vsock: add handshake regression test

This patch has been cherry-picked from the Firecracker tree. The
reference commit is 6dbe8e021a64ba3742081741a7538cdfd93a102e.

Signed-off-by: Gabriel Ionescu <gbi@amazon.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

vm-virtio/src/vsock/csm/connection.rs

@@ -764,6 +764,16 @@ mod tests {
         }
     }
 
+    impl<S> VsockConnection<S>
+    where
+        S: Read + Write + AsRawFd,
+    {
+        /// Get the fwd_cnt value from the connection.
+        pub(crate) fn fwd_cnt(&self) -> Wrapping<u32> {
+            self.fwd_cnt
+        }
+    }
+
     fn init_pkt(pkt: &mut VsockPacket, op: u16, len: u32) -> &mut VsockPacket {
         for b in pkt.hdr_mut() {
             *b = 0;

vm-virtio/src/vsock/unix/muxer.rs

@@ -1356,4 +1356,26 @@ mod tests {
 
         assert!(!ctx.muxer.has_pending_rx());
     }
+
+    #[test]
+    fn test_regression_handshake() {
+        // Address one of the issues found while fixing the following issue:
+        // https://github.com/firecracker-microvm/firecracker/issues/1751
+        // This test checks that the handshake message is not accounted for
+        let mut ctx = MuxerTestContext::new("regression_handshake");
+        let peer_port = 1025;
+
+        // Create a local connection.
+        let (_, local_port) = ctx.local_connect(peer_port);
+
+        // Get the connection from the connection map.
+        let key = ConnMapKey {
+            local_port,
+            peer_port,
+        };
+        let conn = ctx.muxer.conn_map.get_mut(&key).unwrap();
+
+        // Check that fwd_cnt is 0 - "OK ..." was not accounted for.
+        assert_eq!(conn.fwd_cnt().0, 0);
+    }
 }