Add AtomicCell::fetch_update

crossbeam-utils/src/atomic/atomic_cell.rs

@@ -258,6 +258,40 @@ impl<T: Copy + Eq> AtomicCell<T> {
     pub fn compare_exchange(&self, current: T, new: T) -> Result<T, T> {
         unsafe { atomic_compare_exchange_weak(self.value.get(), current, new) }
     }
+
+    /// Fetches the value, and applies a function to it that returns an optional
+    /// new value. Returns a `Result` of `Ok(previous_value)` if the function returned `Some(_)`, else
+    /// `Err(previous_value)`.
+    ///
+    /// Note: This may call the function multiple times if the value has been changed from other threads in
+    /// the meantime, as long as the function returns `Some(_)`, but the function will have been applied
+    /// only once to the stored value.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use crossbeam_utils::atomic::AtomicCell;
+    ///
+    /// let a = AtomicCell::new(7);
+    /// assert_eq!(a.fetch_update(|_| None), Err(7));
+    /// assert_eq!(a.fetch_update(|a| Some(a + 1)), Ok(7));
+    /// assert_eq!(a.fetch_update(|a| Some(a + 1)), Ok(8));
+    /// assert_eq!(a.load(), 9);
+    /// ```
+    #[inline]
+    pub fn fetch_update<F>(&self, mut f: F) -> Result<T, T>
+    where
+        F: FnMut(T) -> Option<T>,
+    {
+        let mut prev = self.load();
+        while let Some(next) = f(prev) {
+            match self.compare_exchange(prev, next) {
+                x @ Ok(_) => return x,
+                Err(next_prev) => prev = next_prev,
+            }
+        }
+        Err(prev)
+    }
 }
 
 macro_rules! impl_arithmetic {