Add support for user-supplied executors

any_spawner/src/lib.rs

@@ -32,8 +32,10 @@
 use std::{future::Future, pin::Pin, sync::OnceLock};
 use thiserror::Error;
 
-pub(crate) type PinnedFuture<T> = Pin<Box<dyn Future<Output = T> + Send>>;
-pub(crate) type PinnedLocalFuture<T> = Pin<Box<dyn Future<Output = T>>>;
+/// A future that has been pinned.
+pub type PinnedFuture<T> = Pin<Box<dyn Future<Output = T> + Send>>;
+/// A future that has been pinned.
+pub type PinnedLocalFuture<T> = Pin<Box<dyn Future<Output = T>>>;
 
 static SPAWN: OnceLock<fn(PinnedFuture<()>)> = OnceLock::new();
 static SPAWN_LOCAL: OnceLock<fn(PinnedLocalFuture<()>)> = OnceLock::new();
@@ -284,63 +286,42 @@ impl Executor {
             .map_err(|_| ExecutorError::AlreadySet)?;
         Ok(())
     }
-}
 
-#[cfg(test)]
-mod tests {
-    #[cfg(feature = "futures-executor")]
-    #[test]
-    fn can_spawn_local_future() {
-        use crate::Executor;
-        use std::rc::Rc;
-        _ = Executor::init_futures_executor();
-        let rc = Rc::new(());
-        Executor::spawn_local(async {
-            _ = rc;
-        });
-        Executor::spawn(async {});
-    }
+    /// Globally sets a custom executor as the executor used to spawn tasks.
+    ///
+    /// Returns `Err(_)` if an executor has already been set.
+    pub fn init_custom_executor(
+        custom_executor: impl CustomExecutor + 'static,
+    ) -> Result<(), ExecutorError> {
+        static EXECUTOR: OnceLock<Box<dyn CustomExecutor>> = OnceLock::new();
+        EXECUTOR
+            .set(Box::new(custom_executor))
+            .map_err(|_| ExecutorError::AlreadySet)?;
 
-    #[cfg(feature = "futures-executor")]
-    #[test]
-    fn can_make_threaded_progress() {
-        use crate::Executor;
-        use std::sync::{atomic::AtomicUsize, Arc};
-        _ = Executor::init_futures_executor();
-        let counter = Arc::new(AtomicUsize::new(0));
-        Executor::spawn({
-            let counter = Arc::clone(&counter);
-            async move {
-                assert_eq!(
-                    counter.fetch_add(1, std::sync::atomic::Ordering::AcqRel),
-                    0
-                );
-            }
-        });
-        futures::executor::block_on(Executor::tick());
-        assert_eq!(counter.load(std::sync::atomic::Ordering::Acquire), 1);
+        SPAWN
+            .set(|fut| {
+                EXECUTOR.get().unwrap().spawn(fut);
+            })
+            .map_err(|_| ExecutorError::AlreadySet)?;
+        SPAWN_LOCAL
+            .set(|fut| EXECUTOR.get().unwrap().spawn_local(fut))
+            .map_err(|_| ExecutorError::AlreadySet)?;
+        POLL_LOCAL
+            .set(|| EXECUTOR.get().unwrap().poll_local())
+            .map_err(|_| ExecutorError::AlreadySet)?;
+        Ok(())
     }
+}
 
-    #[cfg(feature = "futures-executor")]
-    #[test]
-    fn can_make_local_progress() {
-        use crate::Executor;
-        use std::sync::{atomic::AtomicUsize, Arc};
-        _ = Executor::init_futures_executor();
-        let counter = Arc::new(AtomicUsize::new(0));
-        Executor::spawn_local({
-            let counter = Arc::clone(&counter);
-            async move {
-                assert_eq!(
-                    counter.fetch_add(1, std::sync::atomic::Ordering::AcqRel),
-                    0
-                );
-                Executor::spawn_local(async {
-                    // Should not crash
-                });
-            }
-        });
-        Executor::poll_local();
-        assert_eq!(counter.load(std::sync::atomic::Ordering::Acquire), 1);
-    }
+/// A trait for custom executors.
+/// Custom executors can be used to integrate with any executor that supports spawning futures.
+///  
+/// All methods can be called recursively.
+pub trait CustomExecutor: Send + Sync {
+    /// Spawns a future, usually on a thread pool.
+    fn spawn(&self, fut: PinnedFuture<()>);
+    /// Spawns a local future. May require calling `poll_local` to make progress.
+    fn spawn_local(&self, fut: PinnedLocalFuture<()>);
+    /// Polls the executor, if it supports polling.
+    fn poll_local(&self);
 }

