[New-bugs-announce] [issue35311] exception unpickling error causes `multiprocessing.Pool` to hang forever

[Anthony Sottile]

New submission from Anthony Sottile <asottile at umich.edu>:

```
import multiprocessing

class E(Exception):
    def __init__(self, a1, a2):
        Exception.__init__(self, '{}{}'.format(a1, a2))

def f(_):
    raise E(1, 2)

multiprocessing.Pool(1).map(f, (1,))
```


Running this causes a hang:

```
$ python3.7 t2.py 
Exception in thread Thread-3:
Traceback (most recent call last):
  File "/usr/lib/python3.7/threading.py", line 917, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.7/threading.py", line 865, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.7/multiprocessing/pool.py", line 496, in _handle_results
    task = get()
  File "/usr/lib/python3.7/multiprocessing/contrnection.py", line 251, in recv
    return _ForkingPickler.loads(buf.getbuffer())
TypeError: __init__() missing 1 required positional argument: 'a2'

```

Upon eventual `^C`

```
^CTraceback (most recent call last):
Process ForkPoolWorker-1:
  File "t2.py", line 10, in <module>
    multiprocessing.Pool(1).map(f, (1,))
  File "/usr/lib/python3.7/multiprocessing/pool.py", line 290, in map
    return self._map_async(func, iterable, mapstar, chunksize).get()
  File "/usr/lib/python3.7/multiprocessing/pool.py", line 677, in get
    self.wait(timeout)
  File "/usr/lib/python3.7/multiprocessing/pool.py", line 674, in wait
Traceback (most recent call last):
  File "/usr/lib/python3.7/multiprocessing/process.py", line 297, in _bootstrap
    self.run()
  File "/usr/lib/python3.7/multiprocessing/process.py", line 99, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib/python3.7/multiprocessing/pool.py", line 110, in worker
    task = get()
  File "/usr/lib/python3.7/multiprocessing/queues.py", line 352, in get
    res = self._reader.recv_bytes()
  File "/usr/lib/python3.7/multiprocessing/connection.py", line 216, in recv_bytes
    buf = self._recv_bytes(maxlength)
  File "/usr/lib/python3.7/multiprocessing/connection.py", line 407, in _recv_bytes
    buf = self._recv(4)
  File "/usr/lib/python3.7/multiprocessing/connection.py", line 379, in _recv
    chunk = read(handle, remaining)
KeyboardInterrupt
    self._event.wait(timeout)
  File "/usr/lib/python3.7/threading.py", line 552, in wait
    signaled = self._cond.wait(timeout)
  File "/usr/lib/python3.7/threading.py", line 296, in wait
    waiter.acquire()
KeyboardInterrupt
Traceback (most recent call last):
  File "/usr/lib/python3.7/multiprocessing/util.py", line 265, in _run_finalizers
    finalizer()
  File "/usr/lib/python3.7/multiprocessing/util.py", line 189, in __call__
    res = self._callback(*self._args, **self._kwargs)
  File "/usr/lib/python3.7/multiprocessing/pool.py", line 611, in _terminate_pool
    "Cannot have cache with result_hander not alive")
AssertionError: Cannot have cache with result_hander not alive
```

(I've also tried this against master at 158695817d736df8b18682866033c87e46252309)

----------
components: Library (Lib)
messages: 330397
nosy: Anthony Sottile
priority: normal
severity: normal
status: open
title: exception unpickling error causes `multiprocessing.Pool` to hang forever
versions: Python 3.7, Python 3.8

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue35311>
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