"Diagonal" Columns of MPIAIJ Matrix?

[Andrew Colombi]

I'm letting PETSC_DECIDE how to distribute the rows and columns of my
non-square matrix, and I want to retrieve the columns it's decided to
make the "Diagonal" for a particular process.  How can I do that?  I'm
probably missing something obvious here, but I checked the docs.

In case some context helps (or inspires a work-around):

I want this information to count the number of d_nnz and o_nnz to
preallocate perfectly.  This shouldn't be hard to do, but I need to
know the boundary between off-diagonal and diagonal to do it.

Thanks,
-Andrew