vhost_user_backend: Add client mode support

Introduce some new code to support running a vhost-user backend as a
client instead of the default server mode.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

vhost_user_backend/src/lib.rs

@@ -20,6 +20,7 @@ use vhost::vhost_user::message::{
     VhostUserSingleMemoryRegion, VhostUserVirtioFeatures, VhostUserVringAddrFlags,
     VhostUserVringState,
 };
+use vhost::vhost_user::SlaveReqHandler;
 use vhost::vhost_user::{
     Error as VhostUserError, Listener, Result as VhostUserResult, SlaveFsCacheReq, SlaveListener,
     VhostUserSlaveReqHandlerMut,
@@ -153,7 +154,7 @@ impl<S: VhostUserBackend> VhostUserDaemon<S> {
         })
     }
 
-    /// Connect to the vhost-user socket and run a dedicated thread handling
+    /// Listen to the vhost-user socket and run a dedicated thread handling
     /// all requests coming through this socket. This runs in an infinite loop
     /// that should be terminating once the other end of the socket (the VMM)
     /// disconnects.
@@ -178,6 +179,27 @@ impl<S: VhostUserBackend> VhostUserDaemon<S> {
         Ok(())
     }
 
+    /// Connect to the vhost-user socket and run a dedicated thread handling
+    /// all requests coming through this socket. This runs in an infinite loop
+    /// that should be terminating once the other end of the socket (the VMM)
+    /// hangs up.
+    pub fn start_client(&mut self, socket_path: &str) -> Result<()> {
+        let mut slave_handler = SlaveReqHandler::connect(socket_path, self.handler.clone())
+            .map_err(Error::CreateSlaveReqHandler)?;
+        let handle = thread::Builder::new()
+            .name(self.name.clone())
+            .spawn(move || loop {
+                slave_handler
+                    .handle_request()
+                    .map_err(Error::HandleRequest)?;
+            })
+            .map_err(Error::StartDaemon)?;
+
+        self.main_thread = Some(handle);
+
+        Ok(())
+    }
+
     /// Wait for the thread handling the vhost-user socket connection to
     /// terminate.
     pub fn wait(&mut self) -> Result<()> {