Wasm Actor

The swactor-wasm-actor crate runs WebAssembly guest code inside a swactor actor. The Wasm instance is sandboxed by wasmtime.

Architecture

  ┌─ Runtime ──────────────────────────────────────────────────────────────┐
  │                                                                        │
  │  ┌─ WasmActor ──────────────────────────────────────────────────────┐  │
  │  │                                                                  │  │
  │  │  Store<HostState>    -- wasmtime store with outbox               │  │
  │  │  Memory              -- guest linear memory                      │  │
  │  │  alloc: TypedFunc    -- guest allocator                          │  │
  │  │  handle: TypedFunc   -- guest message handler                    │  │
  │  │                                                                  │  │
  │  │  impl ActorInterface for WasmActor                               │  │
  │  │    Incoming = ByteMessage                                        │  │
  │  │    Response = ()                                                 │  │
  │  │                                                                  │  │
  │  └──────────────────────────────────────────────────────────────────┘  │
  │                                                                        │
  │  ┌─ Native Actors ─────────────────────────────────────────────────┐   │
  │  │  (can exchange ByteMessage with WasmActors normally)            │   │
  │  └─────────────────────────────────────────────────────────────────┘   │
  │                                                                        │
  └────────────────────────────────────────────────────────────────────────┘

Message Flow

  Host                          Guest (Wasm)
  ────                          ────────────

  ByteMessage arrives
       │
       ├─1─ call alloc(len) ──────────► bump-allocate, return ptr
       │
       ├─2─ write bytes at ptr ───────► (memory updated)
       │
       ├─3─ call handle(ptr, len) ────► process message
       │                                   │
       │    ◄── swactor.send() ────────────┤  (0..N times)
       │    (buffered in HostState.outbox)  │
       │                                   │
       ├─4─ drain outbox ◄────────────── handle returns
       │
       v
  ctx.send(dest, ByteMessage) for each outbox entry

Guest Contract

Guests are standalone wasm32-unknown-unknown modules. They export three symbols and may import one:

Direction	Module	Symbol	Signature
export	—	`memory`	linear memory
export	—	`alloc`	`(i32) -> i32`
export	—	`handle`	`(i32, i32) -> ()`
import	`swactor`	`send`	`(i32, i32, i32) -> ()`

The send import takes (dest_ptr, payload_ptr, payload_len) where dest_ptr points to a 32-byte ActorAddress in guest memory.

Usage

use swactor::runtime::{Runtime, RuntimeConfig};
use swactor_wasm_actor::{ByteMessage, SharedEngine, WasmActorBuilder};

// Create a shared engine (once)
let engine = SharedEngine::new().unwrap();

// Build an actor from .wasm bytes
let wasm_bytes = std::fs::read("my_guest.wasm").unwrap();
let actor = WasmActorBuilder::new(engine, wasm_bytes)
    .build()
    .unwrap();

// Use it like any other actor
let rt = Runtime::new(RuntimeConfig::default());
let addr = rt.spawn(actor).unwrap();
rt.send_to(addr, ByteMessage(b"hello".to_vec())).unwrap();
rt.tick();

Sandboxing

The SharedEngine disables all optional Wasm proposals:

Threads — disabled
SIMD / relaxed SIMD — disabled
Reference types — disabled
Multi-value — disabled
Bulk memory — enabled (required by most Rust/LLVM toolchains)

No WASI imports are linked. Guests have no access to the filesystem, network, clock, or random number generator. The only host function available is swactor.send.

Where Things Live

File	Purpose
`crates/wasm-actor/src/lib.rs`	`ByteMessage` + re-exports
`crates/wasm-actor/src/engine.rs`	`SharedEngine` — sandboxed wasmtime config
`crates/wasm-actor/src/builder.rs`	`WasmActorBuilder` — compile, link, instantiate
`crates/wasm-actor/src/actor.rs`	`WasmActor` — `ActorInterface` impl
`crates/wasm-actor/src/error.rs`	`WasmActorError`
`crates/wasm-actor/tests/guests/`	Three test guest crates (echo, double, silent)
`crates/wasm-actor/tests/wasm_actor.rs`	7 integration tests

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