Timeout handlers "timeout" when certain objects are returned

[cicdw]

The timeout handlers in prefect.utilities.executors generally work well. However, I've stumbled across an edge case in which they incorrectly signify a timeout occurred, when clearly one didn't occur:

import prefect
from prefect.utilities.executors import multiprocessing_timeout

multiprocessing_timeout(lambda: prefect.context, timeout=2)
# >> TimeoutError: Execution timed out.

The timeout handlers work by placing objects into Queues, so for some reason it seems that certain objects disappear from the Queue.

This is almost certainly related to the fact that context is thread.local as seen by the following example:

import prefect
from prefect.utilities.executors import multiprocessing_timeout

multiprocessing_timeout(lambda: prefect.context.to_dict(), timeout=2)
# returns a dictionary

I think returning objects from context is a reasonable enough thing that we should either find a way to detect this or raise a more informative error.

[cicdw]

In addressing this, we should also add tests to ensure the logger survives the timeout handling process (#887)

[cicdw]

For the multiprocessing timeout handler, this might be unavoidable. The best we could do is use multiprocess instead of multiprocessing, which uses the dill serializer instead of pickle, but either way prefect.context isn't handled well. It appears that customizing the serializer isn't really an exposed option right now, otherwise we could use cloudpickle. :/