How to throw exceptions from rust

[onelson]

I'm working on a rust crate with some functions that return Result - I'm hoping use the Err values to generate exceptions for the Java caller, but I'm not confident I'm using the provided API in the intended way.

The reason I'm doubtful is I expected to be able to do something like this:

#[no_mangle]                                                                   
#[allow(non_snake_case)]                                                       
pub extern "system" fn Java_MyLib_getMaybeBool(                               
    env: JNIEnv,                                                               
    class: JClass,                                                             
    input: JString,                                                            
) -> jboolean {                                                                
    let input: String = env.get_string(input)                                  
        .expect("Couldn't get java string!")                                   
        .into();                                                               

    match get_maybe_bool(&input) {
        Ok(output) => output as u8, 
        Err(err) => {                                                          
            env.throw(err.to_string()).unwrap();
            unreachable!();
        }                                                                      
    }                                                                          
} 

When I write this, the java caller segfaults in the Err case, hitting the unreachable!(); and then panics.

Instead, I'm returning a 0u8 following the throw, which I guess could be correct from a certain point of view (but not really). With the 0u8 return value, I see the caller get the exception and everything works how I'd think it should.

Am I missing some aspect of this? An example of throwing exceptions would be nice to have.

[jrobsonchase]

You still have to return to the JVM, even if you've thrown an exception. Remember that unwinding across the ffi boundary is always undefined behavior, so any panics need to be caught and recovered from in your extern functions. I can give more of an example tomorrow when I'm not replying from my phone

…

On Mon, Feb 5, 2018, 18:24 Owen Nelson ***@***.***> wrote: I'm working on a rust crate with some functions that return Result - I'm hoping use the Err values to generate exceptions for the Java caller, but I'm not confident I'm using the provided API in the intended way. The reason I'm doubtful is I expected to be able to do something like this: #[no_mangle] #[allow(non_snake_case)] pub extern "system" fn Java_MyLib_getMaybeBool( env: JNIEnv, class: JClass, input: JString, ) -> jboolean { let input: String = env.get_string(input) .expect("Couldn't get java string!") .into(); match get_maybe_bool(&input) { Ok(output) => output as u8, Err(err) => { env.throw(err.to_string()).unwrap(); unreachable!(); } } } When I write this, the java caller segfaults in the Err case, hitting the unreachable!(); and then panics. Instead, I'm returning a 0u8 which I guess could be correct from a certain point of view (but not really). With the 0u8 return value, I see the caller get the exception and everything works how I'd think it should. I'm I missing some aspect of this? An example of throwing exceptions would be nice to have. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#76>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABe0ra4Lt-C66cXeaz0e8jymDCxsPHIZks5tR42mgaJpZM4R6QfR> .

[onelson]

OK, that makes some good sense.

I suppose I could aim to document this with a const BOOL_NOPE: u8 = 255; and just return BOOL_NOPE in these situations.

I was just wondering if there was some way to model the throwing of the exception in the function signature (like you might with a Result).

[dmitry-timofeev]

That's a good question because this thing is tricky to handle properly. We ended up with the following (@DarkEld3r, please correct me if I'm wrong):

• If a native function panics -- throw a Java runtime exception.
• If a native function returns Err:
  • check if there is a pending (= already thrown) Java exception, do nothing if that's the case.
  • otherwise -- throw a proper Java exception to let the caller know about the error.
• If a native function returns Ok: return to the Java side.

I can't say if this library can help with handling panics and errors, or simplify it.

[jrobsonchase]

There's nothing in this lib as of right now that can help with that, but it could be possible with the help of some macro magic.

Some time ago, I played around with wrapping the wren in Rust and came up with a macro system for doing some extra automatic translation of wren types -> rust types and adding safety wrappers. Example here.

Something similar could conceivably be included here if someone had the motivation to build it 😉

Edit: on second look, it doesn't appear that I ever got around to wrapping everything in a panic catch in my wren crate, but it wouldn't be hard to add.

[jrobsonchase]

I think the question here has effectively been answered, and I filed an enhancement issue for some of the ergonomics improvements proposed here, so I'm closing this one.

Feel free to discuss more ergonomics improvements in #80!