add test for when read count exceeds total size

We cannot mark the returned usize as must use unfortunately. This
is what triggered the bug in linux-loader:
rust-vmm/linux-loader#125. The least we can
do is to validate that when there are more bytes requested than what
the reader has available, we do return the correct number of the read.
Unfortunately, when this result is not used it can lead to bugs.

Signed-off-by: Andreea Florescu <fandree@amazon.com>

src/volatile_memory.rs

@@ -1375,6 +1375,8 @@ mod tests {
     use super::*;
 
     use std::fs::File;
+    use std::io::Cursor;
+    use std::mem;
     use std::mem::size_of_val;
     use std::path::Path;
     use std::sync::atomic::{AtomicUsize, Ordering};
@@ -1905,6 +1907,30 @@ mod tests {
         assert_eq!(buf, sample_buf);
     }
 
+    #[test]
+    fn test_read_from_exceeds_size() {
+        #[derive(Debug, Default, Copy, Clone)]
+        struct BytesToRead {
+            _val1: u128, // 16 bytes
+            _val2: u128, // 16 bytes
+        }
+        unsafe impl ByteValued for BytesToRead {}
+        let cursor_size = 20;
+        let mut image = Cursor::new(vec![1u8; cursor_size]);
+
+        // Trying to read more bytes than we have available in the cursor should
+        // make the read_from function return maximum cursor size (i.e. 20).
+        let mut bytes_to_read = BytesToRead::default();
+        let size_of_bytes = mem::size_of_val(&bytes_to_read);
+        assert_eq!(
+            bytes_to_read
+                .as_bytes()
+                .read_from(0, &mut image, size_of_bytes)
+                .unwrap(),
+            cursor_size
+        );
+    }
+
     #[test]
     fn ref_array_from_slice() {
         let mut a = [2, 4, 6, 8, 10];