[PATCH STABLE] worker: wait worker pid explicitly

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2016-07-24 15:32:10 +0900:
>  but that change wouldn't be trivial enough for stable.

Yes. I think so and didn't work on V2.

> I think writing status/debug messages would be cheaper than the I/O jobs
> running in workers. Currently the master process does nothing other than
> yielding progress messages, so the read loop should run fast.

I was aware of that pipe.

> Also it doesn't work under vanilla commandserver which replaces ui.fout by
> a pseudo file object.

I think commandserver needs some lock mechanism to protect its I/O. flock
is a good choice wherever supported. The problem is not only limited to
workers but also any other places using threads / processes.