https://github.com/GoogleCloudPlatform/google-cloud-python/blob/b2c7c11e4e24cf571cd5601803a883542136552c/pubsub/google/cloud/pubsub_v1/subscriber/policy/thread.py#L338

Since the future will block indefinitely, it would be nice to be able to specify a timeout. The current work-around we use is to ensure that the callback function doesn't block forever, but that requires introducing yet another thread construct into the situation.