Re: background sessions

On Sat, Mar 18, 2017 at 10:59 AM, Petr Jelinek
<petr(dot)jelinek(at)2ndquadrant(dot)com> wrote:
>> shm_redirect_to_shm_mq() wasn't really designed to be used this way;
>> it's designed for use by the worker, not the process that launched it.
>> If an error occurs while output is redirected, bad things will happen.
>> I think it would be better to find a way of sending that message to
>> the queue without doing this.
>
> Couldn't we just create special version of pq_endmessage that sends to
> shm_mq?

Yes, I think that sounds good.

>> Also, I suspect this is deadlock-prone. If we get stuck trying to
>> send a message to the background session while the queue is full, and
>> at the same time the session is stuck trying to send us a long error
>> message, we will have an undetected deadlock. That's why
>> pg_background() puts the string being passed to the worker into the
>> DSM segment in its entirety, rather than sending it through a shm_mq.
>
> Yeah I think this will need to use the nowait = true when sending to
> shm_mq and chunk the message if necessary...

Hmm, yeah. If you take breaks while sending to check for data that
you need to receive, then you should be fine.

--
Robert Haas
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