any_spawner/tests/custom_runtime.rs

@@ -0,0 +1,55 @@
+#[cfg(feature = "futures-executor")]
+use any_spawner::{CustomExecutor, Executor, PinnedFuture, PinnedLocalFuture};
+#[cfg(feature = "futures-executor")]
+#[test]
+fn can_create_custom_executor() {
+    use futures::{
+        executor::{LocalPool, LocalSpawner},
+        task::LocalSpawnExt,
+    };
+    use std::{
+        cell::RefCell,
+        sync::{
+            atomic::{AtomicUsize, Ordering},
+            Arc,
+        },
+    };
+
+    thread_local! {
+        static LOCAL_POOL: RefCell<LocalPool> = RefCell::new(LocalPool::new());
+        static SPAWNER: LocalSpawner = LOCAL_POOL.with(|pool| pool.borrow().spawner());
+    }
+
+    struct CustomFutureExecutor;
+    impl CustomExecutor for CustomFutureExecutor {
+        fn spawn(&self, _fut: PinnedFuture<()>) {
+            panic!("not supported in this test");
+        }
+
+        fn spawn_local(&self, fut: PinnedLocalFuture<()>) {
+            SPAWNER.with(|spawner| {
+                spawner.spawn_local(fut).expect("failed to spawn future");
+            });
+        }
+
+        fn poll_local(&self) {
+            LOCAL_POOL.with(|pool| {
+                if let Ok(mut pool) = pool.try_borrow_mut() {
+                    pool.run_until_stalled();
+                }
+                // If we couldn't borrow_mut, we're in a nested call to poll, so we don't need to do anything.
+            });
+        }
+    }
+
+    Executor::init_custom_executor(CustomFutureExecutor)
+        .expect("couldn't set executor");
+
+    let counter = Arc::new(AtomicUsize::new(0));
+    let counter_clone = Arc::clone(&counter);
+    Executor::spawn_local(async move {
+        counter_clone.store(1, Ordering::Release);
+    });
+    Executor::poll_local();
+    assert_eq!(counter.load(Ordering::Acquire), 1);
+}

any_spawner/tests/futures_runtime.rs

@@ -0,0 +1,38 @@
+#[cfg(feature = "futures-executor")]
+use any_spawner::Executor;
+// All tests in this file use the same executor.
+
+#[cfg(feature = "futures-executor")]
+#[test]
+fn can_spawn_local_future() {
+    use std::rc::Rc;
+
+    let _ = Executor::init_futures_executor();
+    let rc = Rc::new(());
+    Executor::spawn_local(async {
+        _ = rc;
+    });
+    Executor::spawn(async {});
+}
+
+#[cfg(feature = "futures-executor")]
+#[test]
+fn can_make_local_progress() {
+    use std::sync::{
+        atomic::{AtomicUsize, Ordering},
+        Arc,
+    };
+    let _ = Executor::init_futures_executor();
+    let counter = Arc::new(AtomicUsize::new(0));
+    Executor::spawn_local({
+        let counter = Arc::clone(&counter);
+        async move {
+            assert_eq!(counter.fetch_add(1, Ordering::AcqRel), 0);
+            Executor::spawn_local(async {
+                // Should not crash
+            });
+        }
+    });
+    Executor::poll_local();
+    assert_eq!(counter.load(Ordering::Acquire), 1);
+}