Support racy initialization of an Executor's state

Fixes smol-rs#89. Uses @notgull's suggestion of using a `AtomicPtr` with a racy initialization instead of a `OnceCell`.

For the addition of more `unsafe`, I added the `clippy::undocumented_unsafe_blocks` lint at a warn, and fixed a few of the remaining open clippy issues (i.e. `Waker::clone_from` already handling the case where they're equal).

Removing `async_lock` as a dependency shouldn't be a SemVer breaking change.

Author: james7132

Cargo.toml

@@ -15,7 +15,6 @@ categories = ["asynchronous", "concurrency"]
 exclude = ["/.*"]
 
 [dependencies]
-async-lock = "3.0.0"
 async-task = "4.4.0"
 concurrent-queue = "2.0.0"
 fastrand = "2.0.0"

src/lib.rs

@@ -25,7 +25,12 @@
 //! future::block_on(ex.run(task));
 //! ```
 
-#![warn(missing_docs, missing_debug_implementations, rust_2018_idioms)]
+#![warn(
+    missing_docs,
+    missing_debug_implementations,
+    rust_2018_idioms,
+    clippy::undocumented_unsafe_blocks
+)]
 #![doc(
     html_favicon_url = "https://raw.githubusercontent.com/smol-rs/smol/master/assets/images/logo_fullsize_transparent.png"
 )]
@@ -37,11 +42,10 @@ use std::fmt;
 use std::marker::PhantomData;
 use std::panic::{RefUnwindSafe, UnwindSafe};
 use std::rc::Rc;
-use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicPtr, Ordering};
 use std::sync::{Arc, Mutex, RwLock, TryLockError};
 use std::task::{Poll, Waker};
 
-use async_lock::OnceCell;
 use async_task::{Builder, Runnable};
 use concurrent_queue::ConcurrentQueue;
 use futures_lite::{future, prelude::*};
@@ -76,13 +80,15 @@ pub use async_task::Task;
 /// ```
 pub struct Executor<'a> {
     /// The executor state.
-    state: OnceCell<Arc<State>>,
+    state: AtomicPtr<State>,
 
     /// Makes the `'a` lifetime invariant.
     _marker: PhantomData<std::cell::UnsafeCell<&'a ()>>,
 }
 
+// SAFETY: Executor stores no thread local state that can be accessed via other thread.
 unsafe impl Send for Executor<'_> {}
+// SAFETY: Executor internally synchronizes all of it's operations internally.
 unsafe impl Sync for Executor<'_> {}
 
 impl UnwindSafe for Executor<'_> {}
@@ -106,7 +112,7 @@ impl<'a> Executor<'a> {
     /// ```
     pub const fn new() -> Executor<'a> {
         Executor {
-            state: OnceCell::new(),
+            state: AtomicPtr::new(std::ptr::null_mut()),
             _marker: PhantomData,
         }
     }
@@ -231,7 +237,7 @@ impl<'a> Executor<'a> {
         // Remove the task from the set of active tasks when the future finishes.
         let entry = active.vacant_entry();
         let index = entry.key();
-        let state = self.state().clone();
+        let state = self.state_as_arc();
         let future = async move {
             let _guard = CallOnDrop(move || drop(state.active.lock().unwrap().try_remove(index)));
             future.await
@@ -361,7 +367,7 @@ impl<'a> Executor<'a> {
 
     /// Returns a function that schedules a runnable task when it gets woken up.
     fn schedule(&self) -> impl Fn(Runnable) + Send + Sync + 'static {
-        let state = self.state().clone();
+        let state = self.state_as_arc();
 
         // TODO: If possible, push into the current local queue and notify the ticker.
         move |runnable| {
@@ -370,34 +376,73 @@ impl<'a> Executor<'a> {
         }
     }
 
-    /// Returns a reference to the inner state.
-    fn state(&self) -> &Arc<State> {
-        #[cfg(not(target_family = "wasm"))]
-        {
-            return self.state.get_or_init_blocking(|| Arc::new(State::new()));
+    /// Returns a pointer to the inner state.
+    #[inline]
+    fn state_ptr(&self) -> *const State {
+        #[cold]
+        fn alloc_state(atomic_ptr: &AtomicPtr<State>) -> *mut State {
+            let state = Arc::new(State::new());
+            // TODO: Switch this to use cast_mut once the MSRV can be bumped past 1.65
+            let ptr = Arc::into_raw(state) as *mut State;
+            if let Err(actual) = atomic_ptr.compare_exchange(
+                std::ptr::null_mut(),
+                ptr,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            ) {
+                // SAFETY: This was just created from Arc::into_raw.
+                drop(unsafe { Arc::from_raw(ptr) });
+                actual
+            } else {
+                ptr
+            }
         }
 
