Minimal async-await foundations

This sets the foundations for async-await support in godot-rust, based on the
original idea in #284. However, although the tests work, this is not a full
implementation:

- Async methods can only be registered manually using `build_method`. Macro
  syntax and implementation are out of the scope of this PR.
- The runtime types aren't registered automatically yet.
  Users need to manually call `register_runtime` and `terminate_runtime`
  functions in their library lifecycle hooks. Improving this is out of the
  scope of this PR for now.
- The crate is currently re-exported as `gdnative::asn`, instead of the much
  longer `async_yield`. The name is open to discussion -- I don't like it very
  much.
- Only local spawners are supported, due to issues with thread safety. Users
  may off-load tasks that don't contain `yield`-likes to thread pool spawners
  using something like `futures::future::Remote`, however.
- Panics in async methods don't currently behave very well. Their
  `FunctionState`-likes simply block forever and any outstanding bridge
  objects for futures can be leaked.

Cargo.toml

@@ -1,6 +1,7 @@
 [workspace]
 members = [
     "gdnative",
+    "gdnative-async",
     "gdnative-bindings",
     "gdnative-core",
     "gdnative-derive",

gdnative-async/Cargo.toml

@@ -0,0 +1,26 @@
+[package]
+name = "gdnative-async"
+authors = ["The godot-rust developers"]
+description = "Runtime async support for godot-rust."
+documentation = "https://docs.rs/crate/gdnative-async"
+repository = "https://github.com/godot-rust/godot-rust"
+homepage = "https://godot-rust.github.io/"
+version = "0.9.3"
+license = "MIT"
+workspace = ".."
+edition = "2018"
+
+[features]
+
+[dependencies]
+gdnative-derive = { path = "../gdnative-derive", version = "=0.9.3" }
+gdnative-core = { path = "../gdnative-core", version = "=0.9.3" }
+gdnative-bindings = { path = "../gdnative-bindings", version = "=0.9.3" }
+futures-task = "0.3.13"
+atomic-waker = "1.0.0"
+once_cell = "1.7.2"
+thiserror = "1.0"
+parking_lot = "0.11.0"
+crossbeam-channel = "0.5.0"
+
+[build-dependencies]

gdnative-async/src/executor.rs

@@ -0,0 +1,45 @@
+use futures_task::LocalSpawn;
+use once_cell::unsync::OnceCell as UnsyncCell;
+use thiserror::Error;
+
+thread_local!(
+    static LOCAL_SPAWN: UnsyncCell<&'static dyn LocalSpawn> = UnsyncCell::new();
+);
+
+/// Error returned by `set_*_executor` if an executor of the kind has already been set.
+#[derive(Error, Debug)]
+#[error("an executor is already set")]
+pub struct SetExecutorError {
+    _private: (),
+}
+
+impl SetExecutorError {
+    fn new() -> Self {
+        SetExecutorError { _private: () }
+    }
+}
+
+pub(crate) fn local_spawn() -> Option<&'static dyn LocalSpawn> {
+    LOCAL_SPAWN.with(|cell| cell.get().copied())
+}
+
+/// Sets the global executor for the current thread to a `Box<dyn LocalSpawn>`. This value is leaked.
+pub fn set_boxed_executor(sp: Box<dyn LocalSpawn>) -> Result<(), SetExecutorError> {
+    set_executor(Box::leak(sp))
+}
+
+/// Sets the global executor for the current thread to a `&'static dyn LocalSpawn`.
+pub fn set_executor(sp: &'static dyn LocalSpawn) -> Result<(), SetExecutorError> {
+    LOCAL_SPAWN.with(|cell| cell.set(sp).map_err(|_| SetExecutorError::new()))
+}
+
+/// Sets the global executor for the current thread with a function that will only be called
+/// if an executor isn't set yet.
+pub fn ensure_executor_with<F>(f: F)
+where
+    F: FnOnce() -> &'static dyn LocalSpawn,
+{
+    LOCAL_SPAWN.with(|cell| {
+        cell.get_or_init(f);
+    });
+}

gdnative-async/src/future.rs

@@ -0,0 +1,56 @@
+use std::future::Future;
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::{Context, Poll};
+
+use atomic_waker::AtomicWaker;
+use crossbeam_channel::{Receiver, Sender};
+
+pub(crate) fn make<T>() -> (Yield<T>, Resume<T>) {
+    let (arg_send, arg_recv) = crossbeam_channel::bounded(1);
+    let waker = Arc::default();
+
+    let future = Yield {
+        waker: Arc::clone(&waker),
+        arg_recv,
+    };
+
+    let resume = Resume { waker, arg_send };
+
+    (future, resume)
+}
+
+/// Signal
+pub struct Yield<T> {
+    waker: Arc<AtomicWaker>,
+    arg_recv: Receiver<T>,
+}
+
+impl<T: Send> Future for Yield<T> {
+    type Output = T;
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        match self.arg_recv.try_recv() {
+            Ok(arg) => Poll::Ready(arg),
+            Err(_) => {
+                self.waker.register(cx.waker());
+                Poll::Pending
+            }
+        }
+    }
+}
+
+pub(crate) struct Resume<T> {
+    waker: Arc<AtomicWaker>,
+    arg_send: Sender<T>,
+}
+
+impl<T: Send> Resume<T> {
+    /// Resume the task with a given argument from GDScript.
+    pub fn resume(self, arg: T) {
+        self.arg_send
+            .send(arg)
+            .expect("sender should not become disconnected");
+
+        self.waker.wake();
+    }
+}

