Because a message we queue from the foreground may cause the background to complete, exit, and tear down the AsyncTask, we have to block queue processing until we've finished our call to AsyncTask::send().
Broadening the scope of a mutex is scary, but I audited the code of our four implementations of AsyncTask and I don't see any way this could cause a deadlock.
The current ::stop() implementation was not thread safe and we
should queue the ::stop() for the next loop iteration so even
if we call ::stop() before ::run(), it gets processed.
Not supported, because we don't have control over the Looper
timeout like we do for Loopers we create. On the main thread,
Android takes care of making the application sleep and sets
an arbitrary timeout. This could be properly fixed by implementing
timers in Java, but we don't really have the use case of timers
in the main thread.
This is needed for plugging into an existing Looper because
we won't call ::run() on the main thread (this is called
on the Java side of the application automatically by Android).
But we still need to wake up and process the events. For that
we create a pipe and write to the pipe to wake up the main
loop and we process events on the callback of the fd handler.
Fixes#4682
