Re: HOT chain validation in verify_heapam()

On Wed, Mar 22, 2023 at 8:38 PM Andres Freund <andres@anarazel.de> wrote:
> skink / valgrind reported in a while back and found another issue:
>
> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=skink&dt=2023-03-22%2021%3A53%3A41
>
> ==2490364== VALGRINDERROR-BEGIN
> ==2490364== Conditional jump or move depends on uninitialised value(s)
> ==2490364==    at 0x11D459F2: check_tuple_visibility (verify_heapam.c:1379)
...
> ==2490364==  Uninitialised value was created by a stack allocation
> ==2490364==    at 0x11D45325: check_tuple_visibility (verify_heapam.c:994)

OK, so this is an interesting one. It's complaining about switch
(xmax_status), because the get_xid_status(xmax, ctx, &xmax_status)
used in the previous switch might not actually initialize xmax_status,
and apparently didn't in this case. get_xid_status() does not set
xmax_status except when it returns XID_BOUNDS_OK, and the previous
switch falls through both in that case and also when get_xid_status()
returns XID_INVALID. That seems like it must be the issue here. As far
as I can see, this isn't related to any of the recent changes but has
been like this since this code was introduced, so I'm a little
confused about why it's only causing a problem now.

Nonetheless, here's a patch. I notice that there's a similar problem
in another place, too. get_xid_status() is called a total of five
times and it looks like only three of them got it right. I suppose
that if this is correct we should back-patch it.

-- 
Robert Haas
EDB: http://www.enterprisedb.com