From 4a8e9cf4d9f682bfe4942559da7e76315216c377 Mon Sep 17 00:00:00 2001
From: Grant Miller <GrantM11235@gmail.com>
Date: Tue, 31 Jan 2023 18:49:32 -0600
Subject: [PATCH] Add internal `AvailableTask` type

---
 embassy-executor/src/raw/mod.rs | 68 +++++++++++++++++++--------------
 1 file changed, 40 insertions(+), 28 deletions(-)

diff --git a/embassy-executor/src/raw/mod.rs b/embassy-executor/src/raw/mod.rs
index 6783c485..e93e6036 100644
--- a/embassy-executor/src/raw/mod.rs
+++ b/embassy-executor/src/raw/mod.rs
@@ -141,25 +141,14 @@ impl<F: Future + 'static> TaskStorage<F> {
     /// Once the task has finished running, you may spawn it again. It is allowed to spawn it
     /// on a different executor.
     pub fn spawn(&'static self, future: impl FnOnce() -> F) -> SpawnToken<impl Sized> {
-        if self.spawn_mark_used() {
-            return unsafe { SpawnToken::<F>::new(self.spawn_initialize(future)) };
+        let task = AvailableTask::claim(self);
+        match task {
+            Some(task) => {
+                let task = task.initialize(future);
+                unsafe { SpawnToken::<F>::new(task) }
+            }
+            None => SpawnToken::new_failed(),
         }
-
-        SpawnToken::<F>::new_failed()
-    }
-
-    fn spawn_mark_used(&'static self) -> bool {
-        let state = STATE_SPAWNED | STATE_RUN_QUEUED;
-        self.raw
-            .state
-            .compare_exchange(0, state, Ordering::AcqRel, Ordering::Acquire)
-            .is_ok()
-    }
-
-    unsafe fn spawn_initialize(&'static self, future: impl FnOnce() -> F) -> TaskRef {
-        // Initialize the task
-        self.future.write(future());
-        TaskRef::new(self)
     }
 
     unsafe fn poll(p: TaskRef) {
@@ -184,6 +173,27 @@ impl<F: Future + 'static> TaskStorage<F> {
 
 unsafe impl<F: Future + 'static> Sync for TaskStorage<F> {}
 
+struct AvailableTask<F: Future + 'static> {
+    task: &'static TaskStorage<F>,
+}
+
+impl<F: Future + 'static> AvailableTask<F> {
+    fn claim(task: &'static TaskStorage<F>) -> Option<Self> {
+        task.raw
+            .state
+            .compare_exchange(0, STATE_SPAWNED | STATE_RUN_QUEUED, Ordering::AcqRel, Ordering::Acquire)
+            .ok()
+            .map(|_| Self { task })
+    }
+
+    fn initialize(self, future: impl FnOnce() -> F) -> TaskRef {
+        unsafe {
+            self.task.future.write(future());
+        }
+        TaskRef::new(self.task)
+    }
+}
+
 /// Raw storage that can hold up to N tasks of the same type.
 ///
 /// This is essentially a `[TaskStorage<F>; N]`.
@@ -207,13 +217,14 @@ impl<F: Future + 'static, const N: usize> TaskPool<F, N> {
     /// is currently free. If none is free, a "poisoned" SpawnToken is returned,
     /// which will cause [`Spawner::spawn()`](super::Spawner::spawn) to return the error.
     pub fn spawn(&'static self, future: impl FnOnce() -> F) -> SpawnToken<impl Sized> {
-        for task in &self.pool {
-            if task.spawn_mark_used() {
-                return unsafe { SpawnToken::<F>::new(task.spawn_initialize(future)) };
+        let task = self.pool.iter().find_map(AvailableTask::claim);
+        match task {
+            Some(task) => {
+                let task = task.initialize(future);
+                unsafe { SpawnToken::<F>::new(task) }
             }
+            None => SpawnToken::new_failed(),
         }
-
-        SpawnToken::<F>::new_failed()
     }
 
     /// Like spawn(), but allows the task to be send-spawned if the args are Send even if
@@ -255,13 +266,14 @@ impl<F: Future + 'static, const N: usize> TaskPool<F, N> {
         // This ONLY holds for `async fn` futures. The other `spawn` methods can be called directly
         // by the user, with arbitrary hand-implemented futures. This is why these return `SpawnToken<F>`.
 
-        for task in &self.pool {
-            if task.spawn_mark_used() {
-                return SpawnToken::<FutFn>::new(task.spawn_initialize(future));
+        let task = self.pool.iter().find_map(AvailableTask::claim);
+        match task {
+            Some(task) => {
+                let task = task.initialize(future);
+                unsafe { SpawnToken::<FutFn>::new(task) }
             }
+            None => SpawnToken::new_failed(),
         }
-
-        SpawnToken::<FutFn>::new_failed()
     }
 }