This commit fixes a race condition discovered by @artpol84. The race
happens when a signalling thread decrements the sync count to 0 then
goes to sleep. If the waiting thread runs and detects the count == 0
before going to sleep on the condition variable it will destroy the
condition variable while the signalling thread is potentially still
processing the completion. The fix is to add a non-atomic member to
the sync structure that indicates another process is handling
completion. Since the member will only be set to false by the
initiating thread and the completing thread the variable does not need
to be protected. When destoying a condition variable the waiting
thread needs to wait until the singalling thread is finished.
Thanks to @artpol84 for tracking this down.
Fixes#1813
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>
fb455f0802