Add util::BoxCloneSyncService (#777)

Closes #770

tower/src/util/boxed_clone_sync.rs

@@ -0,0 +1,101 @@
+use super::ServiceExt;
+use futures_util::future::BoxFuture;
+use std::{
+    fmt,
+    task::{Context, Poll},
+};
+use tower_layer::{layer_fn, LayerFn};
+use tower_service::Service;
+
+/// A [`Clone`] + [`Send`] + [`Sync`] boxed [`Service`].
+///
+/// [`BoxCloneSyncService`] turns a service into a trait object, allowing the
+/// response future type to be dynamic, and allowing the service to be cloned and shared.
+///
+/// This is similar to [`BoxCloneService`](super::BoxCloneService) except the resulting
+/// service implements [`Sync`].
+/// ```
+pub struct BoxCloneSyncService<T, U, E>(
+    Box<
+        dyn CloneService<T, Response = U, Error = E, Future = BoxFuture<'static, Result<U, E>>>
+            + Send
+            + Sync,
+    >,
+);
+
+impl<T, U, E> BoxCloneSyncService<T, U, E> {
+    /// Create a new `BoxCloneService`.
+    pub fn new<S>(inner: S) -> Self
+    where
+        S: Service<T, Response = U, Error = E> + Clone + Send + Sync + 'static,
+        S::Future: Send + 'static,
+    {
+        let inner = inner.map_future(|f| Box::pin(f) as _);
+        BoxCloneSyncService(Box::new(inner))
+    }
+
+    /// Returns a [`Layer`] for wrapping a [`Service`] in a [`BoxCloneSyncService`]
+    /// middleware.
+    ///
+    /// [`Layer`]: crate::Layer
+    pub fn layer<S>() -> LayerFn<fn(S) -> Self>
+    where
+        S: Service<T, Response = U, Error = E> + Clone + Send + Sync + 'static,
+        S::Future: Send + 'static,
+    {
+        layer_fn(Self::new)
+    }
+}
+
+impl<T, U, E> Service<T> for BoxCloneSyncService<T, U, E> {
+    type Response = U;
+    type Error = E;
+    type Future = BoxFuture<'static, Result<U, E>>;
+
+    #[inline]
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), E>> {
+        self.0.poll_ready(cx)
+    }
+
+    #[inline]
+    fn call(&mut self, request: T) -> Self::Future {
+        self.0.call(request)
+    }
+}
+
+impl<T, U, E> Clone for BoxCloneSyncService<T, U, E> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone_box())
+    }
+}
+
+trait CloneService<R>: Service<R> {
+    fn clone_box(
+        &self,
+    ) -> Box<
+        dyn CloneService<R, Response = Self::Response, Error = Self::Error, Future = Self::Future>
+            + Send
+            + Sync,
+    >;
+}
+
+impl<R, T> CloneService<R> for T
+where
+    T: Service<R> + Send + Sync + Clone + 'static,
+{
+    fn clone_box(
+        &self,
+    ) -> Box<
+        dyn CloneService<R, Response = T::Response, Error = T::Error, Future = T::Future>
+            + Send
+            + Sync,
+    > {
+        Box::new(self.clone())
+    }
+}
+
+impl<T, U, E> fmt::Debug for BoxCloneSyncService<T, U, E> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        fmt.debug_struct("BoxCloneSyncService").finish()
+    }
+}

tower/src/util/mod.rs

@@ -3,6 +3,7 @@
 mod and_then;
 mod boxed;
 mod boxed_clone;
+mod boxed_clone_sync;
 mod call_all;
 mod either;
 
@@ -25,6 +26,7 @@ pub use self::{
     and_then::{AndThen, AndThenLayer},
     boxed::{BoxCloneServiceLayer, BoxLayer, BoxService, UnsyncBoxService},
     boxed_clone::BoxCloneService,
+    boxed_clone_sync::BoxCloneSyncService,
     either::Either,
     future_service::{future_service, FutureService},
     map_err::{MapErr, MapErrLayer},