Petsc Shell-Matrix

[BalticPinguin]

I need some assistance with Shell-matrices:

Setup

I have defined a shell-matrix derivative
class HFShellMat : public libMesh::ShellMatrix<libMesh::Number>
for which the matrix-vector product is constructed as "Ax + f \Delta^-1 (f x)" with a given matrix A and a vector f.

   Hamiltonian = new HFShellMat(...);

   PetscLinearSolver<Number> linear_solver(system.comm());
   linear_solver.init();

  // essentially copied from systems_of_equations_ex6.C: sets the following:
  // ierr = KSPSetType (_petsc_linear_solver.ksp(), const_cast<KSPType>(KSPGMRES));
  // ierr = PCSetType (_petsc_linear_solver.pc(), const_cast<PCType>(PCNONE));
  PetscSolverConfiguration solver_conf(linear_solver);
   linear_solver.set_solver_configuration (solver_conf);
  ....
    > furthter setup of  rhs, solution_x, etc.
  ....
  auto rval = linear_solver.solve(*Hamiltonian, *solution_x, rhs, tol, maxits);

Error message

running this yields

[0]PETSC ERROR: Object is in wrong state
[0]PETSC ERROR: Not for unassembled matrix
[0]PETSC ERROR: See https://petsc.org/release/faq/ for trouble shooting.

[0]PETSC ERROR: #1 MatMult() at /data/tm162/bin/petsc-git/petsc_dsk/src/mat/interface/matrix.c:2588
[0]PETSC ERROR: #2 KSP_MatMult() at /data/tm162/bin/petsc-git/petsc_dsk/include/petsc/private/kspimpl.h:347
[0]PETSC ERROR: #3 KSPInitialResidual() at /data/tm162/bin/petsc-git/petsc_dsk/src/ksp/ksp/interface/itres.c:48
[0]PETSC ERROR: #4 KSPSolve_GMRES() at /data/tm162/bin/petsc-git/petsc_dsk/src/ksp/ksp/impls/gmres/gmres.c:226
[0]PETSC ERROR: #5 KSPSolve_Private() at /data/tm162/bin/petsc-git/petsc_dsk/src/ksp/ksp/interface/itfunc.c:899
[0]PETSC ERROR: #6 KSPSolve() at /data/tm162/bin/petsc-git/petsc_dsk/src/ksp/ksp/interface/itfunc.c:1071

Analysis

It boils down to the problem that mat->assembled is not set true; but I didn't find any indication on where/how to set it for shell-matrices correctly.

Since the error occurs before actually calling any of the functions from HFShellMat, I don't think that the error is related to my definition of that function?

The only example using ShellMatrix seems to be miscellaneous_ex4, but I couldn't find a place that looks relevant to me.

Question

Does anybody spot an error in my overall setup? Do I possibly miss something obvious?
I am not even sure if it belongs here, or rather is related to Petsc (I use version 3.19.2)...

[BalticPinguin]

I have created a minimal example. I think, this may help to understand my issue!?

[BalticPinguin]

For the record: I could finally localize the error.
In this gist, I copy-pasted the libMesh-functions and removed those cases that are not called.

When solving a ShellMatrix without preconditioner, the call to KSPSetOperators is missing.
Corresponding PR is in the making, so it should be fixed.