Attempting to interrupt the kernel while autocomplete is thinking crashes the runtime

[craigcitro]

Backstory: we've added some custom completers, some of which do things that amount to making a network request (say to find out the potential arguments to a function). Sometimes, this can be slow, and users are tempted to interrupt their kernel. Doing so then crashes the kernel.

Here's a really goofy repro:

def bad_completer(*args, **kwargs):
    import time
    time.sleep(10)
    return ['houses']

get_ipython().set_hook('complete_command', bad_completer, re_key='%%file')

%%file <TAB>

and then interrupt while you're waiting.

A naive fix would be to simply catch any exception here, but I'm not sure if that should happen in a more central place.

In particular, if a user does interrupt a completer, do we want to continue with the next one, or cancel completion completely?

[Carreau]

Ouch, thanks, we should definitively not crash.

I'm slowly thinking on how we can make a better completer, in particular now that master is Python 3 only I have the vague plan on trying to make everything generator/coroutine. I also want to implement a kind of timeout for completer. If you have any ideas about that that they are welcome.

[chmp]

A very similar issue arises on the code path for non-custom completes (see here).