, self.uid))
+                result = {}
+                cur.execute("select content from solution where problem_id = %s and codeq_user_id = %s", (problem_id, uid))
                 row = cur.fetchone()
                 if row:
-                    result['solution'] = row[0]
+                    result['solution'] = row[0] or ''
                 else:
                     result['solution'] = ''
                 return result
@@ -140,12 +209,13 @@ class UserSession(object):
     def update_solution(self, problem_id, trace, solution):
         if (trace is None) and (solution is None):
             return
+        uid = self.get_uid()
         conn = db.get_connection()
         try:
             cur = conn.cursor()
             try:
                 # TODO: convert to upsert with postgresql 9.5 to eliminate the small window where it's possible for more than one concurrent insert to execute
-                cur.execute('select id, trace, content from solution where codeq_user_id = %s and problem_id = %s for update', (self.uid, problem_id))
+                cur.execute('select id, trace, content from solution where codeq_user_id = %s and problem_id = %s for update', (uid, problem_id))
                 row = cur.fetchone()
                 if row:
                     if row[1]:
@@ -158,7 +228,7 @@ class UserSession(object):
                     cur.execute('update solution set content = %s, trace = %s where id = %s', (new_solution, psycopg2.extras.Json(new_trace), row[0]))
                 else:
                     # this is the first entry
-                    cur.execute('insert into solution (done, content, problem_id, codeq_user_id, trace) values (%s, %s, %s, %s, %s)', (False, solution, problem_id, self.uid, psycopg2.extras.Json(trace)))
+                    cur.execute('insert into solution (done, content, problem_id, codeq_user_id, trace) values (%s, %s, %s, %s, %s)', (False, solution, problem_id, uid, psycopg2.extras.Json(trace)))
             finally:
                 cur.close()
             conn.commit()
@@ -169,11 +239,12 @@ class UserSession(object):
             db.return_connection(conn)
 
     def change_password(self, password):
+        uid = self.get_uid()
         conn = db.get_connection()
         try:
             cur = conn.cursor()
             try:
-                cur.execute('update codeq_user set password = %s  where id = %s', (encrypt_password(password), self.uid,))
+                cur.execute('update codeq_user set password = %s  where id = %s', (encrypt_password(password), uid))
                 affected = cur.rowcount
                 if affected is None:
                     raise PasswordChangeFailed('Password change failed')
@@ -183,7 +254,6 @@ class UserSession(object):
             conn.commit()
             db.return_connection(conn)
 
-
     def send(self, json_obj):
         """Sends a message to the user.
 
@@ -202,6 +272,7 @@ class UserSession(object):
             self._lang_session = None
         # TODO: add any cleanups as features are added!
 
+
 def get_session_by_id(sid):
     with module_access_lock:
         s = sessions.get(sid, None)
@@ -209,62 +280,6 @@ def get_session_by_id(sid):
             raise NoSuchSession('There is no session with SID {}'.format(sid))
         return s
 
-def get_or_create_session(uid, username, sid=None, lan=None):
-    with module_access_lock:
-        if sid is not None:
-            s = sessions.get(sid)
-            if s is not None:
-                return s
-        settings = {}
-        if lan is not None:
-            #settings['lan'] or settings['lang'] ????
-            settings['lan'] = lan
-        s = UserSession(uid, username, settings)
-        sessions[s.sid] = s
-        return s
-
-def authenticate_and_create_session(username, password):
-    now = datetime.datetime.now()
-    conn = db.get_connection()
-    try:
-        cur = conn.cursor()
-        try:
-            cur.execute('select id, password, gui_lang from codeq_user where username = %s', (username,))
-            row = cur.fetchone()
-            if row is None:
-                raise AuthenticationFailed('No such user: {}'.format(username))
-            if verify_password(password, row[1]):
-                cur.execute('update codeq_user set last_login = %s  where id = %s', (str(now), row[0],))
-                return get_or_create_session(row[0], username, None, row[2])
-            raise AuthenticationFailed('Password mismatch')
-        finally:
-            cur.close()
-    finally:
-        conn.commit()
-        db.return_connection(conn)
-
-
-def signup(username, password):
-    now = datetime.datetime.now()
-    conn = db.get_connection()
-    try:
-        cur = conn.cursor()
-        try:
-            cur.execute('select id from codeq_user where username = %s', (username,))
-            row = cur.fetchone()
-            if row:
-                raise UserExists('User exists')
-            else:
-                cur.execute('insert into codeq_user (username, password, name, email, is_admin, is_active, date_joined, last_login, gui_lang) values (%s, %s, %s, %s, %s, %s, %s, %s, %s) returning id', (username, encrypt_password(password),None ,'', False, True, str(now), str(now), None ))
-                affected = cur.rowcount
-                if affected is None:
-                    raise SignupFailed('Signn up failed')
-        finally:
-            cur.close()
-    finally:
-        conn.commit()
-        db.return_connection(conn)
-
 
 def verify_password(plain_password, encrypted_password):
     elts = encrypted_password.split('$')
@@ -279,8 +294,11 @@ def verify_password(plain_password, encrypted_password):
     enc = hashlib.pbkdf2_hmac('sha256', plain_password.encode('utf-8'), elts[2].encode('utf-8'), rounds)
     return base64.b64encode(enc).decode('utf-8') == elts[3]
 
