util: Add then combinator (#500)

Currently, `ServiceExt` and `ServiceBuilder` provide combinators for
mapping successful responses to other responses, and mapping errors to
other errors, but don't provide a way to map between `Ok` and `Err`
results.

For completeness, this branch adds a new `then` combinator, which takes
a function from `Result` to `Result` and applies it when the service's
future completes. This can be used for recovering from some errors or
for rejecting some `Ok` responses. It can also be used for behaviors
that should be run when a service's future completes regardless of
whether it completed successfully or not.

Depends on #499

tower/src/builder/mod.rs

@@ -229,6 +229,24 @@ impl<L> ServiceBuilder<L> {
         self.layer(crate::util::MapErrLayer::new(f))
     }
 
+    /// Apply a function after the service, regardless of whether the future
+    /// succeeds or fails.
+    ///
+    /// This is similar to the [`map_response`] and [`map_err] functions,
+    /// except that the *same* function is invoked when the service's future
+    /// completes, whether it completes successfully or fails. This function
+    /// takes the `Result` returned by the service's future, and returns a
+    /// `Result`.
+    ///
+    /// See the documentation for the [`then` combinator] for details.
+    ///
+    /// [`then` combinator]: crate::util::ServiceExt::then
+    #[cfg(feature = "util")]
+    pub fn then<F>(self, f: F) -> ServiceBuilder<Stack<crate::util::ThenLayer<F>, L>> {
+        self.layer(crate::util::ThenLayer::new(f))
+    }
+
+
     /// Obtains the underlying `Layer` implementation.
     pub fn into_inner(self) -> L {
         self.layer

tower/src/util/map_result.rs

@@ -0,0 +1,73 @@
+use futures_util::FutureExt;
+use std::task::{Context, Poll};
+use tower_layer::Layer;
+use tower_service::Service;
+
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use futures_util::future::Map as MapResultFuture;
+
+/// Service returned by the [`map_result`] combinator.
+///
+/// [`map_result`]: crate::util::ServiceExt::map_result
+#[derive(Clone, Debug)]
+pub struct MapResult<S, F> {
+    inner: S,
+    f: F,
+}
+
+/// A [`Layer`] that produces a [`MapResult`] service.
+///
+/// [`Layer`]: tower_layer::Layer
+#[derive(Debug, Clone)]
+pub struct MapResultLayer<F> {
+    f: F,
+}
+
+impl<S, F> MapResult<S, F> {
+    /// Creates a new `MapResult` service.
+    pub fn new(inner: S, f: F) -> Self {
+        MapResult { f, inner }
+    }
+}
+
+impl<S, F, Request, Response, Error> Service<Request> for MapResult<S, F>
+where
+    S: Service<Request>,
+    Error: From<S::Error>,
+    F: FnOnce(Result<S::Response, S::Error>) -> Result<Response, Error> + Clone,
+{
+    type Response = Response;
+    type Error = Error;
+    type Future = MapResultFuture<S::Future, F>;
+
+    #[inline]
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        self.inner.poll_ready(cx).map_err(Into::into)
+    }
+
+    #[inline]
+    fn call(&mut self, request: Request) -> Self::Future {
+        self.inner.call(request).map(self.f.clone())
+    }
+}
+
+impl<F> MapResultLayer<F> {
+    /// Creates a new [`MapResultLayer`] layer.
+    pub fn new(f: F) -> Self {
+        MapResultLayer { f }
+    }
+}
+
+impl<S, F> Layer<S> for MapResultLayer<F>
+where
+    F: Clone,
+{
+    type Service = MapResult<S, F>;
+
+    fn layer(&self, inner: S) -> Self::Service {
+        MapResult {
+            f: self.f.clone(),
+            inner,
+        }
+    }
+}