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Re: curious regression failures (was Re: [PATCHES] PL/TCL Patch to prevent postgres from becoming multithreaded)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andrew Dunstan <andrew(at)dunslane(dot)net>
Cc: Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: curious regression failures (was Re: [PATCHES] PL/TCL Patch to prevent postgres from becoming multithreaded)
Date: 2007-09-19 16:34:00
Message-ID: 22220.1190219640@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-patches
Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> pgbfprod=# select sysname, stage, snapshot from build_status where log ~ 
> $$read only \d+ of \d+ bytes$$;
>   sysname   |    stage     |      snapshot      
>  -----------+--------------+---------------------
>  zebra      | InstallCheck | 2007-09-11 10:25:03
>  wildebeest | InstallCheck | 2007-09-11 22:00:11
>  baiji      | InstallCheck | 2007-09-12 22:39:24
>  luna_moth  | InstallCheck | 2007-09-19 13:10:01
> (4 rows)

Fascinating.  So I would venture that (1) it's definitely our bug,
not something we could blame on NFS or whatever, and (2) we introduced
it fairly recently.  That specific error message wording exists only
in HEAD, but it's been there since 2007-01-03, so if there were a
pre-existing problem you'd think there would be some more matches.

The patterns I notice here are (1) they're all InstallCheck not Check
failures; (2) though not all at the same place in the tests, it's
a fairly short range; (3) it's all references to system catalogs,
though not all the same one.

My gut feeling is that we're seeing autovacuum truncate off an empty end
block and then a backend tries to reference that block again.  But there
should be enough interlocks in place to prevent such references.  Any
ideas out there?

			regards, tom lane

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