Re: XID-wraparound hazards in LISTEN/NOTIFY

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Mark Dilger <hornschnorter(at)gmail(dot)com>
Cc: Martijn van Oosterhout <kleptog(at)gmail(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: XID-wraparound hazards in LISTEN/NOTIFY
Date: 2019-11-23 17:10:56
Message-ID: 28596.1574529056@sss.pgh.pa.us
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Mark Dilger <hornschnorter(at)gmail(dot)com> writes:
> On 11/23/19 8:34 AM, Tom Lane wrote:
>> It suddenly strikes me to worry that we have an XID wraparound hazard
>> for entries in the notify queue.

> Is it worth checking for this condition in autovacuum?

Dunno, maybe. It's a different avenue to consider, at least.

> There shouldn't be too much reason to back-patch any of this, since
> the change in 51004c717 only applies to v13 and onward. Or do you
> see the risk you described as "pretty minimal" as still being large
> enough to outweigh the risk of anything we might back-patch?

There may not be a risk large enough to worry about before 51004c717,
assuming that we discount cases like a single session staying
idle-in-transaction for long enough for the XID counter to wrap
(which'd cause problems for more than just LISTEN/NOTIFY). I haven't
analyzed this carefully enough to be sure. We'd have to consider
that, as well as the complexity of whatever fix we choose for HEAD,
while deciding if we need a back-patch.

regards, tom lane

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