Implement an algorithm to make this crate no_std, take 3

.github/workflows/ci.yml

@@ -28,7 +28,12 @@ jobs:
       - name: Run cargo check (without dev-dependencies to catch missing feature flags)
         if: startsWith(matrix.rust, 'nightly')
         run: cargo check -Z features=dev_dep
-      - run: cargo test
+      - run: cargo test --features __test
+      - run: cargo build --no-default-features
+      - name: Install cargo-hack
+        uses: taiki-e/install-action@cargo-hack
+      - run: rustup target add thumbv7m-none-eabi
+      - run: cargo hack build --target thumbv7m-none-eabi --no-default-features --no-dev-deps
 
   msrv:
     runs-on: ubuntu-latest
@@ -65,7 +70,7 @@ jobs:
       - uses: actions/checkout@v3
       - name: Install Rust
         run: rustup toolchain install nightly --component miri && rustup default nightly
-      - run: cargo miri test
+      - run: cargo miri test --features __test
         env:
           MIRIFLAGS: -Zmiri-strict-provenance -Zmiri-symbolic-alignment-check -Zmiri-disable-isolation
           RUSTFLAGS: ${{ env.RUSTFLAGS }} -Z randomize-layout

Cargo.toml

@@ -14,8 +14,16 @@ keywords = ["condvar", "eventcount", "wake", "blocking", "park"]
 categories = ["asynchronous", "concurrency"]
 exclude = ["/.*"]
 
+[features]
+default = ["std"]
+std = ["parking"]
+
+# Unstable, test only feature. Do not enable this.
+__test = []
+
 [dependencies]
-parking = "2.0.0"
+crossbeam-utils = { version = "0.8.12", default-features = false }
+parking = { version = "2.0.0", optional = true }
 
 [dev-dependencies]
 criterion = "0.3.4"
@@ -26,4 +34,4 @@ name = "bench"
 harness = false
 
