Re: Incorrect visibility test function assigned to snapshot

On May 30, 2018 9:45:32 AM EDT, Antonin Houska <ah(at)cybertec(dot)at> wrote:
>Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
>
>> On 2018-May-30, Antonin Houska wrote:
>>
>> > In the header comment, SnapBuildInitialSnapshot() claims to set
>> > snapshot->satisfies to the HeapTupleSatisfiesMVCC test function,
>and indeed it
>> > converts the "xid" array to match its semantics (i.e. the xid items
>eventually
>> > represent running transactions as opposed to the committed ones).
>However the
>> > test function remains HeapTupleSatisfiesHistoricMVCC as set by
>> > SnapBuildBuildSnapshot().
>>
>> 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).

What is that extension doing with that snapshot?

Andres
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