Re: [HACKERS] Issues with logical replication

> On 30 Nov 2017, at 03:30, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com> wrote:
>
> On 30/11/17 00:47, Andres Freund wrote:
>> On 2017-11-30 00:45:44 +0100, Petr Jelinek wrote:
>>> I don't understand. I mean sure the SnapBuildWaitSnapshot() can live
>>> with it, but the problematic logic happens inside the
>>> XactLockTableInsert() and SnapBuildWaitSnapshot() has no way of
>>> detecting the situation short of reimplementing the
>>> XactLockTableInsert() instead of calling it.
>>
>> Right. But we fairly trivially can change that. I'm remarking on it
>> because other people's, not yours, suggestions aimed at making this
>> bulletproof. I just wanted to make clear that I don't think that's
>> necessary at all.
>>
>
> Okay, then I guess we are in agreement. I can confirm that the attached
> fixes the issue in my tests. Using SubTransGetTopmostTransaction()
> instead of SubTransGetParent() means 3 more ifs in terms of extra CPU
> cost for other callers. I don't think it's worth worrying about given we
> are waiting for heavyweight lock, but if we did we can just inline the
> code directly into SnapBuildWaitSnapshot().
>
> --
> Petr Jelinek http://www.2ndQuadrant.com/
> PostgreSQL Development, 24x7 Support, Training & Services
> <0001-Make-XactLockTableWait-work-for-transactions-that-ar.patch>

+1

Seems that having busy loop is the best idea out of several discussed.

I thought about small sleep at the bottom of that loop if we reached topmost
transaction, but taking into account low probability of that event may be
it is faster to do just busy wait.

Also some clarifying comment in code would be nice.

Stas Kelvich
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company