It must be marked as completed during that lock hold since the
waiters field is set to null. Thus, any other thread attempting
a cancellation will fail to remove from waiters. Also, any
other thread attempting to cancel may set the completed field
to true which would cause accept() to fail as well.
Completion was always designed to happen while holding the
scheduling lock to prevent these race conditions. The code
was originally set up to complete while not holding the
scheduling lock to avoid invoking callbacks while holding the
lock, however the access to the completion field was not
considered.
Resolve this by marking the callback as completed during the
lock, but invoking the accept() function after releasing
the lock. This will prevent any cancellation attempts to be
blocked, and allow the current thread to complete the callback
without any issues.
