Merge #706

706: Add Atomic::fetch_update r=taiki-e a=PatrickNorton

Equivalent of [`AtomicN::fetch_update`](https://doc.rust-lang.org/nightly/core/sync/atomic/struct.AtomicUsize.html#method.fetch_update) and [`AtomicCell::fetch_update`](#704).

Co-authored-by: Patrick Norton <patrick.147.norton@gmail.com>

Author: bors[bot]

crossbeam-epoch/src/atomic.rs

@@ -562,6 +562,65 @@ impl<T: ?Sized + Pointable> Atomic<T> {
             })
     }
 
+    /// Fetches the pointer, and then applies a function to it that returns a new value.
+    /// Returns a `Result` of `Ok(previous_value)` if the function returned `Some`, else `Err(_)`.
+    ///
+    /// Note that the given function may be called multiple times if the value has been changed by
+    /// 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.
+    ///
+    /// `fetch_update` takes two [`Ordering`] arguments to describe the memory
+    /// ordering of this operation. The first describes the required ordering for
+    /// when the operation finally succeeds while the second describes the
+    /// required ordering for loads. These correspond to the success and failure
+    /// orderings of [`Atomic::compare_exchange`] respectively.
+    ///
+    /// Using [`Acquire`] as success ordering makes the store part of this
+    /// operation [`Relaxed`], and using [`Release`] makes the final successful
+    /// load [`Relaxed`]. The (failed) load ordering can only be [`SeqCst`],
+    /// [`Acquire`] or [`Relaxed`] and must be equivalent to or weaker than the
+    /// success ordering.
+    ///
+    /// [`Relaxed`]: Ordering::Relaxed
+    /// [`Acquire`]: Ordering::Acquire
+    /// [`Release`]: Ordering::Release
+    /// [`SeqCst`]: Ordering::SeqCst
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use crossbeam_epoch::{self as epoch, Atomic};
+    /// use std::sync::atomic::Ordering::SeqCst;
+    ///
+    /// let a = Atomic::new(1234);
+    /// let guard = &epoch::pin();
+    ///
+    /// let res1 = a.fetch_update(SeqCst, SeqCst, guard, |x| Some(x.with_tag(1)));
+    /// assert!(res1.is_ok());
+    ///
+    /// let res2 = a.fetch_update(SeqCst, SeqCst, guard, |x| None);
+    /// assert!(res2.is_err());
+    /// ```
+    pub fn fetch_update<'g, F>(
+        &self,
+        set_order: Ordering,
+        fail_order: Ordering,
+        guard: &'g Guard,
+        mut func: F,
+    ) -> Result<Shared<'g, T>, Shared<'g, T>>
+    where
+        F: FnMut(Shared<'g, T>) -> Option<Shared<'g, T>>,
+    {
+        let mut prev = self.load(fail_order, guard);
+        while let Some(next) = func(prev) {
+            match self.compare_exchange_weak(prev, next, set_order, fail_order, guard) {
+                Ok(shared) => return Ok(shared),
+                Err(next_prev) => prev = next_prev.current,
+            }
+        }
+        Err(prev)
+    }
+
     /// Stores the pointer `new` (either `Shared` or `Owned`) into the atomic pointer if the current
     /// value is the same as `current`. The tag is also taken into account, so two pointers to the
     /// same object, but with different tags, will not be considered equal.