Fix non-terminating loop in tokio_io::length_delimited::FramedWrite (#576)

* tokio-io: fix non-terminating loop in length_delimited::FramedWrite (#497)

tests/length_delimited.rs

@@ -473,6 +473,16 @@ fn write_update_max_frame_len_in_flight() {
     assert!(io.get_ref().calls.is_empty());
 }
 
+#[test]
+fn write_zero() {
+    let mut io = length_delimited::Builder::new()
+        .new_write(mock! { });
+
+    assert!(io.start_send(Bytes::from("abcdef")).unwrap().is_ready());
+    assert_eq!(io.poll_complete().unwrap_err().kind(), io::ErrorKind::WriteZero);
+    assert!(io.get_ref().calls.is_empty());
+}
+
 // ===== Test utils =====
 
 fn would_block() -> io::Error {

tokio-io/src/length_delimited.rs

@@ -455,7 +455,12 @@ impl<T: AsyncWrite, B: IntoBuf> FramedWrite<T, B> {
 
         loop {
             let frame = self.frame.as_mut().unwrap();
-            try_ready!(self.inner.write_buf(frame));
+            if try_ready!(self.inner.write_buf(frame)) == 0 {
+                return Err(io::Error::new(
+                    io::ErrorKind::WriteZero,
+                    "failed to write frame to transport",
+                ));
+            }
 
             if !frame.has_remaining() {
                 break;

tokio-io/tests/length_delimited.rs

@@ -433,6 +433,16 @@ fn write_max_frame_len() {
     assert!(io.get_ref().calls.is_empty());
 }
 
+#[test]
+fn write_zero() {
+    let mut io = Builder::new()
+        .new_write(mock! { });
+
+    assert!(io.start_send("abcdef").unwrap().is_ready());
+    assert_eq!(io.poll_complete().unwrap_err().kind(), io::ErrorKind::WriteZero);
+    assert!(io.get_ref().calls.is_empty());
+}
+
 #[test]
 fn write_update_max_frame_len_at_rest() {
     let mut io = Builder::new()