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service: Clarify subtly around cloning and readiness #548

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2 changes: 2 additions & 0 deletions tower-service/CHANGELOG.md
Original file line number Diff line number Diff line change
@@ -1,5 +1,7 @@
# Unreleased

- Clarify subtly around cloning and readiness in the `Service` docs.

# 0.3.1 (November 29, 2019)

- Improve example in `Service` docs. ([#510])
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77 changes: 77 additions & 0 deletions tower-service/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -231,6 +231,83 @@ use std::task::{Context, Poll};
/// `Service` provides a mechanism by which the caller is able to coordinate
/// readiness. `Service::poll_ready` returns `Ready` if the service expects that
/// it is able to process a request.
///
/// # Be careful when cloning inner services
///
/// Services are permitted to panic if `call` is invoked without obtaining `Poll::Ready(Ok(()))`
/// from `poll_ready`. You should therefore be careful when cloning services for example to move
/// them into boxed futures. Even though the original service is ready, the clone might not be.
///
/// Therefore this kind of code is wrong and might panic:
///
/// ```rust
/// # use std::pin::Pin;
/// # use std::task::{Poll, Context};
/// # use std::future::Future;
/// # use tower_service::Service;
/// #
/// struct Wrapper<S> {
/// inner: S,
/// }
///
/// impl<R, S> Service<R> for Wrapper<S>
/// where
/// S: Service<R> + Clone + 'static,
/// R: 'static,
/// {
/// type Response = S::Response;
/// type Error = S::Error;
/// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
///
/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
/// Poll::Ready(Ok(()))
/// }
///
/// fn call(&mut self, req: R) -> Self::Future {
/// let mut inner = self.inner.clone();
/// Box::pin(async move {
/// // `inner` might not be ready since its a clone
/// inner.call(req).await
/// })
/// }
/// }
/// ```
///
/// You should instead use [`std::mem::replace`] to take the service that was ready:
///
/// ```rust
/// # use std::pin::Pin;
/// # use std::task::{Poll, Context};
/// # use std::future::Future;
/// # use tower_service::Service;
/// #
/// struct Wrapper<S> {
/// inner: S,
/// }
///
/// impl<R, S> Service<R> for Wrapper<S>
/// where
/// S: Service<R> + Clone + 'static,
/// R: 'static,
/// {
/// type Response = S::Response;
/// type Error = S::Error;
/// type Future = Pin<Box<dyn Future<Output = Result<Self::Response, Self::Error>>>>;
///
/// fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
/// Poll::Ready(Ok(()))
/// }
///
/// fn call(&mut self, req: R) -> Self::Future {
/// let clone = self.inner.clone();
/// // take the service that was ready
/// let mut inner = std::mem::replace(&mut self.inner, clone);
/// Box::pin(async move {
/// inner.call(req).await
/// })
/// }
/// }
/// ```
pub trait Service<Request> {
/// Responses given by the service.
type Response;
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