util: Adds a BoxCloneServiceLayer

tower/src/util/boxed/layer_clone.rs

@@ -0,0 +1,128 @@
+use crate::util::BoxCloneService;
+use std::{fmt, sync::Arc};
+use tower_layer::{layer_fn, Layer};
+use tower_service::Service;
+
+/// A [`Clone`] + [`Send`] boxed [`Layer`].
+///
+/// [`BoxCloneServiceLayer`] turns a layer into a trait object, allowing both the [`Layer`] itself
+/// and the output [`Service`] to be dynamic, while having consistent types.
+///
+/// This [`Layer`] produces [`BoxCloneService`] instances erasing the type of the
+/// [`Service`] produced by the wrapped [`Layer`].
+///
+/// This is similar to [`BoxLayer`](super::BoxLayer) except the layer and resulting
+/// service implements [`Clone`].
+///
+/// # Example
+///
+/// `BoxCloneServiceLayer` can, for example, be useful to create layers dynamically that otherwise wouldn't have
+/// the same types, when the underlying service must be clone (for example, when building a MakeService)
+/// In this example, we include a [`Timeout`] layer only if an environment variable is set. We can use
+/// `BoxCloneService` to return a consistent type regardless of runtime configuration:
+///
+/// ```
+/// use std::time::Duration;
+/// use tower::{Service, ServiceBuilder, BoxError};
+/// use tower::util::{BoxCloneServiceLayer, BoxCloneService};
+///
+/// #
+/// # struct Request;
+/// # struct Response;
+/// # impl Response {
+/// #     fn new() -> Self { Self }
+/// # }
+///
+/// fn common_layer<S, T>() -> BoxCloneServiceLayer<S, T, S::Response, BoxError>
+/// where
+///     S: Service<T> + Clone + Send + 'static,
+///     S::Future: Send + 'static,
+///     S::Error: Into<BoxError> + 'static,
+/// {
+///     let builder = ServiceBuilder::new()
+///         .concurrency_limit(100);
+///
+///     if std::env::var("SET_TIMEOUT").is_ok() {
+///         let layer = builder
+///             .timeout(Duration::from_secs(30))
+///             .into_inner();
+///
+///         BoxCloneServiceLayer::new(layer)
+///     } else {
+///         let layer = builder
+///             .map_err(Into::into)
+///             .into_inner();
+///
+///         BoxCloneServiceLayer::new(layer)
+///     }
+/// }
+///
+/// // We can clone the layer (this is true of BoxLayer as well)
+/// let boxed_clone_layer = common_layer();
+///
+/// let cloned_layer = boxed_clone_layer.clone();
+///
+/// // Using the `BoxCloneServiceLayer` we can create a `BoxCloneService`
+/// let service: BoxCloneService<Request, Response, BoxError> = ServiceBuilder::new().layer(boxed_clone_layer)
+///      .service_fn(|req: Request| async {
+///         Ok::<_, BoxError>(Response::new())
+///     });
+///
+/// # let service = assert_service(service);
+///
+/// // And we can still clone the service
+/// let cloned_service = service.clone();
+/// #
+/// # fn assert_service<S, R>(svc: S) -> S
+/// # where S: Service<R> { svc }
+///
+/// ```
+///
+/// [`Layer`]: tower_layer::Layer
+/// [`Service`]: tower_service::Service
+/// [`BoxService`]: super::BoxService
+/// [`Timeout`]: crate::timeout
+pub struct BoxCloneServiceLayer<In, T, U, E> {
+    boxed: Arc<dyn Layer<In, Service = BoxCloneService<T, U, E>> + Send + Sync + 'static>,
+}
+
+impl<In, T, U, E> BoxCloneServiceLayer<In, T, U, E> {
+    /// Create a new [`BoxCloneServiceLayer`].
+    pub fn new<L>(inner_layer: L) -> Self
+    where
+        L: Layer<In> + Send + Sync + 'static,
+        L::Service: Service<T, Response = U, Error = E> + Send + Clone + 'static,
+        <L::Service as Service<T>>::Future: Send + 'static,
+    {
+        let layer = layer_fn(move |inner: In| {
+            let out = inner_layer.layer(inner);
+            BoxCloneService::new(out)
+        });
+
+        Self {
+            boxed: Arc::new(layer),
+        }
+    }
+}
+
+impl<In, T, U, E> Layer<In> for BoxCloneServiceLayer<In, T, U, E> {
+    type Service = BoxCloneService<T, U, E>;
+
+    fn layer(&self, inner: In) -> Self::Service {
+        self.boxed.layer(inner)
+    }
+}
+
+impl<In, T, U, E> Clone for BoxCloneServiceLayer<In, T, U, E> {
+    fn clone(&self) -> Self {
+        Self {
+            boxed: Arc::clone(&self.boxed),
+        }
+    }
+}
+
+impl<In, T, U, E> fmt::Debug for BoxCloneServiceLayer<In, T, U, E> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("BoxCloneServiceLayer").finish()
+    }
+}

tower/src/util/boxed/mod.rs

@@ -1,6 +1,9 @@
 mod layer;
+mod layer_clone;
 mod sync;
 mod unsync;
 
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
-pub use self::{layer::BoxLayer, sync::BoxService, unsync::UnsyncBoxService};
+pub use self::{
+    layer::BoxLayer, layer_clone::BoxCloneServiceLayer, sync::BoxService, unsync::UnsyncBoxService,
+};

tower/src/util/mod.rs

@@ -23,7 +23,7 @@ pub mod rng;
 
 pub use self::{
     and_then::{AndThen, AndThenLayer},
-    boxed::{BoxLayer, BoxService, UnsyncBoxService},
+    boxed::{BoxCloneServiceLayer, BoxLayer, BoxService, UnsyncBoxService},
     boxed_clone::BoxCloneService,
     either::Either,
     future_service::{future_service, FutureService},