 be yielded by the
generator).
</p>
<p>This procedure is intended to be called in a waitable procedure
invoked by a-sync.  It must (like the a-sync procedure) be called in
the same thread as that in which the default glib main loop runs.  As
mentioned above, the generator itself will run in its own thread.
</p>
<p>Exceptions may propagate out of this procedure if they arise while
setting up (that is, before the worker thread starts), which shouldn&rsquo;t
happen unless memory is exhausted or pthread has run