From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Peter Geoghegan <pg@bowt.ie>,
        PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: buildfarm: could not read block 3 in file "base/16384/2662": read
 only 0 of 8192 bytes
In-reply-to: <17825.1535556627@sss.pgh.pa.us>
References: 
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 <1368.1533956577@sss.pgh.pa.us>
 <2e674bd6-3bc2-9d70-7d03-db48f7ea5877@2ndquadrant.com>
 <fcab55ec-660a-bc43-7fe1-09af3665e89a@2ndquadrant.com>
 <20180811140823.gyxqbtjbr5uitgsf@alap3.anarazel.de>
 <20180828075213.yqyl3w2gpd5heopd@alap3.anarazel.de>
 <20180829025658.brk3hfdczo3f73al@alap3.anarazel.de>
 <6970.1535512705@sss.pgh.pa.us>
 <20180829032714.6yu3bxdywcvyj2li@alap3.anarazel.de>
 <20180829032908.o46wdjmti6c6dfep@alap3.anarazel.de>
 <20180829083730.n645apqhb2gyih3g@alap3.anarazel.de>
 <17825.1535556627@sss.pgh.pa.us>
Comments: In-reply-to Tom Lane <tgl@sss.pgh.pa.us>
	message dated "Wed, 29 Aug 2018 11:30:27 -0400"
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Date: Wed, 29 Aug 2018 12:56:07 -0400
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I wrote:
> * We now recursively enter ScanPgRelation, which (again) needs to do a
> search using pg_class_oid_index, so it (again) opens and locks that.
> BUT: LockRelationOid sees that *this process already has share lock on
> pg_class_oid_index*, so it figures it can skip AcceptInvalidationMessages.

BTW, I now have a theory for why we suddenly started seeing this problem
in mid-June: commits a54e1f158 et al added a ScanPgRelation call where
there had been none before (in RelationReloadNailed, for non-index rels).
That didn't create the problem, but it probably increased the odds of
seeing it happen.

Also ... isn't the last "relation->rd_isvalid = true" in
RelationReloadNailed wrong?  If it got cleared during ScanPgRelation,
I do not think we want to believe that we got an up-to-date row.

			regards, tom lane