# Buggy code
def can_create_engine_wrapper(self):
    """
    Periodically checks whether the engine wrapper can be created.
    Notifies when it's ready by calling the callback of the deferred being returned.
    :return: A deferred that will be called when you can create the engine wrapper.
    """
    can_create_deferred = Deferred()

    def do_check():
        with self.dllock:
            if not self.cew_scheduled:
                self.ltmgr = self.session.lm.ltmgr
                dht_ok = (
                    not isinstance(self.tdef, TorrentDefNoMetainfo)
                    or self.ltmgr.is_dht_ready()
                )
                tunnel_community = self.ltmgr.trsession.lm.tunnel_community
                tunnels_ready = (
                    tunnel_community.tunnels_ready(self.get_hops())
                    if tunnel_community
                    else 1
                )

                if not self.ltmgr or not dht_ok or tunnels_ready < 1:
                    self._logger.info(
                        "LTMGR/DHT/session not ready, rescheduling create_engine_wrapper"
                    )

                    if tunnels_ready < 1:
                        self.dlstate = DLSTATUS_CIRCUITS
                        tunnel_community.build_tunnels(self.get_hops())
                    else:
                        self.dlstate = DLSTATUS_METADATA

                    # Schedule this function call to be called again in 5 seconds
                    self.session.lm.threadpool.add_task(do_check, 5)
                else:
                    can_create_deferred.callback(True)
            else:
                # Schedule this function call to be called again in 5 seconds
                self.session.lm.threadpool.add_task(do_check, 5)

    do_check()
    return can_create_deferred


# Traceback (isError)
Regression

Tribler.Test.Core.Modules.RestApi.test_downloads_endpoint.TestDownloadsEndpoint.test_remove_download_no_remove_data_param (from nosetests)
Failing for the past 1 build (Since Failed#2109 )
Took 4.4 sec.
Error Message

Test raised 1 unhandled Twisted exceptions:
[{'log_namespace': u'log_legacy', 'log_time': 1467938979.639, 'format': '%(log_legacy)s', 'log_system': '-', 'log_legacy': <twisted.logger._stdlib.StringifiableFromEvent object at 0x06D22830>, 'log_level': <LogLevel=critical>, 'system': '-', 'isError': 1, 'failure': <twisted.python.failure.Failure exceptions.AttributeError: 'NoneType' object has no attribute 'add_task'>, 'time': 1467938979.639, 'log_text': 'Unhandled Error\nTraceback (most recent call last):\n  File "C:\\Python27\\lib\\threading.py", line 763, in run\n    self.__target(*self.__args, **self.__kwargs)\n  File "C:\\workspace\\jenkins\\workspace\\GH_Tribler_PR_tests_win32\\tribler\\Tribler\\Core\\Utilities\\twisted_thread.py", line 55, in _reactor_runner\n    reactor.run(installSignalHandlers=False)\n  File "C:\\Python27\\lib\\site-packages\\twisted\\internet\\base.py", line 1194, in run\n    self.mainLoop()\n  File "C:\\Python27\\lib\\site-packages\\twisted\\internet\\base.py", line 1203, in mainLoop\n    self.runUntilCurrent()\n--- <exception caught here> ---\n  File "C:\\Python27\\lib\\site-packages\\twisted\\internet\\base.py", line 825, in runUntilCurrent\n    call.func(*call.args, **call.kw)\n  File "C:\\workspace\\jenkins\\workspace\\GH_Tribler_PR_tests_win32\\tribler\\Tribler\\Core\\Libtorrent\\__init__.py", line 27, in <lambda>\n    lambda_f = lambda a = args, kwa = kwargs: invoke_func(*a, **kwa)\n  File "C:\\workspace\\jenkins\\workspace\\GH_Tribler_PR_tests_win32\\tribler\\Tribler\\Core\\Libtorrent\\__init__.py", line 28, in invoke_func\n    download.session.lm.threadpool.add_task(lambda_f, 1)\nexceptions.AttributeError: \'NoneType\' object has no attribute \'add_task\'\n', 'log_format': u'{log_text}', 'message': (), 'why': None}]

# Fixed code
def can_create_engine_wrapper(self):
    """
    Periodically checks whether the engine wrapper can be created.
    Notifies when it's ready by calling the callback of the deferred being returned.
    :return: A deferred that will be called when you can create the engine wrapper.
    """
    can_create_deferred = Deferred()

    def do_check():
        with self.dllock:
            if not self.cew_scheduled:
                self.ltmgr = self.session.lm.ltmgr
                dht_ok = (
                    not isinstance(self.tdef, TorrentDefNoMetainfo)
                    or self.ltmgr.is_dht_ready()
                )
                tunnel_community = self.ltmgr.trsession.lm.tunnel_community
                tunnels_ready = (
                    tunnel_community.tunnels_ready(self.get_hops())
                    if tunnel_community
                    else 1
                )

                if not self.ltmgr or not dht_ok or tunnels_ready < 1:
                    self._logger.info(
                        "LTMGR/DHT/session not ready, rescheduling create_engine_wrapper"
                    )

                    if tunnels_ready < 1:
                        self.dlstate = DLSTATUS_CIRCUITS
                        tunnel_community.build_tunnels(self.get_hops())
                    else:
                        self.dlstate = DLSTATUS_METADATA

                    # Schedule this function call to be called again in 5 seconds
                    self.register_task(
                        "check_create_wrapper", reactor.callLater(5, do_check)
                    )
                else:
                    can_create_deferred.callback(True)
            else:
                # Schedule this function call to be called again in 5 seconds
                self.register_task(
                    "check_create_wrapper", reactor.callLater(5, do_check)
                )

    do_check()
    return can_create_deferred