[New-bugs-announce] [issue42963] [multiprocessing] Calling pool.terminate() from an error_callback causes deadlock

Sammy Jelin report at bugs.python.org
Mon Jan 18 20:08:51 EST 2021

New submission from Sammy Jelin <sjelin at gmail.com>:

As the title says, calling `pool.terminate()` inside an `error_callback` handler causes a deadlock.  The deadlock always happens, so it's not a race condition or thread-safety issue.

Simple repro:

from multiprocessing import Pool
p = Pool()

def error_callback(x):
    print(f'error: {x!r}')
    print('this message is never seen, because p.termiante() deadlocks')

p.apply_async(lambda: None, error_callback=error_callback)

# The following lines are technically aren't threadsafe,
# but I manually verified that that wasn't the problem.
print('this is also never seen, because the task handler is stuck in the deadlock')

This will print the following line and then hang:
error: PicklingError("Can't pickle <function <lambda> at 0x112c55e18>: attribute lookup <lambda> on __main__ failed")

The hanging happens inside `_help_stuff_finish`, when we call `inqueue._rlock.acquire()`.  As far as I can tell, `_handle_tasks` is already holding the lock when it calls the error_callback, so when `_terminate_pool` calls `_help_stuff_finish` a deadlock occurs.

Calling `p.terminate()` from a success callback doesn't appear to be an issue, likely because success callbacks are called via `_handle_results` instead of via `_handle_tasks`.

If calling `p.terminate()` from an error_callback isn't supported, that should be one of those big red warnings in the documentation.

I verified that this bug happens on 3.6 and 3.7, a skimmed the code on the github project to verify that it was likely still an issue.

components: Library (Lib)
messages: 385240
nosy: sjelin
priority: normal
severity: normal
status: open
title: [multiprocessing] Calling pool.terminate() from an error_callback causes deadlock
versions: Python 3.6, Python 3.7

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