feat: Add a loom implementation for event-listener

.github/workflows/ci.yml

@@ -114,3 +114,14 @@ jobs:
       - uses: rustsec/audit-check@master
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
+
+  loom:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install Rust
+        run: rustup update stable
+      - name: Loom tests
+        run: RUSTFLAGS="--cfg=loom" cargo test --release --test loom --features loom  
+
+

Cargo.toml

@@ -18,15 +18,19 @@ exclude = ["/.*"]
 default = ["std"]
 std = ["concurrent-queue/std", "parking"]
 portable-atomic = ["portable-atomic-util", "portable_atomic_crate"]
+loom = ["concurrent-queue/loom", "parking?/loom", "dep:loom"]
 
 [dependencies]
-concurrent-queue = { version = "2.2.0", default-features = false }
+concurrent-queue = { version = "2.4.0", default-features = false }
 pin-project-lite = "0.2.12"
 portable-atomic-util = { version = "0.1.4", default-features = false, optional = true, features = ["alloc"] }
 
 [target.'cfg(not(target_family = "wasm"))'.dependencies]
 parking = { version = "2.0.0", optional = true }
 
+[target.'cfg(loom)'.dependencies]
+loom = { version = "0.7", optional = true }
+
 [dependencies.portable_atomic_crate]
 package = "portable-atomic"
 version = "1.2.0"

src/lib.rs

@@ -105,7 +105,10 @@ use {
 };
 
 use sync::atomic::{AtomicPtr, AtomicUsize, Ordering};
-use sync::{Arc, WithMut};
+use sync::Arc;
+
+#[cfg(not(loom))]
+use sync::WithMut;
 
 use notify::{Internal, NotificationPrivate};
 pub use notify::{IntoNotification, Notification};
@@ -216,13 +219,20 @@ impl<T> Event<T> {
     ///
     /// let event = Event::<usize>::with_tag();
     /// ```
-    #[cfg(feature = "std")]
+    #[cfg(all(feature = "std", not(loom)))]
     #[inline]
     pub const fn with_tag() -> Self {
         Self {
             inner: AtomicPtr::new(ptr::null_mut()),
         }
     }
+    #[cfg(all(feature = "std", loom))]
+    #[inline]
+    pub fn with_tag() -> Self {
+        Self {
+            inner: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
 
     /// Tell whether any listeners are currently notified.
     ///
@@ -543,12 +553,21 @@ impl Event<()> {
     /// let event = Event::new();
     /// ```
     #[inline]
+    #[cfg(not(loom))]
     pub const fn new() -> Self {
         Self {
             inner: AtomicPtr::new(ptr::null_mut()),
         }
     }
 
+    #[inline]
+    #[cfg(loom)]
+    pub fn new() -> Self {
+        Self {
+            inner: AtomicPtr::new(ptr::null_mut()),
+        }
+    }
+
     /// Notifies a number of active listeners without emitting a `SeqCst` fence.
     ///
     /// The number is allowed to be zero or exceed the current number of listeners.
@@ -1119,6 +1138,12 @@ impl<T, B: Borrow<Inner<T>> + Unpin> InnerListener<T, B> {
             match deadline {
                 None => parker.park(),
 
+                #[cfg(loom)]
+                Some(_deadline) => {
+                    panic!("parking does not support timeouts under loom");
+                }
+
+                #[cfg(not(loom))]
                 Some(deadline) => {
                     // Make sure we're not timed out already.
                     let now = Instant::now();
@@ -1330,10 +1355,9 @@ const NEVER_INSERTED_PANIC: &str = "\
 EventListener was not inserted into the linked list, make sure you're not polling \
 EventListener/listener! after it has finished";
 
+#[cfg(not(loom))]
 /// Synchronization primitive implementation.
 mod sync {
-    pub(super) use core::cell;
-
     #[cfg(not(feature = "portable-atomic"))]
     pub(super) use alloc::sync::Arc;
     #[cfg(not(feature = "portable-atomic"))]
@@ -1344,7 +1368,7 @@ mod sync {
     #[cfg(feature = "portable-atomic")]
     pub(super) use portable_atomic_util::Arc;
 