+
 _salt_chars = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'
 _salt_chars_len = len(_salt_chars)
+
+
 def encrypt_password(plain_password):
     rounds = 20000
     chosen_chars = []
@@ -292,4 +310,31 @@ def encrypt_password(plain_password):
 
 random.seed()
 
-# TODO: add a session timeout timer
+
+# a session timeout timer
+def _session_cleaner():
+    try:
+        while True:
+            try:
+                with module_access_lock:
+                    s = dict(sessions)  # make a copy of the collection of sessions, because it's a bad idea to iterate over a collection that is modified as we go, and we must not hold the module lock or else we deadlock
+                now = int(time.time())
+                for sid, session in s.items():
+                    try:
+                        if (now - session.last_access) > 3600:  # a session is considered stale if there's no activity for at least an hour
+                            logging.info('Expiring session: {}'.format(sid))
+                            session.destroy()
+                            server.handlers.send(None, sid, {'sid': sid, 'type': 'session_expire'})  # inform the frontend via system (meta) protocol of the session expiry
+                    except Exception as e:
+                        logging.critical('Error while checking last access or pruning the session {0}: {1}'.format(sid, str(e)))
+                        logging.critical(traceback.format_exc())
+            except Exception as e:
+                logging.critical('Error while pruning stale sessions: ' + str(e))
+                logging.critical(traceback.format_exc())
+            time.sleep(60)  # one minute interval between prune runs
+    finally:
+        logging.critical('The session cleaner thread terminated!')
+
+_session_cleaner_thread = threading.Thread(target=_session_cleaner)
+_session_cleaner_thread.daemon = True
+_session_cleaner_thread.start()
diff --git a/web/main.js b/web/main.js
index 37729b9..b3c096f 100644
--- a/web/main.js
+++ b/web/main.js
@@ -32,12 +32,29 @@ var server = new engine.Server({
 });
 
 http_server.on('request', function (request, response) {
-    var uriParts = request.url.split('/');  // uriParts[0] will be an empty string, because uri must start with a /
+    var uriParts = request.url.split('/'),  // uriParts[0] will be an empty string, because uri must start with a /
+        params, session;
 
     logger.debug('HTTP server request, URL: ' + request.url);
 
     if ((uriParts.length <= 1) || (uriParts[1] === 'ws')) {
-        server.handleRequest(request, response);
+        if ((uriParts.length == 3) && (uriParts[2].substring(0, 7) === 'logout?')) {
+            // special logout service
+            params = uriParts[2].substring(7).split('&')[0].split('=');
+            if ((params.length == 2) && (params[0] === 'sid')) {
+                session = sessions[params[1]];
+                if (session) {
+                    logger.debug('Logging out via AJAX, sid=' + params[1]);
+                    guiHandlers.logout(session, {'sid': params[1], 'action': 'logout'});
+                }
+            }
+            response.writeHead(200, {'Content-Type': 'text/plain'});
+            response.write('OK');
+            response.end();
+        }
+        else {
+            server.handleRequest(request, response);
+        }
     }
     else {
         response.writeHead(404, {'Content-Type': 'text/plain'});
@@ -59,69 +76,83 @@ var sessions = {
 // GUI action handlers, keyed by the action name, values are functions that take the session and the message, or truthy values
 var guiHandlers = {
     // special-case action handlers
-    'login': function actionLogin(session, message) {
-        // first-time connect: login
-        var tid = message['tid']; // remember any TID that is set
-        delete message['tid']; // delete any TID, we must use an internal one
-        logger.debug('Received a login request from GUI');
-        sendDataToPython(message).then(
-            function loginRequestOK(response) {
+    'logout': function actionLogout(session, message) {
+        // logout, the user quit the app
+        logger.debug('Logout GUI');
+        sendDataToPython(message).finally(function () {
+            session.end({'type': 'reset', 'code': 9999, 'message': 'Bye.'});
+        }).done();
+    },
+
+    // actions to use default handling should define truthy values that are not functions
+    // (this is to filter out unnecessary traffic before it hits Python)
+    'login':                true,
+    'signup':               true,
+    'change_password':      true,
+    'activity':             true,
+    'query':                true,
+    'python_exec':          true,
+    'python_push':          true,
+    'python_stop':          true,
+    'hint':                 true,
+    'test':                 true,
+    'get_current_solution': true,
+    'update_settings':      true,
+    'load_problem':         true,
+    'end_problem':          true,
+    'system':               true
+};
+
+var system_handlers = {
+    'create_session': function (session, message) {
+        sendDataToPython({'action': 'create_session'})
+            .then(function (response) {
                 var sid, existingSession;
-                if ((typeof tid !== 'undefined') && (tid !== null)) response['tid'] = tid;
-                else delete response['tid'];
                 if (response.code !== 0) {
-                    logger.debug('Python rejected login request from GUI');
-                    session.end(response);
+                    logger.debug('Python rejected create_session request from GUI');
+                    session.send({'type': 'reset', 'message': response.message});
                 }
                 else {
-                    logger.debug('Python accepted login request from GUI');
+                    logger.debug('Python accepted create_session request from GUI');
                     session.sid = sid = response.sid;
                     existingSession = sessions[sid];
                     sessions[sid] = session;
-                    session.send(response);
-                    if (existingSession) {
-                        existingSession.end({'code': -40, 'message': 'Supplanted with a new connection for the same session', 'sid': sid});
+                    if (existingSession && (existingSession !== session)) {
+                        existingSession.end({'type': 'reset', 'message': 'Supplanted with a new connection for the same session', 'sid': sid});
                     }
+                    session.send({'type': 'sid', 'sid': sid});
                 }
-            },
-            function loginRequestFailed(err) {
-                var response = {
-                        'code': -30
-                    },
-                    reason;
-                logger.debug('Failed to request login from Python');
-                if ((typeof tid !== 'undefined') && (tid !== null)) response['tid'] = tid;
-                if ((typeof err === 'object') && (err !== null)) reason = err.toString();
-                else reason = '' + err;
-                response['message'] = 'Login request failed: ' + reason;
-                session.end(response);
-            }
-        ).done();
-    },
-
-    'logout': function actionLogout(session, message) {
-        // logout, the user quit the app
-        logger.debug('Logout GUI');
-        sendDataToPython(message).finally(function () {
-            session.end({'code': 9999, 'message': 'Bye.'});
-        }).done();
+            })
+            .catch(function (error) {
+                session.send({'type': 'reset', 'message': 'Could not create a new session: ' + (error || 'unknown error')});
+            })
+            .done();
     },
 
