Using Mun in a threaded environment

[baszalmstra]

The mun_runtime::Runtime is not Sync nor Send since the RuntimeBuilder::spawn function returns a Rc<RefCell<Runtime>>. This makes it hard to use in other programs that make use of threads. The reason we wrap the Runtime in an Rc is because the runtime (or better yet, the memory model) is not thread safe. Wrapping the Runtime in a mutex will also not work because the memory model is shared between other structs (like StructRef). Sending the Runtime to a thread and doing modifications while also accessing a StructRef from another thread will cause threading issues.

We've already talked about this a lot. We currently have no proper plan to support multithreading. However, our current solution is also not very ergonomic. I propose we implement a blocking solution that still doesnt do multithreading of the runtime but does allow the runtime and derived data to be shared across threads. In a later version we can then add proper multithreading hopefully using the same API we design now.

A straighforward solution is to wrap all memory modifying operations inside a mutex. This will block any operation performed by other threads but will enable the use of the Runtime and derived data to be send to different threads.

Curious to your thoughts!

[baszalmstra]

Looking at it closer, the StructRef is already safe because it is tied to the lifetime of the runtime. If we ensure that you can only use a rooted struct by first converting it to a StructRef (the RootedStruct::as_ref), which is again tied to the runtime, we can make the runtime Send. The as_ref method is currently unsafe. I would prefer to have an API like:

let rooted_struct:RootedStruct = // ...
let ref:StructRef<'r> = runtime.get_ref(rooted_struct).unwrap();
invoke_fn!("main", ref)

This involves making sure that the get_ref method can be safely called. WDYT @Wodann?

[Wodann]

IIRC, the API can be made safe if we can guarantee that the rooted struct was created in the runtime that we are using to convert to a StructRef. That could also be achieved by storing a "unique identifier" that matches that of a runtime.

[baszalmstra]

I modified the API the remove the need for Rc<RefCell<..>> in #370 , this makes the Runtime Send which should make it more friendly in a threaded/async environment.