[i2c] Avoid using expect()

Be consistent with the use of expect() and unwrap() and use only one of
them, if required. Also add a comment on why usage of unwrap() is safe
in main().

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

src/i2c/src/main.rs

@@ -182,6 +182,14 @@ fn start_backend<D: 'static + I2cDevice + Send + Sync>(config: I2cConfiguration)
         let i2c_map = i2c_map.clone();
 
         let handle = spawn(move || loop {
+            // A separate thread is spawned for each socket and can connect to a separate guest.
+            // These are run in an infinite loop to not require the daemon to be restarted once a
+            // guest exits.
+            //
+            // There isn't much value in complicating code here to return an error from the
+            // threads, and so the code uses unwrap() instead. The panic on a thread won't cause
+            // trouble to other threads/guests or the main() function and should be safe for the
+            // daemon.
             let backend = Arc::new(RwLock::new(
                 VhostUserI2cBackend::new(i2c_map.clone()).unwrap(),
             ));
@@ -211,12 +219,7 @@ fn start_backend<D: 'static + I2cDevice + Send + Sync>(config: I2cConfiguration)
             }
 
             // No matter the result, we need to shut down the worker thread.
-            backend
-                .read()
-                .unwrap()
-                .exit_event
-                .write(1)
-                .expect("Shutting down worker thread");
+            backend.read().unwrap().exit_event.write(1).unwrap();
         });
 
         handles.push(handle);

src/i2c/src/vhu_i2c.rs

@@ -51,6 +51,8 @@ pub enum Error {
     DescriptorWriteFailed,
     #[error("Failed to send notification")]
     NotificationFailed,
+    #[error("Failed to create new EventFd: {0:?}")]
+    EventFdFailed(std::io::Error),
 }
 
 impl convert::From<Error> for io::Error {
@@ -94,7 +96,7 @@ impl<D: I2cDevice> VhostUserI2cBackend<D> {
             i2c_map,
             event_idx: false,
             mem: None,
-            exit_event: EventFd::new(EFD_NONBLOCK).expect("Creating exit eventfd"),
+            exit_event: EventFd::new(EFD_NONBLOCK).map_err(Error::EventFdFailed)?,
         })
     }
 
@@ -295,7 +297,7 @@ impl<D: 'static + I2cDevice + Sync + Send> VhostUserBackendMut<VringRwLock, ()>
     }
 
     fn exit_event(&self, _thread_index: usize) -> Option<EventFd> {
-        Some(self.exit_event.try_clone().expect("Cloning exit eventfd"))
+        self.exit_event.try_clone().ok()
     }
 }