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Re: WIP: remove use of flat auth file for clientauthentication

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: WIP: remove use of flat auth file for clientauthentication
Date: 2009-08-29 18:03:41
Message-ID: 1251569021.4839.1517.camel@ebony.2ndQuadrant (view raw or whole thread)
Lists: pgsql-hackers
On Sat, 2009-08-29 at 11:19 -0400, Tom Lane wrote:
> Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> > Specifically, should I remove the parts of the HS patch that refresh
> > those files?
> Yes.  This was the last part that I was afraid might have insurmountable
> problems.  There are some bits yet to do but they're in the nature of
> crank-turning, I believe.  I think it's a good bet the flat files will
> be gone before the next commitfest starts.

OK, its gone. Shame really, took a while to make that work. :)

> BTW, one of the to-do bits is to find another way to initialize the
> XID wrap limit at database start.  Right now that's set up as a
> side-effect of scanning pg_database to create the flat DB file.
> The idea I had about it was to store the current wrap limit in
> checkpoint WAL records and use the most recent checkpoint's value
> as the initial limit.  Can you tell me what you're doing about the
> wrap limit in HS and whether that solution would be an issue for you?

...quakes in boots...

We're weren't doing anything at all. I guess we will be now; thanks for
alerting me.

In general we don't look at the actual xid values themselves, so going
past the xid max value itself shouldn't present an issue (that I can
see, but I will think longer on this).

A problem could occur if a standby query/snapshot lived long enough to
see the same xid in different epochs. Realistically that's a long time
and a query would be unlikely to survive that long without having first
constrained the flow of cleaning records by holding a snapshot open on
the primary. However, it could present a problem to someone and agree we
must have a clear remedial action. 

The standard remedial action is to kill queries that cause conflicts, so
we would need to kill queries based upon a wrap test. We could do that
as each xid arrives, but it would be best to do that via something rare
like checkpoint records, so that works for me.

 Simon Riggs 

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