 [lib]
-bench = false
+bench = false

src/inner.rs

@@ -0,0 +1,226 @@
+//! The inner mechanism powering the `Event` type.
+
+use crate::list::{Entry, List};
+use crate::node::Node;
+use crate::queue::Queue;
+use crate::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use crate::sync::cell::UnsafeCell;
+use crate::Task;
+
+use alloc::vec;
+use alloc::vec::Vec;
+
+use core::ops;
+use core::ptr::NonNull;
+
+/// Inner state of [`Event`].
+pub(crate) struct Inner {
+    /// The number of notified entries, or `usize::MAX` if all of them have been notified.
+    ///
+    /// If there are no entries, this value is set to `usize::MAX`.
+    pub(crate) notified: AtomicUsize,
+
+    /// A linked list holding registered listeners.
+    list: Mutex<List>,
+
+    /// Queue of nodes waiting to be processed.
+    queue: Queue,
+
+    /// A single cached list entry to avoid allocations on the fast path of the insertion.
+    ///
+    /// This field can only be written to when the `cache_used` field in the `list` structure
+    /// is false, or the user has a pointer to the `Entry` identical to this one and that user
+    /// has exclusive access to that `Entry`. An immutable pointer to this field is kept in
+    /// the `list` structure when it is in use.
+    cache: UnsafeCell<Entry>,
+}
+
+impl Inner {
+    /// Create a new `Inner`.
+    pub(crate) fn new() -> Self {
+        Self {
+            notified: AtomicUsize::new(core::usize::MAX),
+            list: Mutex::new(List::new()),
+            queue: Queue::new(),
+            cache: UnsafeCell::new(Entry::new()),
+        }
+    }
+
+    /// Locks the list.
+    pub(crate) fn lock(&self) -> Option<ListGuard<'_>> {
+        self.list.try_lock().map(|guard| ListGuard {
+            inner: self,
+            guard: Some(guard),
+        })
+    }
+
+    /// Push a pending operation to the queue.
+    #[cold]
+    pub(crate) fn push(&self, node: Node) {
+        self.queue.push(node);
+
+        // Acquire and drop the lock to make sure that the queue is flushed.
+        let _guard = self.lock();
+    }
+
+    /// Returns the pointer to the single cached list entry.
+    #[inline(always)]
+    pub(crate) fn cache_ptr(&self) -> NonNull<Entry> {
+        unsafe { NonNull::new_unchecked(self.cache.get()) }
+    }
+}
+
+/// The guard returned by [`Inner::lock`].
+pub(crate) struct ListGuard<'a> {
+    /// Reference to the inner state.
+    inner: &'a Inner,
+
+    /// The locked list.
+    guard: Option<MutexGuard<'a, List>>,
+}
+
+impl ListGuard<'_> {
+    #[cold]
+    fn process_nodes_slow(
+        &mut self,
+        start_node: Node,
+        tasks: &mut Vec<Task>,
+        guard: &mut MutexGuard<'_, List>,
+    ) {
+        // Process the start node.
+        tasks.extend(start_node.apply(guard, self.inner));
+
+        // Process all remaining nodes.
+        while let Some(node) = self.inner.queue.pop() {
+            tasks.extend(node.apply(guard, self.inner));
+        }
+    }
+}
+
+impl ops::Deref for ListGuard<'_> {
+    type Target = List;
+
+    fn deref(&self) -> &Self::Target {
+        self.guard.as_ref().unwrap()
+    }
+}
+
+impl ops::DerefMut for ListGuard<'_> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.guard.as_mut().unwrap()
+    }
+}
+
+impl Drop for ListGuard<'_> {
+    fn drop(&mut self) {
+        let Self { inner, guard } = self;
+        let mut list = guard.take().unwrap();
+
+        // Tasks to wakeup after releasing the lock.
+        let mut tasks = vec![];
+
+        // Process every node left in the queue.
+        if let Some(start_node) = inner.queue.pop() {
+            self.process_nodes_slow(start_node, &mut tasks, &mut list);
+        }
+
+        // Update the atomic `notified` counter.
+        let notified = if list.notified < list.len {
+            list.notified
+        } else {
+            core::usize::MAX
+        };
+
+        self.inner.notified.store(notified, Ordering::Release);
+
+        // Drop the actual lock.
+        drop(list);
+
+        // Wakeup all tasks.
+        for task in tasks {
+            task.wake();
+        }
+    }
+}
+
+/// A simple mutex type that optimistically assumes that the lock is uncontended.
+struct Mutex<T> {
+    /// The inner value.
+    value: UnsafeCell<T>,
+
+    /// Whether the mutex is locked.
+    locked: AtomicBool,
+}
+
+impl<T> Mutex<T> {
+    /// Create a new mutex.
+    pub(crate) fn new(value: T) -> Self {
+        Self {
+            value: UnsafeCell::new(value),
+            locked: AtomicBool::new(false),
+        }
+    }
+
+    /// Lock the mutex.
+    pub(crate) fn try_lock(&self) -> Option<MutexGuard<'_, T>> {
+        // Try to lock the mutex.
+        if self
+            .locked
+            .compare_exchange(false, true, Ordering::Acquire, Ordering::Relaxed)
+            .is_ok()
+        {
+            // We have successfully locked the mutex.
+            Some(MutexGuard { mutex: self })
+        } else {
+            self.try_lock_slow()
+        }
+    }
+
+    #[cold]
+    fn try_lock_slow(&self) -> Option<MutexGuard<'_, T>> {
+        // Assume that the contention is short-term.
+        // Spin for a while to see if the mutex becomes unlocked.
+        let mut spins = 100u32;
+
+        loop {
+            if self
+                .locked
+                .compare_exchange_weak(false, true, Ordering::Acquire, Ordering::Relaxed)
+                .is_ok()
+            {
+                // We have successfully locked the mutex.
+                return Some(MutexGuard { mutex: self });
+            }
+
+            // Use atomic loads instead of compare-exchange.
+            while self.locked.load(Ordering::Relaxed) {
+                // Return None once we've exhausted the number of spins.
+                spins = spins.checked_sub(1)?;
+            }
+        }
+    }
+}
+
+struct MutexGuard<'a, T> {
+    mutex: &'a Mutex<T>,
+}
+
+impl<'a, T> Drop for MutexGuard<'a, T> {
+    fn drop(&mut self) {
+        self.mutex.locked.store(false, Ordering::Release);
+    }
+}
+
+impl<'a, T> ops::Deref for MutexGuard<'a, T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        unsafe { &*self.mutex.value.get() }
+    }
+}
+
+impl<'a, T> ops::DerefMut for MutexGuard<'a, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.mutex.value.get() }
+    }
+}