-    // actions to use default handling should define truthy values that are not functions
-    // (this is to filter out unnecessary traffic before it hits Python)
-    'signup':           true,
-    'change_password':  true,
-    'activity':         true,
-    'query':            true,
-    'python_exec':      true,
-    'python_push':      true,
-    'python_stop':      true,
-    'hint':             true,
-    'test':             true,
-    'get_problem':      true,
-    'settings':         true,
-    'load_problem':     true,
-    'end_problem':      true
+    'connect_session': function (session, message) {
+        sendDataToPython({'action': 'login', 'sid': session.sid, 'username': message.username, 'password': message.password})
+            .then(function (response) {
+                var sid, existingSession;
+                if (response.code !== 0) {
+                    logger.debug('Python rejected connect_session request from GUI');
+                    session.send({'type': 'reset', 'message': response.message});
+                }
+                else {
+                    logger.debug('Python accepted connect_session request from GUI');
+                    session.sid = sid = response.sid;
+                    existingSession = sessions[sid];
+                    sessions[sid] = session;
+                    if (existingSession && (existingSession !== session)) {
+                        existingSession.end({'type': 'reset', 'message': 'Supplanted with a new connection for the same session', 'sid': sid});
+                    }
+                    session.send({'type': 'sid', 'sid': sid});
+                }
+            })
+            .catch(function (error) {
+                session.send({'type': 'reset', 'message': 'Could not connect the existing session: ' + (error || 'unknown error')});
+            })
+            .done();
+    }
 };
 
 server.on('connection', function (socket) {
@@ -205,6 +236,14 @@ server.on('connection', function (socket) {
             return;
         }
 
+        if (typeof m.type === 'string') {
+            // a system (meta) protocol message
+            handler = system_handlers[m.type];
+            if (!handler) logger.error('Received an unknown system message from client: ' + m.type);
+            else handler(session, m);
+            return;
+        }
+
         tid = m['tid']; // transaction ID
         if (typeof tid !== 'number') {
             fatal({"code": -3, "message": "Transaction ID is missing or is not an integer."});
@@ -266,6 +305,10 @@ server.on('connection', function (socket) {
     });
 });
 
+//var sessionActivityTimeout = setInterval(function () {
+//    var sid;
+//}, 4200000); // once every 70 minutes
+
 
 // ========== Python server connection ==========
 
@@ -304,6 +347,27 @@ var processPacketFromPython = function processPacketFromPythonFunc(packetString)
         return;
     }
 
+    if (typeof m.type === 'string') {
+        // system (meta) protocol message
+        switch (m.type) {
+            case 'session_expire':
+                if (typeof m.sid === 'string') {
+                    session = sessions[m.sid];
+                    if (session) session.end({'type': 'reset', 'message': 'Session expired'}); // send a system message to force the client to reset
+                    else logger.warn('Received the session_expire system message from Python for a sid not handled by me: ' + m.sid);
+                }
+                else {
+                    logger.error('Received the session_expire system message from Python, but no sid was included');
+                }
+                break;
+
+            default:
+                logger.warn('An unknown system message from Python: ' + m.type);
+                break;
+        }
+        return;
+    }
+
     if ((typeof m.tid !== 'undefined') && (m.tid !== null)) {
         if ((typeof m.tid === 'string') && (m.tid.charAt(0) === 'i')) {
             // internal TID: only one way to key this one