Fix Kotlin error handling in async functions

[mhammond]

Seems like it probably fixes #1611 but I'm really not sure what I am doing :) cc @Hywan, does this look OK to you?

[Hywan]

Hmm. The definition of callback_param looks wrong.

In AsyncTypes.py, we have this for the argument:

        {%- match result_type.return_type -%}
        {%- when Some(return_type) -%}
        {{ return_type.ffi_type().borrow()|ffi_type_name }}
        {%- when None -%}
        ctypes.c_uint8
        {%- endmatch -%}

No result type generates ctypes.c_uint8 which seems to accept a null value.

In AsyncType.swift:

 returnValue: {{ result_type.future_callback_param().borrow()|ffi_type_name }},

which fallbacks to Ffi::Uint8, which seems to accept a null value.

The rest is ensured by the when Some(return_type) dance.

Either you change how callback_param is defined, or I would say this patch is valid :-p. I'm really surprised that it works with Swift. Didn't dig that much though.

[bendk]

This seems right to me. The return FFI type is a pointer, since the parameter is an Arc<>. Errors get returned through a out-param and when the Rust function returns an error it needs to return some placeholder value for the pointer, which is null. This is different than either a void return or an Option<Arc<>>.

checkCallStatus() will throw an exception in the error case, so returnValue will never actually be touched. I haven't looked at the Swift code, but maybe Swift allows the null pointer as long as it's not accessed.

[bendk]

This seems right to me. The return FFI type is a pointer, since the parameter is an Arc<>. Errors get returned through a out-param and when the Rust function returns an error it needs to return some placeholder value for the pointer, which is null. This is different than either a void return or an Option<Arc<>>.

checkCallStatus() will throw an exception in the error case, so returnValue will never actually be touched. I haven't looked at the Swift code, but maybe Swift allows the null pointer as long as it's not accessed.