[Utilities] Fix getter in UniversalFallback

[odow]

While thinking about jump-dev/JuMP.jl#2770. The current behavior is:

julia> using JuMP, Gurobi

julia> model = Model(Gurobi.Optimizer)
Academic license - for non-commercial use only - expires 2021-11-06
A JuMP Model
Feasibility problem with:
Variables: 0
Model mode: AUTOMATIC
CachingOptimizer state: EMPTY_OPTIMIZER
Solver name: Gurobi

julia> MOI.get(model, Gurobi.ModelAttribute("IsMIP"))
ERROR: KeyError: key Gurobi.ModelAttribute("IsMIP") not found
Stacktrace:
 [1] getindex
   @ ./dict.jl:482 [inlined]
 [2] _get
   @ ~/.julia/packages/MathOptInterface/D0LUL/src/Utilities/universalfallback.jl:264 [inlined]
 [3] get
   @ ~/.julia/packages/MathOptInterface/D0LUL/src/Utilities/universalfallback.jl:309 [inlined]
 [4] get(model::MathOptInterface.Utilities.CachingOptimizer{MathOptInterface.Bridges.LazyBridgeOptimizer{Gurobi.Optimizer}, MathOptInterface.Utilities.UniversalFallback{MathOptInterface.Utilities.Model{Float64}}}, attr::Gurobi.ModelAttribute)
   @ MathOptInterface.Utilities ~/.julia/packages/MathOptInterface/D0LUL/src/Utilities/cachingoptimizer.jl:808
 [5] get(model::Model, attr::Gurobi.ModelAttribute)
   @ JuMP ~/.julia/dev/JuMP/src/JuMP.jl:1217
 [6] top-level scope
   @ REPL[3]:1

This. is clearly unhelpful.

We return nothing variables and constraints, so it seems sensible to do the same with other attributes:

MathOptInterface.jl/src/Utilities/universalfallback.jl

Lines 260 to 292 in f5cc481

	function _get(uf::UniversalFallback, attr::MOI.AbstractOptimizerAttribute)
	return uf.optattr[attr]
	end
	_get(uf::UniversalFallback, attr::MOI.AbstractModelAttribute) = uf.modattr[attr]
	function _get(
	uf::UniversalFallback,
	attr::MOI.AbstractVariableAttribute,
	vi::MOI.VariableIndex,
	)
	attribute_dict = get(uf.varattr, attr, nothing)
	if attribute_dict === nothing
	# It means the attribute is not set to any variable so in particular, it
	# is not set for `vi`
	return
	end
	return get(attribute_dict, vi, nothing)
	end
	function _get(
	uf::UniversalFallback,
	attr::MOI.AbstractConstraintAttribute,
	ci::MOI.ConstraintIndex,
	)
	attribute_dict = get(uf.conattr, attr, nothing)
	if attribute_dict === nothing
	# It means the attribute is not set to any constraint so in particular,
	# it is not set for `ci`
	return
	end
	return get(attribute_dict, ci, nothing)
	end

But that'd mean that MOI.get(model, Gurobi.ModelAttribute("IsMIP")) returned nothing, which is also unhelpful. We should instead return a meaningful error message.

Thoughts?

[blegat]

But that'd mean that MOI.get(model, Gurobi.ModelAttribute("IsMIP")) returned nothing, which is also unhelpful. We should instead return a meaningful error message.

Gurobi should probably implement is_set_by_optimize and return true. It's not really set by the optimization but is_set_by_optimize is only ever used by the CachingOptimizer to decide whether to dispatch to the cache or the optimizer and by JuMP to know whether it should check the termination status. Is the use case of being able to attach but not optimize and get the attribute important?

If it is, we should probably do what you suggest in jump-dev/JuMP.jl#2587 (comment)

[odow]

So for the Gurobi case, making the attributes is_set_by_optimize is probably okay.

Regardless, there's still a bug here: getting KeyError is unhelpful for users.

[odow]

For example, we used to have to try-catch our NLPBlock checks, but now if the block is missing it returns nothing. That seems better.

[blegat]

Yes, the new behavior with this PR seems better.