vsock: avoid circular references

We have the following circular references found by Li Zebin:
    VhostUserBackend ==> VhostUserVsockThread ==> VringEpollHandler

In addition to causing a resource leak, this causes also an error
after we merged commit 38caab2 ("vsock: Don't allow duplicate CIDs").
When the VM reboot or shutdown, the application exits with the
following error:

    [ERROR vhost_device_vsock] Could not create backend:
        CID already in use by another vsock device

This happened because we have these circular references and
VhostUserVsockThread::drop() is never invoked. So, we don't remove
the cid from the map.

Let's fix this problem by simply removing the reference to
VringEpollHandler from VhostUserVsockThread. In fact, we do not
need to keep the reference for the lifetime of VhostUserVsockThread,
as we only need to add the handlers once.

Let's also rename the fields to follow the current VhostUserDaemon
API.

Closes #438

Reported-by: Li Zebin <cutelizebin@gmail.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

vhost-device-vsock/src/main.rs

@@ -234,13 +234,13 @@ pub(crate) fn start_backend_server(
         )
         .map_err(BackendError::CouldNotCreateDaemon)?;
 
-        let mut vring_workers = daemon.get_epoll_handlers();
+        let mut epoll_handlers = daemon.get_epoll_handlers();
 
         for thread in backend.threads.iter() {
             thread
                 .lock()
                 .unwrap()
-                .set_vring_worker(Some(vring_workers.remove(0)));
+                .register_listeners(epoll_handlers.remove(0));
         }
 
         daemon.start(listener).unwrap();

vhost-device-vsock/src/vhu_vsock_thread.rs

@@ -63,8 +63,6 @@ pub(crate) struct VhostUserVsockThread {
     host_sock_path: String,
     /// Listener listening for new connections on the host.
     host_listener: UnixListener,
-    /// Instance of VringWorker.
-    vring_worker: Option<Arc<VringEpollHandler<ArcVhostBknd, VringRwLock, ()>>>,
     /// epoll fd to which new host connections are added.
     epoll_file: File,
     /// VsockThreadBackend instance.
@@ -151,7 +149,6 @@ impl VhostUserVsockThread {
             host_sock: host_sock.as_raw_fd(),
             host_sock_path: uds_path,
             host_listener: host_sock,
-            vring_worker: None,
             epoll_file,
             thread_backend,
             guest_cid,
@@ -242,20 +239,15 @@ impl VhostUserVsockThread {
         self.epoll_file.as_raw_fd()
     }
 
-    /// Set self's VringWorker.
-    pub fn set_vring_worker(
+    /// Register our listeners in the VringEpollHandler
+    pub fn register_listeners(
         &mut self,
-        vring_worker: Option<Arc<VringEpollHandler<ArcVhostBknd, VringRwLock, ()>>>,
+        epoll_handler: Arc<VringEpollHandler<ArcVhostBknd, VringRwLock, ()>>,
     ) {
-        self.vring_worker = vring_worker;
-        self.vring_worker
-            .as_ref()
-            .unwrap()
+        epoll_handler
             .register_listener(self.get_epoll_fd(), EventSet::IN, u64::from(BACKEND_EVENT))
             .unwrap();
-        self.vring_worker
-            .as_ref()
-            .unwrap()
+        epoll_handler
             .register_listener(
                 self.sibling_event_fd.as_raw_fd(),
                 EventSet::IN,