Re: [HACKERS] [COMMITTERS] pgsql: Fix freezing of a dead HOT-updated tuple

On 2017-11-13 19:03:41 -0800, Andres Freund wrote:
> > Staring at the vacuumlazy hunk I think I might have found a related bug:
> > heap_update_tuple() just copies the old xmax to the new tuple's xmax if
> > a multixact and still running. It does so without verifying liveliness
> > of members. Isn't that buggy? Consider what happens if we have three
> > blocks: 1 has free space, two is being vacuumed and is locked, three is
> > full and has a tuple that's key share locked by a live tuple and is
> > updated by a dead xmax from before the xmin horizon. In that case afaict
> > the multi will be copied from the third page to the first one. Which is
> > quite bad, because vacuum already processed it, and we'll set
> > relfrozenxid accordingly. I hope I'm missing something here?
>
> Trying to write a testcase for that now.

This indeed happens, but I can't quite figure out a way to write an
isolationtester test for this. The problem is that to have something
reproducible one has to make vacuum block on a cleanup lock, but that
currently doesn't register as waiting for the purpose of
isolationtester's logic.

Greetings,

Andres Freund