Rewrite test_buffer_wraparound_tx() by manually prefilling the buffer.

This is still testing the same case which would be masked away by the
upcoming contiguous space optimization (which is important for
PacketBuffers and still helpful to TcpSockets).

Author: astro

src/socket/tcp.rs

@@ -3656,33 +3656,25 @@ mod test {
     #[test]
     fn test_buffer_wraparound_tx() {
         let mut s = socket_established();
-        s.tx_buffer = SocketBuffer::new(vec![0; 6]);
-        assert_eq!(s.send_slice(b"abc"), Ok(3));
+        s.tx_buffer = SocketBuffer::new(vec![b'.'; 9]);
+        assert_eq!(s.tx_buffer.enqueue_many(6).len(), 6);
+        assert_eq!(s.tx_buffer.dequeue_many(3).len(), 3);
+        assert_eq!(s.tx_buffer.len(), 3);
+
+        // "abcdef" not contiguous in tx buffer
+        assert_eq!(s.send_slice(b"abcdef"), Ok(6));
         recv!(s, Ok(TcpRepr {
             seq_number: LOCAL_SEQ + 1,
             ack_number: Some(REMOTE_SEQ + 1),
-            payload:    &b"abc"[..],
+            payload:    &b"...abc"[..],
             ..RECV_TEMPL
         }));
-        send!(s, TcpRepr {
-            seq_number: REMOTE_SEQ + 1,
-            ack_number: Some(LOCAL_SEQ + 1 + 3),
-            ..SEND_TEMPL
-        });
-        assert_eq!(s.send_slice(b"defghi"), Ok(6));
         recv!(s, Ok(TcpRepr {
-            seq_number: LOCAL_SEQ + 1 + 3,
+            seq_number: LOCAL_SEQ + 1 + 6,
             ack_number: Some(REMOTE_SEQ + 1),
             payload:    &b"def"[..],
             ..RECV_TEMPL
         }));
-        // "defghi" not contiguous in tx buffer
-        recv!(s, Ok(TcpRepr {
-            seq_number: LOCAL_SEQ + 1 + 3 + 3,
-            ack_number: Some(REMOTE_SEQ + 1),
-            payload:    &b"ghi"[..],
-            ..RECV_TEMPL
-        }));
     }
 
     // =========================================================================================//