[vsock] try epoll_modify before epoll_register in recv_pkt

in this context, epoll listener can be already registered via other TX
event, so let it try epoll_modify first to avoid 'silent' failure which
possibly drops packets.

Signed-off-by: Jeongik Cha <jeongik@google.com>

vhost-device-vsock/src/vsock_conn.rs

@@ -174,11 +174,21 @@ impl<S: AsRawFd + Read + Write> VsockConnection<S> {
                         pkt.set_op(VSOCK_OP_RW).set_len(read_cnt as u32);
 
                         // Re-register the stream file descriptor for read and write events
-                        VhostUserVsockThread::epoll_register(
+                        if VhostUserVsockThread::epoll_modify(
                             self.epoll_fd,
                             self.stream.as_raw_fd(),
                             epoll::Events::EPOLLIN | epoll::Events::EPOLLOUT,
-                        )?;
+                        )
+                        .is_err()
+                        {
+                            if let Err(e) = VhostUserVsockThread::epoll_register(
+                                self.epoll_fd,
+                                self.stream.as_raw_fd(),
+                                epoll::Events::EPOLLIN | epoll::Events::EPOLLOUT,
+                            ) {
+                                info!("epoll_register failed: {:?}, but proceed further.", e);
+                            }
+                        };
                     }
 
                     // Update the rx_cnt with the amount of data in the vsock packet.
@@ -683,8 +693,7 @@ mod tests {
         conn_local.stream.write_all(b"hello").unwrap();
         conn_local.rx_queue.enqueue(RxOps::Rw);
         let op_zero_read = conn_local.recv_pkt(&mut pkt);
-        // below error due to epoll add
-        assert!(op_zero_read.is_err());
+        assert!(op_zero_read.is_ok());
         assert_eq!(pkt.op(), VSOCK_OP_RW);
         assert!(!conn_local.rx_queue.pending_rx());
         assert_eq!(pkt.len(), 5);