Race condition in threads initialization.

[yuyichao]

When working on #14083, I'm getting a intermittent initialization SegFault during bootstrap. It seems to be caused by a race condition between setting up the thread states and using it.

The jl_all_task_states[tid] is set up inside each threads here. It is used in init.c at here and here. (The first use in particular is very early and is really easy to trigger for me during compilation of inference0.ji)

sleeping for 1 second in init.c before the use make the issue disappear and sleeping for 2 second before the setup in addition to that make the issue happen 100% of the time so these appears to be not synchronized.

Report as an issue instead of a PR since I haven't figure out how do we do thread synchronization yet (especially at this early stage).

@vtjnash

[tkelman]

We're using libuv for our threads, right? Do we just need a uv_barrier http://docs.libuv.org/en/v1.x/threading.html#barriers before the first usage of any thread state to make sure they're ready?

[carnaval]

a fix is also in #12904 by the way but I don't know when this branch is intended to be merged