Make the RingBuffer contiguous memory optimization work.

Author: astro

src/storage/ring_buffer.rs

@@ -221,12 +221,14 @@ impl<'a, T: 'a> RingBuffer<'a, T> {
         let max_size = cmp::min(self.len(), capacity - self.read_at);
         let (size, result) = f(&mut self.storage[self.read_at..self.read_at + max_size]);
         assert!(size <= max_size);
-        self.read_at = if capacity > 0 {
+        self.length -= size;
+        self.read_at = if self.length > 0 {
             (self.read_at + size) % capacity
         } else {
+            // Ring is currently empty, reset `read_at` to optimize
+            // for contiguous space.
             0
         };
-        self.length -= size;
         (size, result)
     }
 
@@ -659,13 +661,13 @@ mod test {
         ring.dequeue_many(6).copy_from_slice(b"ABCDEF");
 
         assert_eq!(ring.write_unallocated(0, b"ghi"), 3);
-        assert_eq!(&ring.storage[..], b"ABCDEFghi...");
+        assert_eq!(&ring.storage[..], b"ghiDEF......");
 
         assert_eq!(ring.write_unallocated(3, b"jklmno"), 6);
-        assert_eq!(&ring.storage[..], b"mnoDEFghijkl");
+        assert_eq!(&ring.storage[..], b"ghijklmno...");
 
         assert_eq!(ring.write_unallocated(9, b"pqrstu"), 3);
-        assert_eq!(&ring.storage[..], b"mnopqrghijkl");
+        assert_eq!(&ring.storage[..], b"ghijklmnopqr");
     }
 
     #[test]