fs: flush on shutdown (#3009)

Fixes: #2950

tokio/src/fs/file.rs

@@ -682,9 +682,8 @@ impl AsyncWrite for File {
         inner.poll_flush(cx)
     }
 
-    fn poll_shutdown(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
-        // Flush is a noop for files so it's fine not to call it.
-        Poll::Ready(Ok(()))
+    fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), io::Error>> {
+        self.poll_flush(cx)
     }
 }
 

tokio/tests/fs_file.rs

@@ -37,6 +37,19 @@ async fn basic_write() {
     assert_eq!(file, HELLO);
 }
 
+#[tokio::test]
+async fn basic_write_and_shutdown() {
+    let tempfile = tempfile();
+
+    let mut file = File::create(tempfile.path()).await.unwrap();
+
+    file.write_all(HELLO).await.unwrap();
+    file.shutdown().await.unwrap();
+
+    let file = std::fs::read(tempfile.path()).unwrap();
+    assert_eq!(file, HELLO);
+}
+
 #[tokio::test]
 async fn coop() {
     let mut tempfile = tempfile();