io: report error on zero-write in write_int (#2334)

* tokio-io: make write_i* same behavior as write_all when poll_write returns Ok(0)

Fixes: #2329

Co-authored-by: lishuo <lishuo.03@bytedance.com>

tokio/src/io/util/write_int.rs

@@ -56,6 +56,9 @@ macro_rules! writer {
                     {
                         Poll::Pending => return Poll::Pending,
                         Poll::Ready(Err(e)) => return Poll::Ready(Err(e.into())),
+                        Poll::Ready(Ok(0)) => {
+                            return Poll::Ready(Err(io::ErrorKind::WriteZero.into()));
+                        }
                         Poll::Ready(Ok(n)) => n as u8,
                     };
                 }
@@ -96,7 +99,7 @@ macro_rules! writer8 {
                 match me.dst.poll_write(cx, &buf[..]) {
                     Poll::Pending => Poll::Pending,
                     Poll::Ready(Err(e)) => Poll::Ready(Err(e.into())),
-                    Poll::Ready(Ok(0)) => Poll::Pending,
+                    Poll::Ready(Ok(0)) => Poll::Ready(Err(io::ErrorKind::WriteZero.into())),
                     Poll::Ready(Ok(1)) => Poll::Ready(Ok(())),
                     Poll::Ready(Ok(_)) => unreachable!(),
                 }

tokio/tests/io_write_int.rs

+#![warn(rust_2018_idioms)]
+#![cfg(feature = "full")]
+
+use tokio::io::{AsyncWrite, AsyncWriteExt};
+
+use std::io;
+use std::pin::Pin;
+use std::task::{Context, Poll};
+
+#[tokio::test]
+async fn write_int_should_err_if_write_count_0() {
+    struct Wr {}
+
+    impl AsyncWrite for Wr {
+        fn poll_write(
+            self: Pin<&mut Self>,
+            _cx: &mut Context<'_>,
+            _buf: &[u8],
+        ) -> Poll<io::Result<usize>> {
+            Ok(0).into()
+        }
+
+        fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Ok(()).into()
+        }
+
+        fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<io::Result<()>> {
+            Ok(()).into()
+        }
+    }
+
+    let mut wr = Wr {};
+
+    // should be ok just to test these 2, other cases actually expanded by same macro.
+    assert!(wr.write_i8(0).await.is_err());
+    assert!(wr.write_i32(12).await.is_err());
+}