If it helps, basically what I need to do:

1) Find the first record in the table with an FK that has null.

2) Update that FK to point to a specified record in the foreign table.

The conflict would occur when two transactions perform #1 before the other
hits #2 as they will retrieve the same record in step 1. I realize that
likely the only way to detect this is try and catch the exception. I was
just soliciting possible other/better ideas.

The other problem is that I just simulated a similar situation, and at least
in the .net provider it comes back as a generic FB exeption. I have to see
if there is some way to detect this other than just looking at the exception
text.