pgsql: amcheck: Don't load the right sibling page into BtreeCheckState

amcheck: Don't load the right sibling page into BtreeCheckState

5ae2087202 implemented a cross-page unique constraint check by loading
the right sibling to the BtreeCheckState.target variable. This is wrong,
because bt_target_page_check() shouldn't change the target page. Also,
BtreeCheckState.target shouldn't be changed alone without
BtreeCheckState.targetblock.

However, the above didn't cause any visible bugs for the following reasons.
1. We do a cross-page unique constraint check only for leaf index pages.
2. The only way target page get accessed after a cross-page unique constraint
check is loading of the lowkey.
3. The only place lowkey is used is bt_child_highkey_check(), and that applies
only to non-leafs.

The reasons above don't diminish the fact that changing BtreeCheckState.target
for a cross-page unique constraint check is wrong. This commit changes this
check to temporarily store the right sibling to the local variable.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzk%2B2116uOXdOViA27SHcr31WKPgmjsxXLBs_aTxAeThzg%40mail.gmail.com
Author: Pavel Borisov

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0b5c161248110b164a3333006004c78f9529a109

Modified Files
--------------
contrib/amcheck/verify_nbtree.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)