add a test

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

src/vmm/src/devices/virtio/iov_deque.rs

@@ -544,4 +544,30 @@ mod tests {
             assert_eq!(iov.iov_len, 2 * copy[i].iov_len);
         }
     }
+
+    #[test]
+    fn test_size_less_than_capacity() {
+        const L: u16 = 16;
+        let mut deque = super::IovDeque::<L>::new().unwrap();
+        assert!(deque.as_mut_slice().is_empty());
+
+        // Number of times need to fill/empty the queue to reach the
+        // pivot point.
+        let fills = deque.capacity / L;
+        // Almost reach the pivot.
+        for _ in 0..(fills - 1) {
+            for _ in 0..L {
+                deque.push_back(make_iovec(0, 100));
+            }
+            deque.pop_front(L);
+        }
+        // Start filling the 'second' page
+        for _ in 0..L {
+            deque.push_back(make_iovec(1, 100));
+        }
+        // Pop one element to trigger the pivot.
+        deque.pop_front(1);
+        // Now the slice will
+        assert_eq!(deque.as_slice(), vec![make_iovec(1, 100); L as usize - 1]);
+    }
 }