---

yaml
---
r: 107978
b: refs/heads/dist-snap
c: 28f277b
h: refs/heads/master
v: v3

Author: bors

[refs]

@@ -6,7 +6,7 @@ refs/heads/try: f64fdf524a434f0e5cd0bc91d09c144723f3c90d
 refs/tags/release-0.1: 1f5c5126e96c79d22cb7862f75304136e204f105
 refs/heads/ndm: f3868061cd7988080c30d6d5bf352a5a5fe2460b
 refs/heads/try2: 147ecfdd8221e4a4d4e090486829a06da1e0ca3c
-refs/heads/dist-snap: 01815b392058dbf862f4287be307821cf00cc964
+refs/heads/dist-snap: 28f277b909bf24728593710a952c65193cfbefe4
 refs/tags/release-0.2: c870d2dffb391e14efb05aa27898f1f6333a9596
 refs/tags/release-0.3: b5f0d0f648d9a6153664837026ba1be43d3e2503
 refs/heads/try3: 9387340aab40a73e8424c48fd42f0c521a4875c0

branches/dist-snap/src/libextra/arc.rs

@@ -554,6 +554,50 @@ impl<'a, T:Freeze + Send> RWReadMode<'a, T> {
     }
 }
 
+/****************************************************************************
+ * Copy-on-write Arc
+ ****************************************************************************/
+
+pub struct CowArc<T> { priv x: UnsafeArc<T> }
+
+/// A Copy-on-write Arc functions the same way as an `arc` except it allows
+/// mutation of the contents if there is only a single reference to
+/// the data. If there are multiple references the data is automatically
+/// cloned and the task modifies the cloned data in place of the shared data.
+impl<T:Clone+Send+Freeze> CowArc<T> {
+    /// Create a copy-on-write atomically reference counted wrapper
+    #[inline]
+    pub fn new(data: T) -> CowArc<T> {
+        CowArc { x: UnsafeArc::new(data) }
+    }
+
+    #[inline]
+    pub fn get<'a>(&'a self) -> &'a T {
+        unsafe { &*self.x.get_immut() }
+    }
+
+    /// get a mutable reference to the contents. If there are more then one
+    /// reference to the contents of the `CowArc` will be cloned
+    /// and this reference updated to point to the cloned data.
+    #[inline]
+    pub fn get_mut<'a>(&'a mut self) -> &'a mut T {
+        if !self.x.is_owned() {
+            *self = CowArc::new(self.get().clone())
+        }
+        unsafe { &mut *self.x.get() }
+    }
+}
+
+impl<T:Clone+Send+Freeze> Clone for CowArc<T> {
+    /// Duplicate a Copy-on-write Arc. See arc::clone for more details.
+    #[inline]
+    fn clone(&self) -> CowArc<T> {
+        CowArc { x: self.x.clone() }
+    }
+}
+
+
+
 /****************************************************************************
  * Tests
  ****************************************************************************/
@@ -963,4 +1007,68 @@ mod tests {
         // and I wasn't sure why :( . This is a mediocre "next best" option.
         for _ in range(0, 8) { test_rw_write_cond_downgrade_read_race_helper(); }
     }
+
+    #[test]
+    fn test_cowarc_clone()
+    {
+        let cow0 = CowArc::new(75u);
+        let cow1 = cow0.clone();
+        let cow2 = cow1.clone();
+
+        assert!(75 == *cow0.get());
+        assert!(75 == *cow1.get());
+        assert!(75 == *cow2.get());
+
+        assert!(cow0.get() == cow1.get());
+        assert!(cow0.get() == cow2.get());
+    }
+
+    #[test]
+    fn test_cowarc_clone_get_mut()
+    {
+        let mut cow0 = CowArc::new(75u);
+        let mut cow1 = cow0.clone();
+        let mut cow2 = cow1.clone();
+
+        assert!(75 == *cow0.get_mut());
+        assert!(75 == *cow1.get_mut());
+        assert!(75 == *cow2.get_mut());
+
+        *cow0.get_mut() += 1;
+        *cow1.get_mut() += 2;
+        *cow2.get_mut() += 3;
+
+        assert!(76 == *cow0.get());
+        assert!(77 == *cow1.get());
+        assert!(78 == *cow2.get());
+
+        // none should point to the same backing memory
+        assert!(cow0.get() != cow1.get());
+        assert!(cow0.get() != cow2.get());
+        assert!(cow1.get() != cow2.get());
+    }
+
+    #[test]
+    fn test_cowarc_clone_get_mut2()
+    {
+        let mut cow0 = CowArc::new(75u);
+        let cow1 = cow0.clone();
+        let cow2 = cow1.clone();
+
+        assert!(75 == *cow0.get());
+        assert!(75 == *cow1.get());
+        assert!(75 == *cow2.get());
+
+        *cow0.get_mut() += 1;
+
+        assert!(76 == *cow0.get());
+        assert!(75 == *cow1.get());
+        assert!(75 == *cow2.get());
+
+        // cow1 and cow2 should share the same contents
+        // cow0 should have a unique reference
+        assert!(cow0.get() != cow1.get());
+        assert!(cow0.get() != cow2.get());
+        assert!(cow1.get() == cow2.get());
+    }
 }

branches/dist-snap/src/libstd/sync/arc.rs

@@ -94,6 +94,14 @@ impl<T: Send> UnsafeArc<T> {
             return &(*self.data).data as *T;
         }
     }
+
+    /// checks if this is the only reference to the arc protected data
+    #[inline]
+    pub fn is_owned(&self) -> bool {
+        unsafe {
+            (*self.data).count.load(Relaxed) == 1
+        }
+    }
 }
 
 impl<T: Send> Clone for UnsafeArc<T> {