devices: console: simplify handle_input()

Instead of handling special conditions by setting a variable and
breaking from the loop into the error handling code, just return the
right error code directly.

BUG=None
TEST=serial console input works.

Change-Id: I02c9b8fd4bc64e4e0f16308d65cd05ace41c5756
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3565299
Reviewed-by: Junichi Uekawa <uekawa@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Keiichi Watanabe <keiichiw@chromium.org>
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>

devices/src/virtio/console.rs

@@ -63,23 +63,17 @@ pub fn handle_input<I: SignalableInterrupt>(
     buffer: &mut VecDeque<u8>,
     receive_queue: &mut Queue,
 ) -> result::Result<(), ConsoleError> {
-    let mut exhausted_queue = false;
-
     loop {
-        let desc = match receive_queue.peek(mem) {
-            Some(d) => d,
-            None => {
-                exhausted_queue = true;
-                break;
-            }
-        };
+        let desc = receive_queue
+            .peek(mem)
+            .ok_or(ConsoleError::RxDescriptorsExhausted)?;
         let desc_index = desc.index;
         // TODO(morg): Handle extra error cases as Err(ConsoleError) instead of just returning.
         let mut writer = match Writer::new(mem.clone(), desc) {
             Ok(w) => w,
             Err(e) => {
                 error!("console: failed to create Writer: {}", e);
-                break;
+                return Ok(());
             }
         };
 
@@ -103,15 +97,9 @@ pub fn handle_input<I: SignalableInterrupt>(
         }
 
         if bytes_written == 0 {
-            break;
+            return Ok(());
         }
     }
-
-    if exhausted_queue {
-        Err(ConsoleError::RxDescriptorsExhausted)
-    } else {
-        Ok(())
-    }
 }
 
 /// Processes the data taken from the given transmit queue into the output sink.