When using multiprocessing.Pool, if the function run in the pool segfaults, the program will simply hang forever. However when using multiprocessing.Process directly, it runs fine, setting the exitcode to -11 as expected.

I would expect the Pool to behave similar to Process, or at the very least an exception to be raised instead of just silently hanging forever.

I was able to reproduce this issue both on Linux (Ubuntu 15.04) and Mac OS X.

So the reason this is happening is very simple:

When using Pool.apply, the task (function) is sent to the task queue, which is consumed by the worker. At this point the task is "in progress". However, the worker dies without being able to finish the task or in any other way tell the Pool that it can't finish the task. The actual process is then ended by the Pool but the task is still in limbo, so any attempt at getting a result will hang forever.

I'm not sure there's a straight forward way to solve this (the ways I can think of from the top of my head involve adding quite a bit of overhead to the Pool so it keeps track of which process/worker is handling which task at a given time, so if it exits prematurely this task can be finished), but at the very least this case should be documented I think.

I've added a patch that would simply warn the user if a worker exits prematurely.

See also issue 22393