Resolve concerns for the original PR

- `set_executor` and `register_runtime` are changed to allow multiple
  calls, at the expense of slightly worse error messages when they aren't
  called as necessary.
- Renamed the top-level re-export to `tasks` instead of `asn`.
- Added documentation for previously undocumented public items.

gdnative-async/Cargo.toml

@@ -16,11 +16,11 @@ edition = "2018"
 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"
+futures-task = "0.3.17"
 atomic-waker = "1.0.0"
-once_cell = "1.7.2"
-thiserror = "1.0"
-parking_lot = "0.11.0"
-crossbeam-channel = "0.5.0"
+once_cell = "1.8.0"
+parking_lot = "0.11.2"
+crossbeam-channel = "0.5.1"
+crossbeam-utils = "0.8.5"
 
 [build-dependencies]

gdnative-async/src/executor.rs

@@ -1,45 +1,21 @@
+use std::cell::Cell;
+
 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();
+    static LOCAL_SPAWN: Cell<Option<&'static dyn LocalSpawn>> = Cell::new(None);
 );
 
-/// 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())
+    LOCAL_SPAWN.with(|cell| cell.get())
 }
 
 /// 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> {
+pub fn set_boxed_executor(sp: Box<dyn LocalSpawn>) {
     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);
-    });
+pub fn set_executor(sp: &'static dyn LocalSpawn) {
+    LOCAL_SPAWN.with(|cell| cell.set(Some(sp)))
 }

gdnative-async/src/future.rs

@@ -20,7 +20,8 @@ pub(crate) fn make<T>() -> (Yield<T>, Resume<T>) {
     (future, resume)
 }
 
-/// Signal
+/// Future that can be `await`ed for a signal or a `resume` call from Godot. See
+/// [`Context`](crate::Context) for methods that return this future.
 pub struct Yield<T> {
     waker: Arc<AtomicWaker>,
     arg_recv: Receiver<T>,

gdnative-async/src/lib.rs

@@ -14,6 +14,7 @@ mod future;
 mod method;
 mod rt;
 
-pub use executor::{ensure_executor_with, set_boxed_executor, set_executor, SetExecutorError};
+pub use executor::{set_boxed_executor, set_executor};
+pub use future::Yield;
 pub use method::{Async, AsyncMethod, Spawner};
-pub use rt::{register_runtime, terminate_runtime, Context, InitError};
+pub use rt::{register_runtime, terminate_runtime, Context};

gdnative-async/src/method.rs

@@ -24,8 +24,8 @@ 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.
+    /// If the `spawner` object is not used, the Godot side of the 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.
@@ -37,6 +37,7 @@ pub trait AsyncMethod<C: NativeClass>: Send + Sync + 'static {
     }
 }
 
+/// A helper structure for working around naming future types. See [`Spawner::spawn`].
 pub struct Spawner<'a, C: NativeClass> {
     sp: &'static dyn LocalSpawn,
     ctx: Context,

gdnative-async/src/rt.rs

@@ -2,8 +2,6 @@ use std::marker::PhantomData;
 
 use gdnative_bindings::Object;
 use gdnative_core::object::SubClass;
-use once_cell::sync::OnceCell;
-use thiserror::Error;
 
 use gdnative_core::core_types::{GodotError, Variant};
 use gdnative_core::nativescript::export::InitHandle;
@@ -18,20 +16,6 @@ mod func_state;
 
 use func_state::FuncState;
 
-static REGISTRATION: OnceCell<()> = OnceCell::new();
-
-#[derive(Debug, Error)]
-#[error("async runtime must only be initialized once")]
-pub struct InitError {
-    _private: (),
-}
-
-impl InitError {
-    fn new() -> Self {
-        InitError { _private: () }
-    }
-}
-
 /// Context for creating `yield`-like futures in async methods.
 pub struct Context {
     func_state: Instance<FuncState, Shared>,
@@ -108,22 +92,15 @@ impl Context {
     }
 }
 
