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Re: postgresql.conf error checking strategy

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: postgresql.conf error checking strategy
Date: 2011-05-08 03:37:07
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Lists: pgsql-hackers
Robert Haas wrote:
> On Wed, Apr 6, 2011 at 5:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > I just spent a rather confused half hour while testing my GUC
> > assign-hook patch, and when I finally figured out what was happening,
> > it made me wonder whether we should redesign the behavior a little bit.
> >
> > The current behavior of ProcessConfigFile is that it runs through all
> > the "name = value" pairs extracted from the file and tries to fully
> > verify each value (by seeing whether set_config_option with changeVal
> > false likes it). ?Only if every one of them checks out does it actually
> > apply any of the settings. ?Now this is nice and conservative --- the
> > aim is to avoid applying settings from a possibly corrupted file, in
> > case somebody fat-fingered their edits in a big way. ?But there's a
> > little problem:
> >
> > 1. It's possible that not all the backends agree on whether a setting
> > is valid. ?The case I was testing involved setting client_encoding
> > from the config file, so whether it succeeds depends on the database
> > encoding (some conversions might exist and others not). ?This means
> > that some backends might apply the postgresql.conf settings and others
> > not. ?That's pretty bad in itself, if something that needs to be
> > consistent system-wide is changing.
> >
> > 2. Only the postmaster reports config file problems at elevel LOG;
> > backends only complain at DEBUG3, to avoid cluttering the log with
> > lots of duplicate messages. ?This means that if you do have a few
> > backends that fail to adopt a setting, there likely won't be anything
> > in the log to tell you so. ?(The reason I was so confused is that I'd
> > raised log_min_messages to DEBUG5 to try to understand what was
> > happening ... but my backend-under-test wasn't adopting that setting,
> > and wasn't logging anything to tell me so either ...)
> >
> > So I'm thinking we should adopt a strategy that's less likely to result
> > in divergent behavior among different backends. ?The idea I have in mind
> > is to have the first "validation" pass only check that each name is a
> > legal GUC variable name, and not look at the values at all. ?If so, try
> > to apply all the values. ?Any that fail to apply we log as usual, but
> > still apply the others. ?ISTM that verifying the names should be enough
> > protection against broken files for practical purposes, and it should be
> > something that all backends will agree on even if there are individual
> > values that are not valid for all.
> >
> > Comments?
> I don't think now is a good time for a major behavior change in this
> area, and I'm not convinced this is the best possible design.
> There are a number of parameters which are currently PGC_POSTMASTER
> rather than PGC_SIGHUP precisely because of the possibility of
> backends being out of step with each other.  wal_level is an obvious
> example, and one that it would be *really* nice to be able to change
> without a server restart.  It would be nice to have a real solution to
> that problem, but this isn't it, and I don't want to engineer it right
> now.

Is this a TODO?

  Bruce Momjian  <bruce(at)momjian(dot)us>

  + It's impossible for everything to be true. +

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