[PATCH] pageinspect function to decode infomasks

Hi

Whenever I'm debugging some kind of corruption incident, possible
visibility bug, etc, I always land up staring at integer infomasks or using
a SQL helper function to decode them.

That's silly, so here's a patch to teach pageinspect how to decode
infomasks to a human readable array of flag names.

Example:

SELECT t_infomask, t_infomask2, flags
FROM heap_page_items(get_raw_page('test1', 0)),
     LATERAL heap_infomask_flags(t_infomask, t_infomask2, true) m(flags);
 t_infomask | t_infomask2 |                                   flags

------------+-------------+----------------------------------------------------------------------------
       2816 |           2 |
{HEAP_XMIN_COMMITTED,HEAP_XMIN_INVALID,HEAP_XMAX_INVALID,HEAP_XMIN_FROZEN}
(1 row)


To decode individual mask integers you can just call it directly. It's
strict, so pass 0 for the other mask if you don't have both, e.g.

SELECT heap_infomask_flags(2816, 0);

The patch backports easily to older pageinspect versions for when you're
debugging something old.

BTW, I used text[] not enums. That costs a fair bit of memory, but it
doesn't seem worth worrying too much about in this context.

For convenience it also tests and reports HEAP_LOCKED_UPGRADED and
HEAP_XMAX_IS_LOCKED_ONLY as pseudo-flags.

I decided not to filter
out HEAP_XMIN_COMMITTED,HEAP_XMIN_INVALID,HEAP_XMAX_INVALID
when HEAP_XMIN_FROZEN is set; that doesn't make sense when we
examine HEAP_XMAX_IS_LOCKED_ONLY or HEAP_LOCKED_UPGRADED, and filtering
them out could be just as confusing as leaving them in.

The infomask2 natts mask is ignored. You can bitwise-and it out in SQL
pretty easily if needed. I could output it here as a constructed text
datum, but it seems mostly pointless.

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services