Subject: Re: logical decoding / rewrite map vs. maxAllocatedDescs
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
References: <20180809174746.aa6rbpsu4zjipzif@alvherre.pgsql>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <2eeb2d11-70be-edbe-198a-a73a30d58664@2ndquadrant.com>
Date: Thu, 9 Aug 2018 19:58:57 +0200
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On 08/09/2018 07:47 PM, Alvaro Herrera wrote:
> On 2018-Aug-09, Tomas Vondra wrote:
> 
>> I suppose there are reasons why it's done this way, and admittedly the test
>> that happens to trigger this is a bit extreme (essentially running pgbench
>> concurrently with 'vacuum full pg_class' in a loop). I'm not sure it's
>> extreme enough to deem it not an issue, because people using many temporary
>> tables often deal with bloat by doing frequent vacuum full on catalogs.
> 
> Actually, it seems to me that ApplyLogicalMappingFile is just leaking
> the file descriptor for no good reason.  There's a different
> OpenTransientFile call in ReorderBufferRestoreChanges that is not
> intended to be closed immediately, but the other one seems a plain bug,
> easy enough to fix.
> 
>> But wait, there's more - what happens after hitting the limit? We restart
>> the decoding process, and end up getting this:
>>
>>      ERROR:  53000: exceeded maxAllocatedDescs (492) while trying to open
>>      file "pg_logical/mappings/map-4000-4eb-1_60DE1E08-5376b5-537c6b"
>>      LOCATION:  OpenTransientFile, fd.c:2161
>>      LOG:  00000: starting logical decoding for slot "s"
>>      DETAIL:  streaming transactions committing after 1/6097DD48, reading
>>      WAL from 1/60275848
>>      LOCATION:  CreateDecodingContext, logical.c:414
>>      LOG:  00000: logical decoding found consistent point at 1/60275848
>>      DETAIL:  Logical decoding will begin using saved snapshot.
>>      LOCATION:  SnapBuildRestore, snapbuild.c:1872
>>      ERROR:  XX000: subtransaction logged without previous top-level txn
>>      record
> 
> Hmm, I wonder if we introduced some bug in f49a80c481f7.
> 

That's possible. I'm running on f85537a88d, i.e. with f49a80c481f7.

>> I'd say this implies the "can't open file" is handled in a way that corrupts
>> the mapping files, making it impossible to restart the decoding. AFAICS the
>> only option at this point is to drop the subscription and start over.
> 
> Wow, that seems pretty serious.  Clearly that's a different bug that
> must be fixed.
> 

Yep.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services