From: Tom Lane <tgl@sss.pgh.pa.us>
To: Chapman Flack <chap@anastigmatix.net>
cc: pgsql-hackers@lists.postgresql.org
Subject: Re: libpq should not look up all host addresses at once
In-reply-to: <641169e3-0410-1317-c997-ceec24ff117a@anastigmatix.net>
References: <4913.1533827102@sss.pgh.pa.us>
 <641169e3-0410-1317-c997-ceec24ff117a@anastigmatix.net>
Comments: In-reply-to Chapman Flack <chap@anastigmatix.net>
	message dated "Thu, 09 Aug 2018 14:18:36 -0400"
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Date: Thu, 09 Aug 2018 14:32:58 -0400
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Chapman Flack <chap@anastigmatix.net> writes:
> On 08/09/2018 11:05 AM, Tom Lane wrote:
>> So I think what this code should do is (1) look up each hostname as it
>> needs it, not all at once, and (2) proceed on to the next hostname
>> if it gets a DNS lookup failure, not fail the whole connection attempt
>> immediately.  As attached.

> Would it be worth the complexity to be a little async about it,
> fling a few DNS requests out, and try the hosts in the order the
> responses come back?

It would be nice if an async connection request didn't have to block during
DNS lookups ... but I don't know of any portable library API for async DNS
requests, and it's most certainly not worth the trouble for us to write
our own version of getaddrinfo(3).

In practice, I think the async connection mode is mostly a legacy API
at this point anyway; surely most people who need that sort of behavior
are handling it nowadays by invoking libpq on a separate thread.  So I
just don't see it being worth a huge amount of work and maintenance
effort to get that to happen.  (Having said that, at least moving the
lookup from connectDBStart into PQconnectPoll, as this patch does,
is a step in the right direction.)

Now that I think about it, there may be some text in the libpq docs
claiming that the lookup happens in PQconnectStart not PQconnectPoll;
that would need adjustment.

			regards, tom lane