-    #[cfg(feature = "std")]
+    #[cfg(all(feature = "std", not(loom)))]
     pub(super) use std::sync::{Mutex, MutexGuard};
 
     pub(super) trait WithMut {
@@ -1366,6 +1390,51 @@ mod sync {
             f(self.get_mut())
         }
     }
+
+    pub(crate) mod cell {
+        pub(crate) use core::cell::Cell;
+
+        /// This newtype around *mut T exists for interoperability with loom::cell::ConstPtr,
+        /// which works as a guard and performs additional logic to track access scope.
+        pub(crate) struct ConstPtr<T>(*mut T);
+        impl<T> ConstPtr<T> {
+            pub(crate) unsafe fn deref(&self) -> &T {
+                &*self.0
+            }
+
+            #[allow(unused)] // std code does not need this
+            pub(crate) unsafe fn deref_mut(&mut self) -> &mut T {
+                &mut *self.0
+            }
+        }
+
+        /// This UnsafeCell wrapper exists for interoperability with loom::cell::UnsafeCell, and
+        /// only contains the interface that is needed for this crate.
+        #[derive(Debug, Default)]
+        pub(crate) struct UnsafeCell<T>(core::cell::UnsafeCell<T>);
+
+        impl<T> UnsafeCell<T> {
+            pub(crate) fn new(data: T) -> UnsafeCell<T> {
+                UnsafeCell(core::cell::UnsafeCell::new(data))
+            }
+
+            pub(crate) fn get(&self) -> ConstPtr<T> {
+                ConstPtr(self.0.get())
+            }
+
+            #[allow(dead_code)] // no_std does not need this
+            pub(crate) fn into_inner(self) -> T {
+                self.0.into_inner()
+            }
+        }
+    }
+}
+
+#[cfg(loom)]
+/// Synchronization primitive implementation.
+mod sync {
+    pub(super) use loom::cell;
+    pub(super) use loom::sync::{atomic, Arc, Mutex, MutexGuard};
 }
 
 fn __test_send_and_sync() {

src/no_std.rs

@@ -16,7 +16,7 @@ use node::{Node, NothingProducer, TaskWaiting};
 
 use crate::notify::{GenericNotify, Internal, Notification};
 use crate::sync::atomic::{AtomicBool, Ordering};
-use crate::sync::cell::{Cell, UnsafeCell};
+use crate::sync::cell::{Cell, ConstPtr, UnsafeCell};
 use crate::sync::Arc;
 use crate::{RegisterResult, State, Task, TaskRef};
 
@@ -771,7 +771,10 @@ impl<T> Mutex<T> {
             .is_ok()
         {
             // We have successfully locked the mutex.
-            Some(MutexGuard { mutex: self })
+            Some(MutexGuard {
+                mutex: self,
+                guard: self.value.get(),
+            })
         } else {
             self.try_lock_slow()
         }
@@ -790,7 +793,10 @@ impl<T> Mutex<T> {
                 .is_ok()
             {
                 // We have successfully locked the mutex.
-                return Some(MutexGuard { mutex: self });
+                return Some(MutexGuard {
+                    mutex: self,
+                    guard: self.value.get(),
+                });
             }
 
             // Use atomic loads instead of compare-exchange.
@@ -804,6 +810,7 @@ impl<T> Mutex<T> {
 
 pub(crate) struct MutexGuard<'a, T> {
     mutex: &'a Mutex<T>,
+    guard: ConstPtr<T>,
 }
 
 impl<'a, T> Drop for MutexGuard<'a, T> {
@@ -816,13 +823,13 @@ impl<'a, T> ops::Deref for MutexGuard<'a, T> {
     type Target = T;
 
     fn deref(&self) -> &T {
-        unsafe { &*self.mutex.value.get() }
+        unsafe { self.guard.deref() }
     }
 }
 
 impl<'a, T> ops::DerefMut for MutexGuard<'a, T> {
     fn deref_mut(&mut self) -> &mut T {
-        unsafe { &mut *self.mutex.value.get() }
+        unsafe { self.guard.deref_mut() }
     }
 }
 

src/std.rs

@@ -73,27 +73,27 @@ impl<T> crate::Inner<T> {
     pub(crate) fn insert(&self, mut listener: Pin<&mut Option<Listener<T>>>) {
         let mut inner = self.lock();
 
