Subject: Re: pgsql: Validate page level checksums in base backups
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
 Magnus Hagander <magnus@hagander.net>,
 Michael Banck <michael.banck@credativ.de>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Tom Lane <tgl@sss.pgh.pa.us>,
 Andres Freund <andres@anarazel.de>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
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From: David Steele <david@pgmasters.net>
Message-ID: <a701f950-30a9-d9b9-d231-82c917420040@pgmasters.net>
Date: Tue, 10 Apr 2018 17:45:12 -0400
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On 4/10/18 5:24 PM, Tomas Vondra wrote:
> 
> I think there's a bug in sendFile(). We do check checksums on all pages
> that pass this LSN check:
> 
>      /*
>       * Only check pages which have not been modified since the
>       * start of the base backup. Otherwise, they might have been
>       * written only halfway and the checksum would not be valid.
>       * However, replaying WAL would reinstate the correct page in
>       * this case.
>       */
>      if (PageGetLSN(page) < startptr)
>      {
>          ...
>      }
> 
> Now, imagine the page is new, i.e. all-zeroes. That means the LSN is 0/0
> too, and we'll try to verify the checksum - but we actually do not set
> checksums on empty pages.
> 
> So I think it should be something like this:
> 
>      if ((!PageIsNew(page)) && (PageGetLSN(page) < startptr))
>      {
>          ...
>      }
> 
> It might be worth verifying that the page is actually all-zeroes (and
> not just with corrupted pd_upper value. Not sure it's worth it.
> 
> I've found this by fairly trivial stress testing - running pgbench and
> pg_basebackup in a loop. It was failing pretty reliably (~75% of runs).
> With the proposed change I see no further failures.

Good catch, Tomas!

I should have seen this since I had the same issue when I implemented 
this feature in pgBackRest.

Anyway, I agree that your fix looks correct.

Thanks,
-- 
-David
david@pgmasters.net