Proper C++ exception handling

[hirschenberger]

I have a problem with some code that has to reconfigure the exposure value after every grabbed image. To do this the grabbing must be stopped, value set, grabbing started...

The problem is, that occasionally the C++ code throws a GenICam_3_1_Basler_pylon::RuntimeException that can't be caught in rust and so my app panics.

I looked at the code and it seems, although fn float_node_set_value(node: &UniquePtr<CFloatParameter>, value: f64) -> Result<()>; returns a Result, it seems it's not really created on errors (exceptions) in the FFI code.

Wouldn't it make sense to try-catch all C++ FFI functions return a Result::Err value on exceptions to make the user handle the error in Rust-land?

[astraw]

Catching all C++ exceptions and translating them to a rust PylonError should happen, but evidently does not.

I notice some cruft which is apparently unused like the trycatch C++ function. It would be great to wire this up to work as you suggest.

[hirschenberger]

It seems as if cxx is doing all the magic of converting an exception to a Result when that trycatch template function is present: https://cxx.rs/binding/result.html

I tried it and it correctly catches and converts a Pylon::GeneicException.

So I think we only have to add that GenICam_3_1_Basler_pylon::RuntimeException and maybe other Pylon exceptions to that catch-chain.

Maybe also add a catch-all arm to be on the safe side.

EDIT: I'll try it out on the system that makes trouble

[hirschenberger]

GenICam_3_1_Basler_pylon::RuntimeException is an Alias for GenICam::RuntimeException which is derived from Pylon::GenericException. So one exception handler is sufficient.

[hirschenberger]

@astraw Can you please look at this, I'd need that fix in a project

[astraw]

Ahh, yes, thanks for the ping. I was very busy with stuff but they are settling down and I should be able to look at this in the next couple of days.