gdnative-async/src/lib.rs

@@ -0,0 +1,19 @@
+//! Runtime async support for godot-rust.
+//!
+//! This crate contains types and functions that enable using async code with godot-rust.
+//!
+//! # Safety assumptions
+//!
+//! This crate assumes that all user non-Rust code follow the official threading guidelines.
+
+// Workaround for macros that expect the `gdnative` crate.
+extern crate gdnative_core as gdnative;
+
+mod executor;
+mod future;
+mod method;
+mod rt;
+
+pub use executor::{ensure_executor_with, set_boxed_executor, set_executor, SetExecutorError};
+pub use method::{Async, AsyncMethod, Spawner};
+pub use rt::{register_runtime, terminate_runtime, Context, InitError};

gdnative-async/src/method.rs

@@ -0,0 +1,126 @@
+use std::future::Future;
+use std::marker::PhantomData;
+use std::sync::Arc;
+
+use futures_task::{LocalFutureObj, LocalSpawn, SpawnError};
+
+use gdnative_core::core_types::{ToVariant, Variant};
+use gdnative_core::log::{self, Site};
+use gdnative_core::nativescript::export::{Method, Varargs};
+use gdnative_core::nativescript::{NativeClass, RefInstance};
+use gdnative_core::object::ownership::Shared;
+
+use crate::rt::Context;
+
+/// Trait for async methods. When exported, such methods return `FunctionState`-like
+/// objects that can be manually resumed or yielded to completion.
+///
+/// Async methods are always spawned locally on the thread where they were created,
+/// and never sent to another thread. This is so that we can ensure the safety of
+/// emitting signals from the `FunctionState`-like object. If you need to off-load
+/// some task to another thread, consider using something like
+/// `futures::future::Remote` to spawn it remotely on a thread pool.
+pub trait AsyncMethod<C: NativeClass>: Send + Sync + 'static {
+    /// Spawns the future for result of this method with `spawner`. This is done so
+    /// that implementors of this trait do not have to name their future types.
+    ///
+    /// If the `spawner` object is not used, the method call will fail, output an error,
+    /// and return a `Nil` variant.
+    fn spawn_with(&self, spawner: Spawner<'_, C>);
+
+    /// Returns an optional site where this method is defined. Used for logging errors in FFI wrappers.
+    ///
+    /// Default implementation returns `None`.
+    #[inline]
+    fn site() -> Option<Site<'static>> {
+        None
+    }
+}
+
+pub struct Spawner<'a, C: NativeClass> {
+    sp: &'static dyn LocalSpawn,
+    ctx: Context,
+    this: RefInstance<'a, C, Shared>,
+    args: Varargs<'a>,
+    result: &'a mut Option<Result<(), SpawnError>>,
+    /// Remove Send and Sync
+    _marker: PhantomData<*const ()>,
+}
+
+impl<'a, C: NativeClass> Spawner<'a, C> {
+    /// Consumes this `Spawner` and spawns a future returned by the closure. This indirection
+    /// is necessary so that implementors of the `AsyncMethod` trait do not have to name their
+    /// future types.
+    pub fn spawn<F, R>(self, f: F)
+    where
+        F: FnOnce(Arc<Context>, RefInstance<'_, C, Shared>, Varargs<'_>) -> R,
+        R: Future<Output = Variant> + 'static,
+    {
+        let ctx = Arc::new(self.ctx);
+        let future = f(Arc::clone(&ctx), self.this, self.args);
+        *self.result = Some(
+            self.sp
+                .spawn_local_obj(LocalFutureObj::new(Box::new(async move {
+                    let value = future.await;
+                    ctx.resolve(value);
+                }))),
+        );
+    }
+}
+
+/// Adapter for async methods that implements `Method` and can be registered.
+#[derive(Clone, Copy, Default, Debug)]
+pub struct Async<F> {
+    f: F,
+}
+
+impl<F> Async<F> {
+    /// Wrap `f` in an adapter that implements `Method`.
+    #[inline]
+    pub fn new(f: F) -> Self {
+        Async { f }
+    }
+}
+
+impl<C: NativeClass, F: AsyncMethod<C>> Method<C> for Async<F> {
+    fn call(&self, this: RefInstance<'_, C, Shared>, args: Varargs<'_>) -> Variant {
+        if let Some(sp) = crate::executor::local_spawn() {
+            let ctx = Context::new();
+            let func_state = ctx.func_state();
+
+            let mut result = None;
+            self.f.spawn_with(Spawner {
+                sp,
+                ctx,
+                this,
+                args,
+                result: &mut result,
+                _marker: PhantomData,
+            });
+
+            match result {
+                Some(Ok(())) => func_state.to_variant(),
+                Some(Err(err)) => {
+                    log::error(
+                        Self::site().unwrap_or_default(),
+                        format_args!("unable to spawn future: {}", err),
+                    );
+                    Variant::new()
+                }
+                None => {
+                    log::error(
+                        Self::site().unwrap_or_default(),
+                        format_args!("implementation did not spawn a future"),
+                    );
+                    Variant::new()
+                }
+            }
+        } else {
+            log::error(
+                Self::site().unwrap_or_default(),
+                "a global executor must be set before any async methods can be called on this thread",
+            );
+            Variant::new()
+        }
+    }
+}