Add fallback inheritance for nested routers (#1521)

* fallback inheritance

* cleanup

* changelog

axum/CHANGELOG.md

@@ -7,8 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+- **fixed:** Nested routers will now inherit fallbacks from outer routers ([#1521])
 - **added:** Add `accept_unmasked_frames` setting in WebSocketUpgrade ([#1529])
 
+[#1521]: https://github.com/tokio-rs/axum/pull/1521
+
 # 0.6.0-rc.4 (9. November, 2022)
 
 - **changed**: The inner error of a `JsonRejection` is now

axum/src/docs/routing/nest.md

@@ -90,8 +90,8 @@ let app = Router::new()
 
 # Fallbacks
 
-When nesting a router, if a request matches the prefix but the nested router doesn't have a matching
-route, the outer fallback will _not_ be called:
+If a nested router doesn't have its own fallback then it will inherit the
+fallback from the outer router:
 
 ```rust
 use axum::{routing::get, http::StatusCode, handler::Handler, Router};
@@ -100,38 +100,43 @@ async fn fallback() -> (StatusCode, &'static str) {
     (StatusCode::NOT_FOUND, "Not Found")
 }
 
-let api_routes = Router::new().nest_service("/users", get(|| async {}));
+let api_routes = Router::new().route("/users", get(|| async {}));
 
 let app = Router::new()
     .nest("/api", api_routes)
     .fallback(fallback);
 # let _: Router = app;
 ```
 
-Here requests like `GET /api/not-found` will go into `api_routes` and then to
-the fallback of `api_routes` which will return an empty `404 Not Found`
-response. The outer fallback declared on `app` will _not_ be called.
+Here requests like `GET /api/not-found` will go into `api_routes` but because
+it doesn't have a matching route and doesn't have its own fallback it will call
+the fallback from the outer router, i.e. the `fallback` function.
 
-Think of nested services as swallowing requests that matches the prefix and
-not falling back to outer router even if they don't have a matching route.
-
-You can still add separate fallbacks to nested routers:
+If the nested router has its own fallback then the outer fallback will not be
+inherited:
 
 ```rust
-use axum::{routing::get, http::StatusCode, handler::Handler, Json, Router};
-use serde_json::{json, Value};
+use axum::{
+    routing::get,
+    http::StatusCode,
+    handler::Handler,
+    Json,
+    Router,
+};
 
 async fn fallback() -> (StatusCode, &'static str) {
     (StatusCode::NOT_FOUND, "Not Found")
 }
 
-async fn api_fallback() -> (StatusCode, Json<Value>) {
-    (StatusCode::NOT_FOUND, Json(json!({ "error": "Not Found" })))
+async fn api_fallback() -> (StatusCode, Json<serde_json::Value>) {
+    (
+        StatusCode::NOT_FOUND,
+        Json(serde_json::json!({ "status": "Not Found" })),
+    )
 }
 
 let api_routes = Router::new()
-    .nest_service("/users", get(|| async {}))
-    // add dedicated fallback for requests starting with `/api`
+    .route("/users", get(|| async {}))
     .fallback(api_fallback);
 
 let app = Router::new()
@@ -140,6 +145,8 @@ let app = Router::new()
 # let _: Router = app;
 ```
 
+Here requests like `GET /api/not-found` will go to `api_fallback`.
+
 # Panics
 
 - If the route overlaps with another route. See [`Router::route`]

axum/src/routing/method_routing.rs

@@ -1,6 +1,6 @@
 //! Route to services and handlers based on HTTP methods.
 
-use super::IntoMakeService;
+use super::{FallbackRoute, IntoMakeService};
 #[cfg(feature = "tokio")]
 use crate::extract::connect_info::IntoMakeServiceWithConnectInfo;
 use crate::{
@@ -744,7 +744,7 @@ where
             post: self.post.into_route(&state),
             put: self.put.into_route(&state),
             trace: self.trace.into_route(&state),
-            fallback: self.fallback.into_route(&state),
+            fallback: self.fallback.into_fallback_route(&state),
             allow_header: self.allow_header,
         }
     }
@@ -1284,7 +1284,7 @@ pub struct WithState<B, E> {
     post: Option<Route<B, E>>,
     put: Option<Route<B, E>>,
     trace: Option<Route<B, E>>,
-    fallback: Route<B, E>,
+    fallback: FallbackRoute<B, E>,
     allow_header: AllowHeader,
 }
 

axum/src/routing/mod.rs

@@ -8,7 +8,7 @@ use crate::{
     handler::{BoxedHandler, Handler},
     util::try_downcast,
 };
-use axum_core::response::IntoResponse;
+use axum_core::response::{IntoResponse, Response};
 use http::Request;
 use matchit::MatchError;
 use std::{
@@ -18,7 +18,10 @@ use std::{
     fmt,
     sync::Arc,
 };
-use tower::{util::MapResponseLayer, ServiceBuilder};
+use tower::{
+    util::{BoxCloneService, MapResponseLayer, Oneshot},
+    ServiceBuilder,
+};
 use tower_layer::Layer;
 use tower_service::Service;
 
