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@autodidaddict
Created December 14, 2019 18:09
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wasmtime chicken and egg
use wasmtime::{Callable, Engine, Extern, FuncType, HostRef, Module, Store, Trap, Val, ValType};
// The goal of this code is that I need a function invoked by the guest wasm module on the
// host to be able to access the memory of that instance, and to be able to call additional
// functions on that instance in response.
//
// When I use wasmer, the call signature includes a Ctx, which allows me to manipulate the instance
// memory and call functions.
struct GuestCallable {}
impl Callable for GuestCallable {
fn call(
&self,
params: &[Val],
results: &mut [Val],
) -> ::std::result::Result<(), HostRef<Trap>> {
// How does this thing access the memory of the instance that invoked it??
Ok(())
}
}
fn main() -> {
let engine = HostRef::new(Engine::default());
let store = HostRef::new(Store::new(&engine));
let module = HostRef::new(Module::new(&store, buf).expect("wasm module"));
let ft = FuncType::new(
Box::new([ValType::I32, ValType::I32, ValType::I32, ValType::I32]),
Box::new([ValType::I32]),
);
let callable = GuestCallable {};
let f = wasmtime::Func::new(&store, ft, Rc::new(callable));
let e = Extern::Func(HostRef::new(f));
let instance = wasmtime::Instance::new(&store, &module, &[e]).expect("wasm instance");
// At this point I've got a callable that's moved out of scope, so I can't
// go back and give it a reference to the instance or the instance handle.
}
@autodidaddict
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The requirements for Func::new() are that something implement Callable... would an Arc also implement Callable? Would that even work given that wasmtime's Instance type is !Send and !Sync ?

@autodidaddict
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Unrelated to the chicken and the egg - this extern has no name - how does the guest module get the right import namespace and function name for dispatch?

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