-        // Some projects use this on WASM for some reason. In this case get_or_init_blocking
-        // doesn't work. Just poll the future once and panic if there is contention.
-        #[cfg(target_family = "wasm")]
-        future::block_on(future::poll_once(
-            self.state.get_or_init(|| async { Arc::new(State::new()) }),
-        ))
-        .expect("encountered contention on WASM")
+        let mut ptr = self.state.load(Ordering::Acquire);
+        if ptr.is_null() {
+            ptr = alloc_state(&self.state);
+        }
+        ptr
+    }
+
+    /// Returns a reference to the inner state.
+    #[inline]
+    fn state(&self) -> &State {
+        // SAFETY: So long as an Executor lives, it's state pointer will always be valid
+        // when accessed through state_ptr.
+        unsafe { &*self.state_ptr() }
+    }
+
+    // Clones the inner state Arc
+    #[inline]
+    fn state_as_arc(&self) -> Arc<State> {
+        // SAFETY: So long as an Executor lives, it's state pointer will always be a valid
+        // Arc when accessed through state_ptr.
+        let arc = unsafe { Arc::from_raw(self.state_ptr()) };
+        let clone = arc.clone();
+        std::mem::forget(arc);
+        clone
     }
 }
 
 impl Drop for Executor<'_> {
     fn drop(&mut self) {
-        if let Some(state) = self.state.get() {
-            let mut active = state.active.lock().unwrap_or_else(|e| e.into_inner());
-            for w in active.drain() {
-                w.wake();
-            }
-            drop(active);
+        let ptr = *self.state.get_mut();
+        if ptr.is_null() {
+            return;
+        }
+
+        // SAFETY: As ptr is not null, it was allocated via Arc::new and converted
+        // via Arc::into_raw in state_ptr.
+        let state = unsafe { Arc::from_raw(ptr) };
 
-            while state.queue.pop().is_ok() {}
+        let mut active = state.active.lock().unwrap_or_else(|e| e.into_inner());
+        for w in active.drain() {
+            w.wake();
         }
+        drop(active);
+
+        while state.queue.pop().is_ok() {}
     }
 }
 
@@ -718,9 +763,7 @@ impl Sleepers {
     fn update(&mut self, id: usize, waker: &Waker) -> bool {
         for item in &mut self.wakers {
             if item.0 == id {
-                if !item.1.will_wake(waker) {
-                    item.1.clone_from(waker);
-                }
+                item.1.clone_from(waker);
                 return false;
             }
         }
@@ -1006,21 +1049,24 @@ fn steal<T>(src: &ConcurrentQueue<T>, dest: &ConcurrentQueue<T>) {
 /// Debug implementation for `Executor` and `LocalExecutor`.
 fn debug_executor(executor: &Executor<'_>, name: &str, f: &mut fmt::Formatter<'_>) -> fmt::Result {
     // Get a reference to the state.
-    let state = match executor.state.get() {
-        Some(state) => state,
-        None => {
-            // The executor has not been initialized.
-            struct Uninitialized;
-
-            impl fmt::Debug for Uninitialized {
-                fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                    f.write_str("<uninitialized>")
-                }
+    let ptr = executor.state.load(Ordering::Acquire);
+    if ptr.is_null() {
+        // The executor has not been initialized.
+        struct Uninitialized;
+
+        impl fmt::Debug for Uninitialized {
+            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+                f.write_str("<uninitialized>")
             }
-
-            return f.debug_tuple(name).field(&Uninitialized).finish();
         }
-    };
+
+        return f.debug_tuple(name).field(&Uninitialized).finish();
+    }
+
+    // SAFETY: If the state pointer is not null, it must have been
+    // allocated properly by Arc::new and converted via Arc::into_raw
+    // in state_ptr.
+    let state = unsafe { &*ptr };
 
     /// Debug wrapper for the number of active tasks.
     struct ActiveTasks<'a>(&'a Mutex<Slab<Waker>>);