[HACKERS] PATCH: pageinspect / add page_checksum and bt_page_items(bytea)

Hi,

while investigating some checksum-related issues, I needed to perform 
some forensics on a copy of a btree page (taken from another instance 
using 'dd').

But I've ran into two pageinspect limitations, hopefully addressed by 
this patch:

1) bt_page_items(bytea) not defined

We have heap_page_items(bytea) but not bt_page_items(bytea). I suspect 
this is partially historical API inconsistence, and partially due to the 
need to handle the btree metapage explicitly.

The original function simply threw an error with blkno=0, the new 
function simply checks for BTP_META page.

I believe this is sufficient, assuming the instance without data 
corruption (which pageinspect assumes anyway). With data corruption all 
bets are off anyway - for example the metapage might be written to a 
different block (essentially what I saw in the investigated issue). 
Actually, the flag check is better in this case - it detects the 
metapage, while the blkno=0 check fails to do that (leading to crash).

2) page_checksum()

When everything is fine, you can do page_header() which also includes 
the checksum. When the checksum gets broken, you may still dump the page 
using 'dd+pg_read_binary_file' to see the header, but clearly that 
checksum is wrong - and it's interesting to see the correct one and 
compare it to the checksum in the header.

This function makes it possible - it accepts the bytea image of the 
page, and blkno (so it's possible to see how would the block look if it 
was written somewhere else, for example).


BTW I've noticed the pageinspect version is 1.6, but we only have 
pageinspect--1.5.sql (and upgrade script to 1.6). Not sure that's 
entirely intentional?

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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