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

Alvaro Herrera wrote:
> Serge Negodyuck wrote:
>
> > 2014-06-02 08:20:55 EEST 172.18.10.4 db PANIC: could not access status of
> > transaction 2080547
> > 2014-06-02 08:20:55 EEST 172.18.10.4 db DETAIL: Could not open file
> > "pg_multixact/members/14078": No such file or directory.
> > 2014-06-02 08:20:55 EEST 172.18.10.4 db CONTEXT: SQL statement "  UPDATE
> > ....."
>
> So as it turns out, this was caused because the arithmetic to handle the
> wraparound case neglected to handle multixacts with more members than
> the number that fit in the last page(s) of the last segment, leading to
> a number of pages in the 14078 segment (or whatever the last segment is
> for a given BLCKSZ) to fail to be initialized.  This patch is a rework
> of that arithmetic, although it seems little bit too obscure.

After some simplification I think it should be clearer.  Thanks Andres
for commenting offlist.

There is a different way to compute the "difference" proposed by Andres,
without using if/else; the idea is to cast the values to int64 and then
clamp.  It would be something like

    uint64    diff64;

    diff64 = Min((uint64) offset + nmembers,
            (uint64) offset + MULTIXACT_MEMBERS_PER_PAGE);
    difference = (uint32) Min(diff64, MaxMultiXactOffset);

(There are other ways to formulate this, of course, but this seems to me
to be the most readable one).  I am undecided between this and the one I
propose in the patch, so I've stuck with the patch.

--
Álvaro Herrera                http://www.2ndQuadrant.com/
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