Re: [HACKERS] Proposal for async support in libpq

>
> Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us> writes:
> > This all looks good. Another thing we really need it to be able to
> > cancel queries. This would be a big win, and looks like it could fit
> > into the scheme here.
>
> I thought about proposing a PQcancelAsync that would cancel the active
> query-in-progress. But that would require support on the backend side,
> and I am far from competent to make it happen. (libpq is simple enough
> that I'm not afraid to rewrite it, but making major mods to the backend
> is another story. I just got here this week...)
>
> If anyone who does know what they're doing is willing to make the
> necessary backend mods, I'm all for it. The libpq side would be
> easy enough.
>
> How would such cancellation interact with transactions, btw? Would
> you expect it to roll back only the current command, or abort the
> whole transaction? We'd also have to consider corner cases, like
> when the backend has already finished the query by the time it gets
> the cancel request.

It is pretty easy, just an elog(ERROR) would do it. The issue is
allowing the backend to see the request. We can put some checks in
tcop/postgres.c as it moves from module to module, and something in the
executor to check for the cancel, and do an elog(ERROR). It would be
nice if it arrived as out-of-band data, so we could check for input
quickly without having to actually process it if it is not a cancel
notification.

The out-of-band data will send a SIGURG signal to the backend, and we
can set a global variable, and check the variable at various places.

To do all this, we need to be able to send a query, and not have it
block, and it seems you are giving us this capability.

You supply the indication to the backend, and I will see that the
backend processes it properly.

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