From: Antonin Houska <ah@cybertec.at>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
cc: pgsql-hackers@postgresql.org
Subject: Re: Incorrect visibility test function assigned to snapshot
In-reply-to: <20180530132854.tkv37qiamepu3slh@alvherre.pgsql>
References: <20180530132854.tkv37qiamepu3slh@alvherre.pgsql>
Comments: In-reply-to Alvaro Herrera <alvherre@2ndquadrant.com>
   message dated "Wed, 30 May 2018 09:28:54 -0400."
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Date: Wed, 30 May 2018 15:45:32 +0200
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Alvaro Herrera <alvherre@2ndquadrant.com> wrote:

> On 2018-May-30, Antonin Houska wrote:
>=20
> > In the header comment, SnapBuildInitialSnapshot() claims to set
> > snapshot->satisfies to the HeapTupleSatisfiesMVCC test function, and in=
deed it
> > converts the "xid" array to match its semantics (i.e. the xid items eve=
ntually
> > represent running transactions as opposed to the committed ones). Howev=
er the
> > test function remains HeapTupleSatisfiesHistoricMVCC as set by
> > SnapBuildBuildSnapshot().
>=20
> Interesting.  While this sounds like an oversight that should have
> horrible consequences, it's seems not to because the current callers
> don't seem to care about the ->satisfies function.  Are you able to come
> up with some scenario in which it causes an actual problem?

Right, the current callers in the core do not seem to use that function. I =
hit
the issue when doing and testing some changes in an extension (pg_squeeze).

--=20
Antonin Houska
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