1 files changed, 102 insertions, 0 deletions
diff --git a/third_party/rust/futures-channel/src/lock.rs b/third_party/rust/futures-channel/src/lock.rs
new file mode 100644
index 0000000000..b328d0f7dd
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+++ b/third_party/rust/futures-channel/src/lock.rs
@@ -0,0 +1,102 @@
+//! A "mutex" which only supports `try_lock`
+//!
+//! As a futures library the eventual call to an event loop should be the only
+//! thing that ever blocks, so this is assisted with a fast user-space
+//! implementation of a lock that can only have a `try_lock` operation.
+
+use core::cell::UnsafeCell;
+use core::ops::{Deref, DerefMut};
+use core::sync::atomic::AtomicBool;
+use core::sync::atomic::Ordering::SeqCst;
+
+/// A "mutex" around a value, similar to `std::sync::Mutex<T>`.
+///
+/// This lock only supports the `try_lock` operation, however, and does not
+/// implement poisoning.
+#[derive(Debug)]
+pub(crate) struct Lock<T> {
+    locked: AtomicBool,
+    data: UnsafeCell<T>,
+}
+
+/// Sentinel representing an acquired lock through which the data can be
+/// accessed.
+pub(crate) struct TryLock<'a, T> {
+    __ptr: &'a Lock<T>,
+}
+
+// The `Lock` structure is basically just a `Mutex<T>`, and these two impls are
+// intended to mirror the standard library's corresponding impls for `Mutex<T>`.
+//
+// If a `T` is sendable across threads, so is the lock, and `T` must be sendable
+// across threads to be `Sync` because it allows mutable access from multiple
+// threads.
+unsafe impl<T: Send> Send for Lock<T> {}
+unsafe impl<T: Send> Sync for Lock<T> {}
+
+impl<T> Lock<T> {
+    /// Creates a new lock around the given value.
+    pub(crate) fn new(t: T) -> Self {
+        Self { locked: AtomicBool::new(false), data: UnsafeCell::new(t) }
+    }
+
+    /// Attempts to acquire this lock, returning whether the lock was acquired or
+    /// not.
+    ///
+    /// If `Some` is returned then the data this lock protects can be accessed
+    /// through the sentinel. This sentinel allows both mutable and immutable
+    /// access.
+    ///
+    /// If `None` is returned then the lock is already locked, either elsewhere
+    /// on this thread or on another thread.
+    pub(crate) fn try_lock(&self) -> Option<TryLock<'_, T>> {
+        if !self.locked.swap(true, SeqCst) {
+            Some(TryLock { __ptr: self })
+        } else {
+            None
+        }
+    }
+}
+
+impl<T> Deref for TryLock<'_, T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        // The existence of `TryLock` represents that we own the lock, so we
+        // can safely access the data here.
+        unsafe { &*self.__ptr.data.get() }
+    }
+}
+
+impl<T> DerefMut for TryLock<'_, T> {
+    fn deref_mut(&mut self) -> &mut T {
+        // The existence of `TryLock` represents that we own the lock, so we
+        // can safely access the data here.
+        //
+        // Additionally, we're the *only* `TryLock` in existence so mutable
+        // access should be ok.
+        unsafe { &mut *self.__ptr.data.get() }
+    }
+}
+
+impl<T> Drop for TryLock<'_, T> {
+    fn drop(&mut self) {
+        self.__ptr.locked.store(false, SeqCst);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Lock;
+
+    #[test]
+    fn smoke() {
+        let a = Lock::new(1);
+        let mut a1 = a.try_lock().unwrap();
+        assert!(a.try_lock().is_none());
+        assert_eq!(*a1, 1);
+        *a1 = 2;
+        drop(a1);
+        assert_eq!(*a.try_lock().unwrap(), 2);
+        assert_eq!(*a.try_lock().unwrap(), 2);
+    }
+}