Why does VACUUM FULL bother locking pages?

Hackers,

I'm reading the vacuum code and I just noticed that the routines
move_plain_tuple and move_chain_tuple expect the dest and source blocks
to be locked, and unlock them at exit.  The only caller of both is
repair_frag, whose only caller in turn is full_vacuum_rel.  Same thing
for update_hint_bits.

So, the only callers of both has already acquired appropiate locks at
the relation level -- nobody is going to be modifying the blocks while
they proceed.  So why bother locking the pages at all?  Is there a
reason or is this an historical accident?

-- 
Alvaro Herrera -- Valdivia, Chile         Architect, www.EnterpriseDB.com
"Puedes vivir solo una vez, pero si lo haces bien, una vez es suficiente"