Function invoke_c_async_callback

crates/c-api/src/async.rs:96–161

(
    cb: wasmtime_func_async_callback_t,
    data: CallbackDataPtr,
    mut caller: WasmtimeCaller<'a>,
    params: &'a [Val],
    results: &'a mut [Val],
)

pub type wasmtime_func_async_continuation_callback_t = extern "C" fn(*mut c_void) -> bool;

async fn invoke_c_async_callback<'a>(
    cb: wasmtime_func_async_callback_t,
    data: CallbackDataPtr,
    mut caller: WasmtimeCaller<'a>,
    params: &'a [Val],
    results: &'a mut [Val],
) -> Result<()> {
    // Convert `params/results` to `wasmtime_val_t`. Use the previous
    // storage in `hostcall_val_storage` to help avoid allocations all the
    // time.
    let mut hostcall_val_storage = mem::take(&mut caller.data_mut().hostcall_val_storage);
    debug_assert!(hostcall_val_storage.is_empty());
    hostcall_val_storage.reserve(params.len() + results.len());
    hostcall_val_storage.extend(
        params
            .iter()
            .cloned()
            .map(|p| wasmtime_val_t::from_val_unscoped(&mut caller, p)),
    );
    hostcall_val_storage.extend((0..results.len()).map(|_| wasmtime_val_t {
        kind: WASMTIME_I32,
        of: wasmtime_val_union { i32: 0 },
    }));
    let (params, out_results) = hostcall_val_storage.split_at_mut(params.len());

    // Invoke the C function pointer.
    // The result will be a continuation which we will wrap in a Future.
    let mut caller = wasmtime_caller_t { caller };
    let mut trap = None;
    extern "C" fn panic_callback(_: *mut c_void) -> bool {
        panic!("callback must be set")
    }
    let mut continuation = wasmtime_async_continuation_t {
        callback: panic_callback,
        env: ptr::null_mut(),
        finalizer: None,
    };
    cb(
        data.ptr,
        &mut caller,
        params.as_ptr(),
        params.len(),
        out_results.as_mut_ptr(),
        out_results.len(),
        &mut trap,
        &mut continuation,
    );
    continuation.await;

    if let Some(trap) = trap {
        return Err(trap.error);
    }

    // Translate the `wasmtime_val_t` results into the `results` space
    for (i, result) in out_results.iter().enumerate() {
        unsafe {
            results[i] = result.to_val_unscoped(&mut caller.caller);
        }