RE: Perform streaming logical transactions by background workers and parallel apply

Dear Hou,

Thanks for updating patch. I will review yours soon, but I reply to your comment.

> > 04. applyparallelworker.c - LogicalParallelApplyLoop()
> >
> > ```
> > + shmq_res = shm_mq_receive(mqh, &len, &data, false);
> > ...
> > + if (ConfigReloadPending)
> > + {
> > + ConfigReloadPending = false;
> > + ProcessConfigFile(PGC_SIGHUP);
> > + }
> > ```
> >
> >
> > Here the parallel apply worker waits to receive messages and after dispatching
> > it ProcessConfigFile() is called.
> > It means that .conf will be not read until the parallel apply worker receives new
> > messages and apply them.
> >
> > It may be problematic when users change log_min_message to debugXXX for
> > debugging but the streamed transaction rarely come.
> > They expected that detailed description appears on the log from next
> > streaming chunk, but it does not.
> >
> > This does not occur in leader worker when it waits messages from publisher,
> > because it uses libpqrcv_receive(), which works asynchronously.
> >
> > I 'm not sure whether it should be documented that the evaluation of GUCs may
> > be delayed, how do you think?
>
> I changed the shm_mq_receive to asynchronous mode which is also consistent
> with
> what we did for Gather node when reading data from parallel query workers.

I checked your implementation, but it seemed that the parallel apply worker will not sleep
even if there are no messages or signals. It might be very inefficient.

In gather node - gather_readnext(), the same way is used, but I think there is a premise
that the wait-time is short because it is related with only one gather node.
In terms of parallel apply worker, however, we cannot predict the wait-time because
it is related with the streamed transactions. If such transactions rarely come, parallel apply workers may spend many CPU time.

I think we should wait during short time or until leader notifies, if shmq_res == SHM_MQ_WOULD_BLOCK.
How do you think?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED