How to return a optional string from native methods?

[tison]

Here is my native method written in Rust:

#[no_mangle]
pub unsafe extern "C" fn datafusion_version() -> *const c_char {
    CString::new(datafusion::DATAFUSION_VERSION).unwrap().into_raw()
}

And below is my attempt to bind it with FFM APIs:

    static MethodHandle createMethodHandle(String name, FunctionDescriptor
descriptor) {
        final MemorySegment fp = LOADER.lookup.find(name).orElseThrow();
        return LOADER.linker.downcallHandle(fp, descriptor);
    }

    private static final MethodHandle datafusionVersionMethodHandle =
NativeLoader.createMethodHandle(datafusionVersionMethodName,
datafusionVersionMethodDesc);

    @SneakyThrows
    public static String datafusionVersion() {
        final MemorySegment version = (MemorySegment)
datafusionVersionMethodHandle.invokeExact();
        return version.getUtf8String(0);
    }

I noticed that the returned MemorySegment is always with length 0 and thus
any access with return OutOfBoundException.

In the demos from JEP, all memory segments are allocated from the Java side
and the native code only move or modify those allocated memory segments
instead of allocate/shrink memory.

I wonder what is the formal method to allocate a string from native methods
and pass back to the Java world.

And, if we think of error handling, how do we properly return a NULL from
native methods, and how do we throw exceptions from native methods?

MemorySegment.NULL seems an Object without overriding equals, and JNIEnv's
throw counterpart is missing or I don't find it (perhaps upcall helps but
it's still a bit away from real-world usage).

Best,
tison.
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