Locks in functions

I have been reading in the dox that pgsql does not support cursors
from inside any of the interfaces, well the ones I care about anyways
which are PL and SPI...

I'm using SPI to spawn an external process to run... (image
manipulator), and I want to be able to control concurrent executions
of the process. I figure setting up a semaphore using an exclusive
record lock is my best bet...

Can anyone tell me if there is a way of doing this, maintaining a
lock
while its away processing while using either PL/pgsql or SPI...

I mention the cursors because it is clear that they cannot be done
without a patch, but it is unclear whether locks and
transactions/commits/rollbacks are supported here.

Thankyou..
scott