Race conditions in shm_mq.c

During my experiments with parallel workers I sometimes saw the "master" and
worker process blocked. The master uses shm queue to send data to the worker,
both sides nowait==false. I concluded that the following happened:

The worker process set itself as a receiver on the queue after
shm_mq_wait_internal() has completed its first check of "ptr", so this
function left sender's procLatch in reset state. But before the procLatch was
reset, the receiver still managed to read some data and set sender's procLatch
to signal the reading, and eventually called its (receiver's) WaitLatch().

So sender has effectively missed the receiver's notification and called
WaitLatch() too (if the receiver already waits on its latch, it does not help
for sender to call shm_mq_notify_receiver(): receiver won't do anything
because there's no new data in the queue).

Below is my patch proposal.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at