ServeDir: convert io::ErrorKind::NotADirectory to not_found (#331)

* ServeDir: convert io::ErrorKind::NotADirectory to not_found

* only check raw OS error on unix OS, add comment.

* mention "is a directory" handling in ServeDir in documentation

tower-http/src/services/fs/serve_dir/future.rs

@@ -124,8 +124,19 @@ where
                     }
 
                     Err(err) => {
-                        if let io::ErrorKind::NotFound | io::ErrorKind::PermissionDenied =
-                            err.kind()
+                        #[cfg(unix)]
+                        // 20 = libc::ENOTDIR => "not a directory
+                        // when `io_error_more` landed, this can be changed
+                        // to checking for `io::ErrorKind::NotADirectory`.
+                        // https://github.com/rust-lang/rust/issues/86442
+                        let error_is_not_a_directory = err.raw_os_error() == Some(20);
+                        #[cfg(not(unix))]
+                        let error_is_not_a_directory = false;
+
+                        if matches!(
+                            err.kind(),
+                            io::ErrorKind::NotFound | io::ErrorKind::PermissionDenied
+                        ) || error_is_not_a_directory
                         {
                             if let Some((mut fallback, request)) = fallback_and_request.take() {
                                 call_fallback(&mut fallback, request)

tower-http/src/services/fs/serve_dir/mod.rs

@@ -35,6 +35,8 @@ const DEFAULT_CAPACITY: usize = 65536;
 /// - The file doesn't exist
 /// - Any segment of the path contains `..`
 /// - Any segment of the path contains a backslash
+/// - On unix, any segment of the path referenced as directory is actually an
+///   existing file (`/file.html/something`)
 /// - We don't have necessary permissions to read the file
 ///
 /// # Example

tower-http/src/services/fs/serve_dir/tests.rs

@@ -289,6 +289,27 @@ async fn not_found() {
     assert!(body.is_empty());
 }
 
+#[cfg(unix)]
+#[tokio::test]
+async fn not_found_when_not_a_directory() {
+    let svc = ServeDir::new("../test-files");
+
+    // `index.html` is a file, and we are trying to request
+    // it as a directory.
+    let req = Request::builder()
+        .uri("/index.html/some_file")
+        .body(Body::empty())
+        .unwrap();
+    let res = svc.oneshot(req).await.unwrap();
+
+    // This should lead to a 404
+    assert_eq!(res.status(), StatusCode::NOT_FOUND);
+    assert!(res.headers().get(header::CONTENT_TYPE).is_none());
+
+    let body = body_into_text(res.into_body()).await;
+    assert!(body.is_empty());
+}
+
 #[tokio::test]
 async fn not_found_precompressed() {
     let svc = ServeDir::new("../test-files").precompressed_gzip();