Threadsafe Handles

[kjvalencik]

Two new Send + Sync features are introduced: EventQueue and Persistent. These features work together to allow multi-threaded modules to interact with the Javascript main thread.

• EventQueue: Threadsafe handle that allows sending closures to be executed on the main thread.
• Persistent<T>: Opaque handle that can be sent across threads, but only dereferenced on the main thread.

[dherman]

Finished my review! This is going to be a great addition to the core primitives of Neon. I'm psyched.

[kjvalencik]

@dherman This is close to stabilization. One change I would like to make prior to merging is the addition of a return value. Two APIs to take inspiration from:

• std::thread::spawn
• tokio::spawn

In both of these APIs, a JoinHandle is returned. On a thread::spawn, a user can call join to block until the thread exits and get the return value. Similarly, in tokio, the handle can be awaited to get the resolved value.

I propose the following changes:

struct Channel
    pub fn send<T, F>(&self, f: F) -> JoinHandle<T>
    where
        T: Send + 'static,
        F: FnOnce(TaskContext) -> NeonResult<T>;

We don't need to initially do anything with the returned JoinHandle, but by adding it there, it allows extension without breaking changes to introduce JoinHandle::join and impl Future for JoinHandle.

Also, I'm still not feeling certain send is the right choice. Maybe it should be renamed spawn? If it is renamed spawn, Channel probably isn't the best name either.

[dherman]

This is a great idea @kjvalencik.

I think I do like send better than spawn, though, because we aren't spawning a thread (not even conceptually like a green thread), the user should be thinking of it as sending information back to an existing thread. As far as mixing traditions (channel / join) I think I'm comfortable with it: conceptually we're dealing with a channel because we're communicating between threads, and conceptually we're dealing with joining because we're blocking on another thread's computation.

[kjvalencik]

Merged in https://github.com/neon-bindings/rfcs/commit/c6396817a56f323ae1d452d373c22513f6d7425b