marionette: Handle unexpected disconnects

If the Marionette server closes the connection unexpectedly, it will
manifest as an empty message from the socket.  We need to handle this
and return an error, instead of causing a panic by attempting to read
from the empty buffer (by trying to create a slice with a negative
length).

src/marionette/error.rs

@@ -8,6 +8,7 @@ pub enum MessageError {
     Io(std::io::Error),
     Parse(ParseIntError),
     Utf8(Utf8Error),
+    Disconnected,
 }
 
 impl From<std::io::Error> for MessageError {
@@ -34,6 +35,7 @@ impl std::fmt::Display for MessageError {
             Self::Io(e) => write!(f, "I/O error: {}", e),
             Self::Parse(e) => write!(f, "Error parsing message: {}", e),
             Self::Utf8(e) => write!(f, "Error parsing message: {}", e),
+            Self::Disconnected => write!(f, "Disconnected"),
         }
     }
 }
@@ -44,6 +46,7 @@ impl std::error::Error for MessageError {
             Self::Io(e) => Some(e),
             Self::Parse(e) => Some(e),
             Self::Utf8(e) => Some(e),
+            Self::Disconnected => None,
         }
     }
 }

src/marionette/mod.rs

@@ -150,6 +150,9 @@ impl MarionetteConnection {
     {
         let mut buf = vec![];
         stream.read_until(b':', &mut buf).await?;
+        if buf.is_empty() {
+            return Err(MessageError::Disconnected);
+        }
         let length: usize =
             std::str::from_utf8(&buf[..buf.len() - 1])?.parse()?;
         trace!("Message length: {:?}", length);