test(tap): Check return value of writev()

We are experiencing intermittent failures of test_write_iovec() due to
the rx packet not matching the ones written. To ensure the write has
been completed successfully, check the return value of writev() matches
the number of bytes that were supposed to be written.

Signed-off-by: Takahiro Itazuri <[email protected]>

Author: zulinx86

src/vmm/src/devices/virtio/net/tap.rs

@@ -306,7 +306,8 @@ pub mod tests {
             fragment3.as_slice(),
         ]);
 
-        tap.write_iovec(&scattered).unwrap();
+        let num_bytes = tap.write_iovec(&scattered).unwrap();
+        assert_eq!(num_bytes, scattered.len() as usize);
 
         let mut read_buf = vec![0u8; scattered.len() as usize];
         assert!(tap_traffic_simulator.pop_rx_packet(&mut read_buf));