-        // SAFETY: We are locked, so we can access the inner `link`.
-        let entry = unsafe {
-            listener.as_mut().set(Some(Listener {
-                link: UnsafeCell::new(Link {
-                    state: Cell::new(State::Created),
-                    prev: Cell::new(inner.tail),
-                    next: Cell::new(None),
-                }),
-                _pin: PhantomPinned,
-            }));
-            let listener = listener.as_pin_mut().unwrap();
-
-            // Get the inner pointer.
-            &*listener.link.get()
-        };
-
-        // Replace the tail with the new entry.
-        match mem::replace(&mut inner.tail, Some(entry.into())) {
-            None => inner.head = Some(entry.into()),
-            Some(t) => unsafe { t.as_ref().next.set(Some(entry.into())) },
-        };
+        listener.as_mut().set(Some(Listener {
+            link: UnsafeCell::new(Link {
+                state: Cell::new(State::Created),
+                prev: Cell::new(inner.tail),
+                next: Cell::new(None),
+            }),
+            _pin: PhantomPinned,
+        }));
+        let listener = listener.as_pin_mut().unwrap();
+
+        {
+            let entry_guard = listener.link.get();
+            // SAFETY: We are locked, so we can access the inner `link`.
+            let entry = unsafe { entry_guard.deref() };
+
+            // Replace the tail with the new entry.
+            match mem::replace(&mut inner.tail, Some(entry.into())) {
+                None => inner.head = Some(entry.into()),
+                Some(t) => unsafe { t.as_ref().next.set(Some(entry.into())) },
+            };
+        }
 
         // If there are no unnotified entries, this is the first one.
         if inner.next.is_none() {
@@ -129,15 +129,12 @@ impl<T> crate::Inner<T> {
         task: TaskRef<'_>,
     ) -> RegisterResult<T> {
         let mut inner = self.lock();
-
-        // SAFETY: We are locked, so we can access the inner `link`.
-        let entry = unsafe {
-            let listener = match listener.as_mut().as_pin_mut() {
-                Some(listener) => listener,
-                None => return RegisterResult::NeverInserted,
-            };
-            &*listener.link.get()
+        let entry_guard = match listener.as_mut().as_pin_mut() {
+            Some(listener) => listener.link.get(),
+            None => return RegisterResult::NeverInserted,
         };
+        // SAFETY: We are locked, so we can access the inner `link`.
+        let entry = unsafe { entry_guard.deref() };
 
         // Take out the state and check it.
         match entry.state.replace(State::NotifiedTaken) {
@@ -175,12 +172,8 @@ impl<T> Inner<T> {
         mut listener: Pin<&mut Option<Listener<T>>>,
         propagate: bool,
     ) -> Option<State<T>> {
-        let entry = unsafe {
-            let listener = listener.as_mut().as_pin_mut()?;
-
-            // Get the inner pointer.
-            &*listener.link.get()
-        };
+        let entry_guard = listener.as_mut().as_pin_mut()?.link.get();
+        let entry = unsafe { entry_guard.deref() };
 
         let prev = entry.prev.get();
         let next = entry.next.get();
@@ -216,7 +209,11 @@ impl<T> Inner<T> {
                 .into_inner()
         };
 
-        let mut state = entry.state.into_inner();
+        // This State::Created is immediately dropped and exists as a workaround for the absence of
+        // loom::cell::Cell::into_inner. The intent is `let mut state = entry.state.into_inner();`
+        //
+        // refs: https://github.com/tokio-rs/loom/pull/341
+        let mut state = entry.state.replace(State::Created);
 
         // Update the notified count.
         if state.is_notified() {