How to return error from chain extension?

Question

I saw how to handle error from chain extension on smart contract side https://paritytech.github.io/ink-docs/macros-attributes/chain-extension#error-code

But I can't find example and can't understand how to return error from chain extension implementation?

return Err(DispatchError::Other("Unimplemented func_id"))
or 
Ok(RetVal::Converging(1))

What I need return from chain extension call function to have possibility get this error:

#[ink(extension = 1106, returns_result = true)]
fn some_func(subject: PalletAssetBalanceRequest) -> Result([u8; 16], SomeError);

Answer 1

You need to SCALE encode the Result([u8; 16], SomeError) and write it to the output buffer:

impl ChainExtension for MyExtension {
    fn call(func_id: u32, env: Environment) -> Result<RetVal> {
        // use buffers for input and output (as opposed to primitives)
        let env = env.buf_in_buf_out();

        /// do stuff with the input ...
        let input: PalletAssetBalanceRequest = env.read_as()?;

        /// write your error to the output buffer
        let err: Result([u8; 16], SomeError) = ...;
        env.write(err.encode().as_ref(), false, None);

        // 0 usually means success which means your output buffer will be decoded by ink!.
        // depends on your `FromStatusCode` implementation, though
        Ok(RetVal::Converging(0))
    }   
}

Also, check out the chain extension documentation.