@@ -639,11 +642,29 @@ where
         }
     }
 
-    fn into_route(self, state: &S) -> Route<B, E> {
+    fn into_fallback_route(self, state: &S) -> FallbackRoute<B, E> {
         match self {
-            Self::Default(route) => route,
-            Self::Service(route) => route,
-            Self::BoxedHandler(handler) => handler.into_route(state.clone()),
+            Self::Default(route) => FallbackRoute::Default(route),
+            Self::Service(route) => FallbackRoute::Service(route),
+            Self::BoxedHandler(handler) => {
+                FallbackRoute::Service(handler.into_route(state.clone()))
+            }
+        }
+    }
+
+    fn map<F, B2, E2>(self, f: F) -> Fallback<S, B2, E2>
+    where
+        S: 'static,
+        B: 'static,
+        E: 'static,
+        F: FnOnce(Route<B, E>) -> Route<B2, E2> + Clone + Send + 'static,
+        B2: 'static,
+        E2: 'static,
+    {
+        match self {
+            Self::Default(route) => Fallback::Default(f(route)),
+            Self::Service(route) => Fallback::Service(f(route)),
+            Self::BoxedHandler(handler) => Fallback::BoxedHandler(handler.map(f)),
         }
     }
 }
@@ -668,20 +689,38 @@ impl<S, B, E> fmt::Debug for Fallback<S, B, E> {
     }
 }
 
-impl<S, B, E> Fallback<S, B, E> {
-    fn map<F, B2, E2>(self, f: F) -> Fallback<S, B2, E2>
-    where
-        S: 'static,
-        B: 'static,
-        E: 'static,
-        F: FnOnce(Route<B, E>) -> Route<B2, E2> + Clone + Send + 'static,
-        B2: 'static,
-        E2: 'static,
-    {
+/// Like `Fallback` but without the `S` param so it can be stored in `RouterService`
+pub(crate) enum FallbackRoute<B, E = Infallible> {
+    Default(Route<B, E>),
+    Service(Route<B, E>),
+}
+
+impl<B, E> fmt::Debug for FallbackRoute<B, E> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::Default(inner) => f.debug_tuple("Default").field(inner).finish(),
+            Self::Service(inner) => f.debug_tuple("Service").field(inner).finish(),
+        }
+    }
+}
+
+impl<B, E> Clone for FallbackRoute<B, E> {
+    fn clone(&self) -> Self {
+        match self {
+            Self::Default(inner) => Self::Default(inner.clone()),
+            Self::Service(inner) => Self::Service(inner.clone()),
+        }
+    }
+}
+
+impl<B, E> FallbackRoute<B, E> {
+    pub(crate) fn oneshot_inner(
+        &mut self,
+        req: Request<B>,
+    ) -> Oneshot<BoxCloneService<Request<B>, Response, E>, Request<B>> {
         match self {
-            Self::Default(inner) => Fallback::Default(f(inner)),
-            Self::Service(inner) => Fallback::Service(f(inner)),
-            Self::BoxedHandler(inner) => Fallback::BoxedHandler(inner.map(f)),
+            FallbackRoute::Default(inner) => inner.oneshot_inner(req),
+            FallbackRoute::Service(inner) => inner.oneshot_inner(req),
         }
     }
 }

axum/src/routing/service.rs

@@ -1,4 +1,6 @@
-use super::{future::RouteFuture, url_params, Endpoint, Node, Route, RouteId, Router};
+use super::{
+    future::RouteFuture, url_params, Endpoint, FallbackRoute, Node, Route, RouteId, Router,
+};
 use crate::{
     body::{Body, HttpBody},
     response::Response,
@@ -11,14 +13,15 @@ use std::{
     sync::Arc,
     task::{Context, Poll},
 };
+use sync_wrapper::SyncWrapper;
 use tower::Service;
 
 /// A [`Router`] converted into a [`Service`].
 #[derive(Debug)]
 pub struct RouterService<B = Body> {
     routes: HashMap<RouteId, Route<B>>,
     node: Arc<Node>,
-    fallback: Route<B>,
+    fallback: FallbackRoute<B>,
 }
 
 impl<B> RouterService<B>
@@ -52,7 +55,7 @@ where
         Self {
             routes,
             node: router.node,
-            fallback: router.fallback.into_route(&state),
+            fallback: router.fallback.into_fallback_route(&state),
         }
     }
 
@@ -121,12 +124,35 @@ where
         let path = req.uri().path().to_owned();
 
         match self.node.at(&path) {
-            Ok(match_) => self.call_route(match_, req),
+            Ok(match_) => {
+                match &self.fallback {
+                    FallbackRoute::Default(_) => {}
+                    FallbackRoute::Service(fallback) => {
+                        req.extensions_mut()
+                            .insert(SuperFallback(SyncWrapper::new(fallback.clone())));
+                    }
+                }
+
+                self.call_route(match_, req)
+            }
             Err(
                 MatchError::NotFound
                 | MatchError::ExtraTrailingSlash
                 | MatchError::MissingTrailingSlash,
-            ) => self.fallback.clone().call(req),
+            ) => match &mut self.fallback {
+                FallbackRoute::Default(fallback) => {
+                    if let Some(super_fallback) = req.extensions_mut().remove::<SuperFallback<B>>()
+                    {
+                        let mut super_fallback = super_fallback.0.into_inner();
+                        super_fallback.call(req)
+                    } else {
+                        fallback.call(req)
+                    }
+                }
+                FallbackRoute::Service(fallback) => fallback.call(req),
+            },
         }
     }
 }
+
+struct SuperFallback<B>(SyncWrapper<Route<B>>);