New submission from Sammy Jelin :
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}')
p.terminate()
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.
p.close()
p.join()
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 at 0x112c55e18>:
attribute lookup 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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<https://bugs.python.org/issue42963>
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