Mesh deformation failure due to vector collinearity

[bobmyhill]

During a run of tests/hill_diffusion.prm with a larger end time, ASPECT failed at Timestep 192: t=0.096 seconds, dt=0.0005 seconds.

The error was:

An error occurred in line <1095> of file <.../deal.II-v9.7.0/include/deal.II/numerics/vector_tools_constraints.templates.h> in function
void dealii::VectorTools::internal::compute_nonzero_normal_flux_constraints_active_or_level(const DoFHandler<dim, spacedim> &, const unsigned int, const std::settypes::boundary_id &, const std::map<types::boundary_id, const Function<spacedim, number> *> &, AffineConstraints &, const Mapping<dim, spacedim> &, const bool, const IndexSet &, const unsigned int) [dim = 2, spacedim = 2, number = double]
The violated condition was:
std::fabs(determinant(t)) > 1e-3
Additional information:
Found a set of normal vectors that are nearly collinear.

@tjhei , @gassmoeller suggested I tag you in this one!

[tjhei]

Is the deformation large or about to blow up?

[bobmyhill]

There's nothing that I can see that would blow up. The topography is diffusing away, the material velocity is zero. The only thing I can see that could conceivably be an issue is that the top left and right cells are getting pretty deformed:

[tjhei]

I can reproduce the problem and it happens on one MPI rank as well. This is one the finest multigrid level (6), which surprised me. I will need to debug when I have more time...

[tjhei]

call stack:

#8  0x00007fffeadaaaa4 in dealii::deal_II_exceptions::internals::issue_error_noreturn<dealii::StandardExceptions::ExcMessage> (
    handling=dealii::deal_II_exceptions::internals::ExceptionHandling::abort_or_throw_on_exception, 
    file=0x7fffe73fa250 "/ssd/deal-git9/include/deal.II/numerics/vector_tools_constraints.templates.h", line=1102, 
    function=0x7fffe73fb0c0 "void dealii::VectorTools::internal::compute_nonzero_normal_flux_constraints_active_or_level(const dealii::DoFHandler<dim, spacedim>&, unsigned int, const std::set<unsigned int>&, const std::map<unsign"..., cond=0x7fffe73fb5f0 "std::fabs(determinant(t)) > 1e-3", 
    exc_name=0x7fffe73fb5a8 "ExcMessage( \"Found a set of normal vectors that are nearly collinear.\")", e=...)
    at /ssd/deal-git9/include/deal.II/base/exceptions.h:1078
#9  0x00007fffeb0a3f32 in dealii::VectorTools::internal::compute_nonzero_normal_flux_constraints_active_or_level<2, 2, double> (dof_handler=..., 
    first_vector_component=0, boundary_ids=std::set with 4 elements = {...}, function_map=std::map with 4 elements = {...}, constraints=..., 
    mapping=..., use_manifold_for_normal=true, refinement_edge_indices=..., level=6)
    at /ssd/deal-git9/include/deal.II/numerics/vector_tools_constraints.templates.h:1102
#10 0x00007fffeb09db13 in dealii::VectorTools::compute_no_normal_flux_constraints_on_level<2, 2> (dof_handler=..., first_vector_component=0, 
    boundary_ids=std::set with 4 elements = {...}, constraints=..., mapping=..., refinement_edge_indices=..., level=6, use_manifold_for_normal=true)
    at /ssd/deal-git9/include/deal.II/numerics/vector_tools_constraints.templates.h:1625
#11 0x000055555ac2bef4 in aspect::MeshDeformation::MeshDeformationHandler<2>::compute_mesh_displacements_gmg (this=0x55555bfb8c00)
    at /ssd/aspect-git-clean/source/mesh_deformation/interface.cc:1119
#12 0x000055555ac1128b in aspect::MeshDeformation::MeshDeformationHandler<2>::execute (this=0x55555bfb8c00)
    at /ssd/aspect-git-clean/source/mesh_deformation/interface.cc:539
#13 0x000055555a27df1e in aspect::Simulator<2>::solve_timestep (this=0x7fffffff8fc0) at /ssd/aspect-git-clean/source/simulator/core.cc:1918
#14 0x000055555a27ecce in aspect::Simulator<2>::run (this=0x7fffffff8fc0) at /ssd/aspect-git-clean/source/simulator/core.cc:2182
#15 0x000055555b51bd5e in (anonymous namespace)::run_simulator<2> (
    raw_input_as_string="set Dimension", ' ' <repeats 30 times>, "= 2\nset Use years instead of seconds", ' ' <repeats 11 times>, "= false\nset End time", ' ' <repeats 31 times>, "= 0.2\nset Maximum time step", ' ' <repeats 22 times>, "= 0.0005\ns"..., 
    input_as_string="set Dimension", ' ' <repeats 30 times>, "= 2\nset Use years instead of seconds", ' ' <repeats 11 times>, "= false\nset End time", ' ' <repeats 31 times>, "= 0.2\nset Maximum time step", ' ' <repeats 22 times>, "= 0.0005\ns"..., output_json=false, output_xml=false, 
    output_plugin_graph=false, validate_only=false) at /ssd/aspect-git-clean/source/main.cc:675
#16 0x000055555b4290c2 in main (argc=2, argv=0x7fffffffd528) at /ssd/aspect-git-clean/source/main.cc:961

The two colinear vectors are: (-1,0) and (-1,0), level=6.