VST3 com interface issue / fix

[PolyVector]

Hello,
I found an issue with the VST3 com interface where any calls to create_instance() appear to return an IComponent, ignoring the IID that was requested. Typically this isn't a problem as the IComponent requested first, but the Steinberg AUWrapper requests an IAudioProcessor first, which wreaks havoc. For example, calls to audioProcessor->setProcessing() actually invoke get_bus_info() with nonsensical parameters. This was pretty difficult to figure out!

The vst3-sys project seems aware of this which I assume is why macros require IComponent to be listed first. Related issue for reference: RustAudio/vst3-sys#14

Here's a quick-and-dirty fix in wrapper/vst3/factory.rs. I'm sure there's a better way to do this, but I'm new to Rust.

    unsafe fn create_instance(
        &self,
        cid: *const vst3_sys::IID,
        iid: *const vst3_sys::IID,
        obj: *mut *mut vst3_sys::c_void,
    ) -> tresult {
        check_null_ptr!(cid, obj);

        if (*cid).data != P::PLATFORM_VST3_CLASS_ID {
            return kNoInterface;
        }

        //Fix: Returns the requested interface:
        let wrapper = Box::into_raw(Wrapper::<P>::new());
        if (*wrapper).query_interface(iid, obj) == vst3_com::sys::NOERROR {
            kResultOk
        } else {
            kResultFalse
        }
    }

Thanks for the wonderful work you're doing on NIH-plug, I'm really loving the combo with egui. 🍻

[robbert-vdh]

That's an oversight on my part, sorry! Don't know how I missed this, normally I'm super thorough with these kinds of things. I fixed this in a97c8ea. The caller can now properly create a properly object for any interface supported by the plugin.