axum-core: fix nested error downcast (#3858)

axum-core/src/extract/rejection.rs

@@ -19,17 +19,19 @@ impl FailedToBufferBody {
     where
         E: Into<BoxError>,
     {
-        // two layers of boxes here because `with_limited_body`
-        // wraps the `http_body_util::Limited` in an `axum_core::Body`
-        // which also wraps the error type
-        let box_error = match err.into().downcast::<Error>() {
-            Ok(err) => err.into_inner(),
-            Err(err) => err,
-        };
-        let box_error = match box_error.downcast::<Error>() {
-            Ok(err) => err.into_inner(),
-            Err(err) => err,
-        };
+        // peel any number of `axum_core::Error` wrappers to reach the underlying source error
+        let mut box_error = err.into();
+        loop {
+            match box_error.downcast::<Error>() {
+                Ok(unwrapped) => {
+                    box_error = unwrapped.into_inner();
+                }
+                Err(err) => {
+                    box_error = err;
+                    break;
+                }
+            }
+        }
         match box_error.downcast::<http_body_util::LengthLimitError>() {
             Ok(err) => Self::LengthLimitError(LengthLimitError::from_err(err)),
             Err(err) => Self::UnknownBodyError(UnknownBodyError::from_err(err)),

axum/src/routing/tests/mod.rs

@@ -1352,3 +1352,28 @@ async fn middleware_adding_body() {
 
     assert_eq!(res.text().await, "…");
 }
+
+#[crate::test]
+async fn nested_body_limit_rejection() {
+    use crate::{
+        extract::Request,
+        middleware::{self, Next},
+        response::Response,
+    };
+    use axum_core::RequestExt;
+
+    async fn limit_middleware(req: Request, next: Next) -> Response {
+        let req = req.with_limited_body();
+        next.run(req).await
+    }
+
+    let app = Router::new()
+        .route("/", post(|_: Bytes| async {}))
+        .layer(middleware::from_fn(limit_middleware))
+        .layer(middleware::from_fn(limit_middleware))
+        .layer(DefaultBodyLimit::max(2));
+
+    let client = TestClient::new(app);
+    let res = client.post("/").body("123").await;
+    assert_eq!(res.status(), StatusCode::PAYLOAD_TOO_LARGE);
+}