io: do not treat zero-length reads as EOF in Chain (#8251)

tokio/src/io/util/chain.rs

@@ -94,7 +94,9 @@ where
         if !*me.done_first {
             let rem = buf.remaining();
             ready!(me.first.poll_read(cx, buf))?;
-            if buf.remaining() == rem {
+            // A read that fills nothing only indicates EOF if the buffer
+            // had capacity to begin with.
+            if buf.remaining() == rem && rem != 0 {
                 *me.done_first = true;
             } else {
                 return Poll::Ready(Ok(()));

tokio/tests/io_chain.rs

@@ -14,3 +14,15 @@ async fn chain() {
     assert_ok!(rd.read_to_end(&mut buf).await);
     assert_eq!(buf, b"hello world");
 }
+
+#[tokio::test]
+async fn empty_read_does_not_skip_first() {
+    let mut buf = Vec::new();
+    let rd1: &[u8] = b"hello ";
+    let rd2: &[u8] = b"world";
+
+    let mut rd = rd1.chain(rd2);
+    assert_eq!(assert_ok!(rd.read(&mut []).await), 0);
+    assert_ok!(rd.read_to_end(&mut buf).await);
+    assert_eq!(buf, b"hello world");
+}