gblues 97e09d179f Fix deadlocks when device is unplugged
== DETAILS
TIL that it's bad to call synchronization code from callbacks.

To avoid that, I made the following changes:

- Implemented an atomic swap (see previous commit) to avoid explicit
  locking when working with the event list
- ensure locks are only acquired in either the main thread or the
  I/O polling thread
- use an explicit polling loop; we still use async reads, but the
  read doesn't immediately re-invoke itself.
- remove the sleep in the polling thread.
- remove unnecessary locking in the thread cleanup call--verified that
  the list can't be modified while it is being executed.

== TESTING
I tested locally, and was able to disconnect/reconnect USB devices several times without the worker thread getting deadlocked.
2018-04-14 13:30:34 -07:00
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