_bt_delitems: change before WAL?

Hi,

It's often said that the golden rule of WAL is that we must log the
changes before actually doing the changes.  However it seems to me that
in _bt_delitems we're modifying the page (calling PageIndexMultiDelete)
before actually logging what's going to happen.  Why is this OK?

I see that we're holding the "super exclusive" lock on the page, but
does this promise that the bgwriter won't write the page early?  Another
thing that I notice is that the buffer is not marked dirty until much
later in btbulkdelete.  But what if the buffer was dirty beforehand?

-- 
Alvaro Herrera                                http://www.CommandPrompt.com/
PostgreSQL Replication, Consulting, Custom Development, 24x7 support