Merge remote-tracking branch 'origin/master' into const-fn-feature

Author: Felix Stegmaier

src/lib.rs

@@ -84,7 +84,6 @@
 #![deny(warnings)]
 #![cfg_attr(feature = "const-fn", feature(const_fn))]
 #![feature(core_intrinsics)]
-#![feature(nonzero)]
 #![feature(untagged_unions)]
 #![no_std]
 

src/ring_buffer/mod.rs

@@ -212,7 +212,7 @@ where
         let tail = self.tail.load_relaxed().into();
 
         if head > tail {
-            head - tail
+            self.capacity().into() + 1 - head + tail
         } else {
             tail - head
         }
@@ -343,7 +343,7 @@ macro_rules! impl_ {
                 let tail = self.tail.load_relaxed();
 
                 if head > tail {
-                    head - tail
+                    self.capacity() + 1 - head + tail
                 } else {
                     tail - head
                 }
@@ -573,4 +573,32 @@ mod tests {
 
         assert_eq!(rb.len(), 2);
     }
+
+    #[test]
+    fn ready_flag() {
+        let mut rb: RingBuffer<i32, U2> = RingBuffer::new();
+        let (mut p, mut c) = rb.split();
+        assert_eq!(c.ready(), false);
+        assert_eq!(p.ready(), true);
+
+        p.enqueue(0).unwrap();
+
+        assert_eq!(c.ready(), true);
+        assert_eq!(p.ready(), true);
+
+        p.enqueue(1).unwrap();
+
+        assert_eq!(c.ready(), true);
+        assert_eq!(p.ready(), false);
+
+        c.dequeue().unwrap();
+
+        assert_eq!(c.ready(), true);
+        assert_eq!(p.ready(), true);
+
+        c.dequeue().unwrap();
+
+        assert_eq!(c.ready(), false);
+        assert_eq!(p.ready(), true);
+    }
 }

src/ring_buffer/spsc.rs

@@ -79,6 +79,14 @@ macro_rules! impl_ {
             N: Add<U1> + Unsigned,
             Sum<N, U1>: ArrayLength<T>,
         {
+            /// Returns if there are any items to dequeue. When this returns true, at least the
+            /// first subsequent dequeue will succeed.
+            pub fn ready(&self) -> bool {
+                let tail = unsafe { self.rb.as_ref().tail.load_acquire() };
+                let head = unsafe { self.rb.as_ref().head.load_relaxed() };
+                return head != tail;
+            }
+
             /// Returns the item in the front of the queue, or `None` if the queue is empty
             pub fn dequeue(&mut self) -> Option<T> {
                 let tail = unsafe { self.rb.as_ref().tail.load_acquire() };
@@ -119,6 +127,21 @@ macro_rules! impl_ {
             N: Add<U1> + Unsigned,
             Sum<N, U1>: ArrayLength<T>,
         {
+            /// Returns if there is any space to enqueue a new item. When this returns true, at
+            /// least the first subsequent enqueue will succeed.
+            pub fn ready(&self) -> bool {
+                let n = unsafe { self.rb.as_ref().capacity() + 1 };
+                let tail = unsafe { self.rb.as_ref().tail.load_relaxed() };
+                // NOTE we could replace this `load_acquire` with a `load_relaxed` and this method
+                // would be sound on most architectures but that change would result in UB according
+                // to the C++ memory model, which is what Rust currently uses, so we err on the side
+                // of caution and stick to `load_acquire`. Check issue google#sanitizers#882 for
+                // more details.
+                let head = unsafe { self.rb.as_ref().head.load_acquire() };
+                let next_tail = (tail + 1) % n;
+                return next_tail != head;
+            }
+
             /// Adds an `item` to the end of the queue
             ///
             /// Returns back the `item` if the queue is full

tests/tsan.rs

@@ -157,3 +157,38 @@ fn unchecked() {
 
     assert_eq!(rb.len(), N::to_usize() / 2);
 }
+
+#[test]
+fn len_properly_wraps() {
+    type N = U3;
+    let mut rb: RingBuffer<u8, N> = RingBuffer::new();
+
+    rb.enqueue(1).unwrap();
+    assert_eq!(rb.len(), 1);
+    rb.dequeue();
+    assert_eq!(rb.len(), 0);
+    rb.enqueue(2).unwrap();
+    assert_eq!(rb.len(), 1);
+    rb.enqueue(3).unwrap();
+    assert_eq!(rb.len(), 2);
+    rb.enqueue(4).unwrap();
+    assert_eq!(rb.len(), 3);
+}
+
+#[test]
+fn iterator_properly_wraps() {
+    type N = U3;
+    let mut rb: RingBuffer<u8, N> = RingBuffer::new();
+
+    rb.enqueue(1).unwrap();
+    rb.dequeue();
+    rb.enqueue(2).unwrap();
+    rb.enqueue(3).unwrap();
+    rb.enqueue(4).unwrap();
+    let expected = [2, 3, 4];
+    let mut actual = [0, 0, 0];
+    for (idx, el) in rb.iter().enumerate() {
+        actual[idx] = *el;
+    }
+    assert_eq!(expected, actual)
+}