[Utilities] throw correct error if get_fallback fails

[odow]

This is a possible fix for #2410 (comment)

https://github.com/jump-dev/MathOptInterface.jl/actions/runs/7635259102

[blegat]

Why is this needed ?

[odow]

I added the try defensively to all methods. But the issue is that get_fallback might be used by something that doesn't support getting the ConstraintFunction. That might be because it isn't implemented, or it might be because the model uses a ZerosBridge (or similar). We should throw the proper error, instead of throwing whatever error actually threw, like "can't unbridge the Zeros bridge."

In the Cbc case, it's something like CachingOptimizer(LazyBridgeOptimizer(CachingOptimizer(Cbc))

The outer cache says "can you get ConstraintPrimal." The bridge says yes. Ends up erroring, falling back to get_fallback, and then errors when it tries to unbridge.

If it thew GetAttributeNotAllowed, then the outer cache would fallback to get_fallback and this would succeed.

Even if it didn't have an outer cache to fall back do, we should throw the correct GetAttributeNotAllowed for consistency.

[blegat]

Isn't this also fixed by throwing a GetAttributeNotAllowed for the ZerosBridge ?

[blegat]

If we don't see the error of the zerosbridge then the user won't know he just has to remove this bridge, that's an issue

[blegat]

What we could do is throw a custom error here:

MathOptInterface.jl/src/Bridges/Variable/map.jl

Lines 582 to 586 in 0249cfa

	error(
	"Cannot unbridge function because some variables are bridged by",
	" variable bridges that do not support reverse mapping, for example,",
	" `ZerosBridge`.",
	)

Here we try catch:

MathOptInterface.jl/src/Bridges/bridge_optimizer.jl

Line 2384 in 0249cfa

f = unbridged_function(b, func)::typeof(func)

If we see exactly this error, we thro a GetAttributeNotAllowed for ConstraintFunction with the error message of:

MathOptInterface.jl/src/Bridges/Variable/map.jl

Lines 582 to 586 in 0249cfa

	error(
	"Cannot unbridge function because some variables are bridged by",
	" variable bridges that do not support reverse mapping, for example,",
	" `ZerosBridge`.",
	)

otherwise, we rethrow

[odow]

This would address this particular issue with ZerosBridge, but it ignores with wider issue. get_fallback may fail, and instead of throwing a nice error, we may throw some arbitrary error. But perhaps you're right, we can just disable ZerosBridge for now in Cbc.

[blegat]

The issue with this error is that it's difficult to debug since it provides no information on the initial error. Also, I'm not sure all the issues with ZerosBridge will go through get_fallback so I feel it's easier to fix at the source as I suggested in my previous comment

[odow]

Closing in favor of #2415