Re: BUG #8673: Could not open file "pg_multixact/members/xxxx" on slave during hot_standby

On 2014-06-06 13:04:29 -0400, Tom Lane wrote:
> Alvaro Herrera <alvherre@2ndquadrant.com> writes:
> > After some simplification I think it should be clearer.  Thanks Andres
> > for commenting offlist.
>
> I find this not only unreadable but a bit scary:
>
> !         if (offset + MAX_MEMBERS_IN_LAST_MEMBERS_PAGE < offset)
>
> I'm worried whether some compilers might not decide that that's a
> can't-happen situation, and optimize the test away entirely.

offset is a (via typedefs) uint32 and unsigned overflow is defined in
the C standard, so that shouldn't be a allowed optimization. Don't we
already have a fair amount of similar tests around? Around TransactionId
wraparound for exzmple:
    xidWrapLimit = oldest_datfrozenxid + (MaxTransactionId >> 1);
    if (xidWrapLimit < FirstNormalTransactionId)
        xidWrapLimit += FirstNormalTransactionId;
and similar things.

Greetings,

Andres Freund

--
 Andres Freund                       http://www.2ndQuadrant.com/
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