Why can't I get this two-way ev_async example to work?

[Åke Kullenberg]

I have attached a very simple ev_async example where I start one event
loop, and another loop in a thread. Both loops contain an async watcher.
When this watcher receives an event, a timer is kicked off. When the timer
fires, an async event is sent the other way. Hence I would expect these two
to bounce off each other indefinitely.

However when I run the program the slave thread's call to ev_async_send
doesn't seem to register. The callback to master_cb_async is never invoked.

$ bin/async
main - starting loop
    slave_loop - starting loop2
master_cb_timeout - sending async_send to loop2
    slave_cb_async - received async_send starting timer for loop2
    slave_cb_timeout - sending async_send to loop

Why doesn't this ev_async_send register? What am I doing wrong? If If this
is a clear case of RTFM just point me in the right direction. If not, I'd
appreciate some help.
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