Command-line help for input options for factory created types

[ZedThree]

Replaces #2336 with a much simpler implementation. It does a very similar thing: pass an empty Options to a type to see what options it expects, and the associated default values and docstrings, and then print all that information.

#2336 tries to avoid actually constructing types, using a companion template class that just reads Options, which has some distinct advantages: we only need an Options to construct this companion class, so we don't need to set anything else up; we don't need to worry about any preconditions; we don't do any unnecessary work.

However, it does also have some glaring downsides: it requires either lots of duplication or ugly indirection; inheritance is painful to deal with; if default values depended on e.g. Mesh values, we have to be able to defer setting the real default value somehow.

So what we do here is just create a real object instead. Much simpler.

We need to make sure all the factories can take the explicit name of the type we want plus an Options, but this is not very much code. We also need to set up a global Mesh, but making a very small one is pretty cheap.

I was worried that there were some types that can only be constructed with >2 processors, but that doesn't seem to be the case.

Also, all the Solvers set most of their options in their init functions, which makes them a little bit pointless:

$  ./blob2d --help-solver arkode
Input options for Solver 'arkode':

solver:is_nonsplit_model_diffusive = true               # type: bool, doc: If not a split operator, treat RHS as diffusive?
solver:mms = false              # type: bool
solver:mms_initialise = false           # type: bool
solver:monitor_timestep = false         # type: bool

If we're happy with the direction of this PR, I'll change types to read the Options in their constructors instead.

[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: the parameter region_in is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]

Region<Ind3D> region_in = {}) const {
              ^
const        &

[github-actions]

warning: the parameter region_in is copied for each invocation but only used as a const reference; consider making it a const reference [performance-unnecessary-value-param]

Region<Ind3D> region_in = {}) const {
              ^
const        &

[github-actions]

warning: assigning newly created gsl::owner<> to non-owner MpiWrapper * [cppcoreguidelines-owning-memory]

bout::globals::mpi = new MpiWrapper();
^

[github-actions]

warning: do not use pointer arithmetic [cppcoreguidelines-pro-bounds-pointer-arithmetic]

throw BoutException(_("Usage is {} {} <name>\n"), argv[0], help_arg);
                                                  ^

[github-actions]

warning: do not use pointer arithmetic [cppcoreguidelines-pro-bounds-pointer-arithmetic]

printTypeOptions<Factory>(argv[i + 1]);
                          ^

[johnomotani]

Neat!

[bendudson]

Thanks @ZedThree (and happy New Year!). This looks like a great way to get this functionality in a simple way without lots of code changes.

The two-stage construction of the solvers is a bit odd, and moving everything into the constructor would be good. I think init was needed to resolve some kind of circular dependency when adding PETSc support around 2013, but perhaps there is a way to remove it now.

[johnomotani]

👍

[ZedThree]

Happy new year to you too @bendudson @johnomotani !

I'll have a look at squashing the solver init functions into the ctors in a separate PR. Might look at doing so for Mesh too? I'm less sure that will work though.

The other QoL missing from this PR is that the arguments must be separated with a space: it would be nice to split on equals too, but then we need to do more parsing of the command line, and at that point I almost want to find a library.
I'll investigate this more if people make noise about it.

[ZedThree]

(Also, this PR is a factor 10 smaller than #2336 😶)