-pub fn register_runtime(handle: &InitHandle) -> Result<(), InitError> {
-    let mut called = false;
-
-    REGISTRATION.get_or_init(|| {
-        handle.add_class::<bridge::SignalBridge>();
-        handle.add_class::<func_state::FuncState>();
-        called = true;
-    });
-
-    if called {
-        Ok(())
-    } else {
-        Err(InitError::new())
-    }
+/// Adds required supporting NativeScript classes to `handle`. This must be called once and
+/// only once per initialization.
+pub fn register_runtime(handle: &InitHandle) {
+    handle.add_class::<bridge::SignalBridge>();
+    handle.add_class::<func_state::FuncState>();
 }
 
+/// Releases all observers still in use. This should be called in the
+/// `godot_gdnative_terminate` callback.
 pub fn terminate_runtime() {
     bridge::terminate();
 }

gdnative-async/src/rt/bridge.rs

@@ -43,8 +43,7 @@ struct Entry {
     resume: Resume<Vec<Variant>>,
 
     // Just need to keep this alive.
-    #[allow(dead_code)]
-    obj: Instance<SignalBridge, Shared>,
+    _obj: Instance<SignalBridge, Shared>,
 }
 
 pub(super) struct SignalBridge {
@@ -69,11 +68,6 @@ impl SignalBridge {
         signal: &str,
         resume: Resume<Vec<Variant>>,
     ) -> Result<(), GodotError> {
-        assert!(
-            super::REGISTRATION.get().is_some(),
-            "async API must be registered before any async methods can be called"
-        );
-
         let mut pool = BRIDGES.get_or_init(Mutex::default).lock();
         let (id, bridge) = pool.free.pop().unwrap_or_else(|| {
             let id = pool.next_id();
@@ -90,8 +84,8 @@ impl SignalBridge {
         )?;
 
         let entry = Entry {
-            obj: bridge,
             resume,
+            _obj: bridge,
         };
 
         assert!(pool.busy.insert(id, entry).is_none());

gdnative-async/src/rt/func_state.rs

@@ -53,11 +53,6 @@ impl NativeClass for FuncState {
 
 impl FuncState {
     pub fn new() -> Instance<Self, Unique> {
-        assert!(
-            super::REGISTRATION.get().is_some(),
-            "async API must be registered before any async methods can be called"
-        );
-
         Instance::emplace(FuncState {
             kind: Kind::Pending,
         })

gdnative/src/lib.rs

@@ -88,4 +88,4 @@ pub use gdnative_bindings as api;
 #[doc(inline)]
 #[cfg(feature = "async")]
 /// Support for async code
-pub use gdnative_async as asn;
+pub use gdnative_async as tasks;

test/src/lib.rs

@@ -272,7 +272,7 @@ fn init(handle: InitHandle) {
 }
 
 fn terminate(_term_info: &gdnative::TerminateInfo) {
-    gdnative::asn::terminate_runtime();
+    gdnative::tasks::terminate_runtime();
 }
 
 gdnative::macros::godot_gdnative_init!();

test/src/test_async.rs

@@ -1,7 +1,7 @@
 use std::cell::RefCell;
 
-use gdnative::asn::{Async, AsyncMethod, Spawner};
 use gdnative::prelude::*;
+use gdnative::tasks::{Async, AsyncMethod, Spawner};
 
 pub(crate) fn run_tests() -> bool {
     // Relevant tests in GDScript
@@ -15,8 +15,8 @@ thread_local! {
 }
 
 pub(crate) fn register(handle: InitHandle) {
-    gdnative::asn::register_runtime(&handle).unwrap();
-    gdnative::asn::set_executor(EXECUTOR.with(|e| *e)).unwrap();
+    gdnative::tasks::register_runtime(&handle);
+    gdnative::tasks::set_executor(EXECUTOR.with(|e| *e));
 
     handle.add_class::<AsyncMethods>();
     handle.add_class::<AsyncExecutorDriver>();