[BUG] ConnectionRefusedError: [Errno 111] Connection refused

[chaokunyang]

Describe the bug When using mars web client, if we cancel a task, then executing a new task ,we got Connection refused. To Reproduce To help us reproducing this bug, please provide information below:

1. Your Python version: python3.7.9
2. The version of Mars you use: mars 0.8.1
3. Versions of crucial packages, such as numpy, scipy and pandas: numpy==1.17.3 scipy==1.5.0 pands==1.3.0
4. Full stack of the error.

---------------------------------------------------------------------------
ConnectionRefusedError                    Traceback (most recent call last)
/tmp/ipykernel_14654/104550561.py in <module>
      1 df = df.astype({'idx': 'string','trd_latitude':'float','trd_longitude':'float'})
----> 2 df.execute()

~/miniconda3/lib/python3.7/site-packages/mars/core/entity/tileables.py in execute(self, session, **kw)
    460 
    461     def execute(self, session=None, **kw):
--> 462         result = self.data.execute(session=session, **kw)
    463         if isinstance(result, TILEABLE_TYPE):
    464             return self

~/miniconda3/lib/python3.7/site-packages/mars/core/entity/executable.py in execute(self, session, **kw)
     96 
     97         session = _get_session(self, session)
---> 98         return execute(self, session=session, **kw)
     99 
    100     def _check_session(self, session: SessionType, action: str):

~/miniconda3/lib/python3.7/site-packages/mars/deploy/oscar/session.py in execute(tileable, session, wait, new_session_kwargs, show_progress, progress_update_interval, *tileables, **kwargs)
   1796         show_progress=show_progress,
   1797         progress_update_interval=progress_update_interval,
-> 1798         **kwargs,
   1799     )
   1800 

~/miniconda3/lib/python3.7/site-packages/mars/deploy/oscar/session.py in execute(self, tileable, show_progress, *tileables, **kwargs)
   1589         try:
   1590             execution_info: ExecutionInfo = fut.result(
-> 1591                 timeout=self._isolated_session.timeout
   1592             )
   1593         except KeyboardInterrupt:  # pragma: no cover

~/miniconda3/lib/python3.7/concurrent/futures/_base.py in result(self, timeout)
    433                 raise CancelledError()
    434             elif self._state == FINISHED:
--> 435                 return self.__get_result()
    436             else:
    437                 raise TimeoutError()

~/miniconda3/lib/python3.7/concurrent/futures/_base.py in __get_result(self)
    382     def __get_result(self):
    383         if self._exception:
--> 384             raise self._exception
    385         else:
    386             return self._result

~/miniconda3/lib/python3.7/site-packages/mars/deploy/oscar/session.py in _execute(session, wait, show_progress, progress_update_interval, cancelled, *tileables, **kwargs)
   1750                 t._attach_session(session)
   1751 
-> 1752     execution_info = await session.execute(*tileables, **kwargs)
   1753     execution_info.add_done_callback(_attach_session)
   1754     cancelled = cancelled or asyncio.Event()

~/miniconda3/lib/python3.7/site-packages/mars/deploy/oscar/session.py in execute(self, *tileables, **kwargs)
    952             task_name=task_name,
    953             fuse_enabled=fuse_enabled,
--> 954             extra_config=extra_config,
    955         )
    956 

~/miniconda3/lib/python3.7/site-packages/mars/services/task/api/web.py in submit_tileable_graph(self, graph, task_name, fuse_enabled, extra_config)
    224             method="POST",
    225             headers={"Content-Type": "application/octet-stream"},
--> 226             data=body,
    227         )
    228         return res.body.decode()

~/miniconda3/lib/python3.7/site-packages/mars/services/web/core.py in _request_url(self, method, path, wrap_timeout_exception, **kwargs)
    194         try:
    195             res = await self._client.fetch(
--> 196                 path, method=method, raise_error=False, **kwargs
    197             )
    198         except HTTPTimeoutError as ex:

ConnectionRefusedError: [Errno 111] Connection refused

[wjsi]

Cannot reproduce with latest master branch and Jupyter Lab. Code cells:

import mars
import mars.remote as mr

start_sess = mars.new_session(web=True)
mars.new_session(start_sess.get_web_endpoint()).as_default()

# cancel and rerun
import time

def func():
    time.sleep(60)
    
mr.spawn(func).execute()