btree shrinking again

Hello,

I'm just taking the btree shrinking problem, and saw in the README:

+ Deletions are handled by getting a super-exclusive lock on the target
page, so that no other backend has a pin on the page when the deletion
starts. This means no scan is pointing at the page. This is OK for
deleting leaf items, probably not OK for deleting internal nodes;
will need to think harder when it's time to support index compaction.

In what cases is not OK to delete an item from an internal node, holding
a super-exclusive lock? (I assume this means LockBufferForCleanup()).

--
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"Los dioses no protegen a los insensatos. Éstos reciben protección de
otros insensatos mejor dotados" (Luis Wu, Mundo Anillo)