Shutdown interacts badly with requestSync()
I've been investigating #10860 (closed), and the problem goes pretty deep, so I'm going to record what I know here and come back to fix it properly later.
We have this mechanism
requestSync() for operations that need
to seize control of the whole runtime to do something. It is used by
requestSync() ensures that only one of these operations wins,
the others will
yieldCapability() to the winner, before
continuing with their own sync.
The problem is that this interacts badly with shutdown. Shutdown
might start at any time (initiated by
exitScheduler()). If it
starts during a sync, then a deadlock is likely: some
capabilities will be already shut down, and cannot be acquired by
acquireAllCapabilities(). This happens in #10860 (closed).
Really, shutdown should play the
requestSync() game too, but
that requires a lot of thought.