[petsc-dev] Creation of snes->vec_func

[Jed Brown]

Problem: User calls SNESSetDM() instead of SNESSetFunction(), then calls
SNESVISetVariableBounds(). The latter function checks for snes->vec_func
and errors.

We can't make SNESSetDM() create vec_func because (a) it is only logically
collective and (b) the DM might not be set up yet.

We can't call SNESSetUp() because that requires snes->vec_sol to have been
created (with a confusing error message about aliasing). We can work around
this by adding a new function SNESSetSolution(), but that's an awkward
interface and SNESSetUp() does other things like call SNESSetUp_VI() which
doesn't have the bounds yet.

It would be simple and consistent (I think) for SNESGetFunction() to create
vec_func if it is requested, does not already exist, and can be created.
Then we can simply call

  SNESGetFunction(snes,&snes->vec_func,PETSC_NULL,PETSC_NULL);

before checking whether vec_func is available yet. The problem with this is
that SNESGetFunction() is currently Not Collective, and this would make it
collective in that case.

If it's not acceptable for SNESGetFunction() to create vec_func, then we
need something like SNESSetUpFunction() that is collective and ensures that
vec_func exists. I'd rather not add this extra API if it's not necessary.


What should we do? (I'm asking in such a verbose way because I think we see
similar issues in other places, so we can reuse the solution here in other
places.)


Also, why does SNESVISetVariableBounds() call SNESSetType(snes,SNSVIRS)? I
didn't think we ever did this, but Barry wrote this line as part of the
huge http://petsc.cs.iit.edu/petsc/petsc-dev/rev/31c8ce41 , so I don't know
what to think.
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