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feature: expose redirect policy through extension #583

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31 changes: 29 additions & 2 deletions tower-http/src/follow_redirect/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -197,7 +197,7 @@ impl<ReqBody, ResBody, S, P> Service<Request<ReqBody>> for FollowRedirect<S, P>
where
S: Service<Request<ReqBody>, Response = Response<ResBody>> + Clone,
ReqBody: Body + Default,
P: Policy<ReqBody, S::Error> + Clone,
P: Policy<ReqBody, S::Error> + Clone + Send + Sync + 'static,
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This would be considered breaking changes, since some types might not fit the bounds.

Can we insert a custom extension without the need to change this layer?

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I'm not sure what you mean by layer ? the tower middleware Layer ?

The extra constraints come from the signature of http::extensions::Extensions

pub fn insert<T: Clone + Send + Sync + 'static>(&mut self, val: T) -> Option<T>

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@unusual-thoughts unusual-thoughts Jun 12, 2025 •
edited
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One way to avoid the breaking change would be something like:

trait MaybeAddExtension {
    fn add_policy_ext<ResBody>(&self, _res: &mut Response<ResBody>) {}
}

impl<T> MaybeAddExtension for T
where
    T: Clone + Send + Sync + 'static,
{
    fn add_policy_ext<ResBody>(&self, res: &mut Response<ResBody>) {
        res.extensions_mut().insert(FollowedPolicy(self.clone()));
    }
}

and then we can have

-    P: Policy<ReqBody, S::Error> + Clone,
+    P: Policy<ReqBody, S::Error> + Clone + MaybeAddExtension,

         res.extensions_mut().insert(RequestUri(this.uri.clone()));
+        this.policy.add_policy_ext(&mut res);

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You'd still be adding a bound where there wasn't one before, it would be exactly the same breaking change. The problem is somebody could have implemented a custom policy that's not Send or not Sync, and passed that to with_policy. The change to this line would then result in FollowRedirect<_, CustomPolicy> no longer implementing Service.

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oh you're right, somehow I mistakenly thought that the default empty implementation would be generated for non-Sync/Send policies.
So is the only way to avoid the breaking change to define a second layer variant that adds the extension?

{
type Response = Response<ResBody>;
type Error = S::Error;
Expand Down Expand Up @@ -250,14 +250,16 @@ impl<S, ReqBody, ResBody, P> Future for ResponseFuture<S, ReqBody, P>
where
S: Service<Request<ReqBody>, Response = Response<ResBody>> + Clone,
ReqBody: Body + Default,
P: Policy<ReqBody, S::Error>,
P: Policy<ReqBody, S::Error> + Clone + Send + Sync + 'static,
{
type Output = Result<Response<ResBody>, S::Error>;

fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut this = self.project();
let mut res = ready!(this.future.as_mut().poll(cx)?);
res.extensions_mut().insert(RequestUri(this.uri.clone()));
res.extensions_mut()
.insert(FollowedPolicy(this.policy.clone()));

let drop_payload_headers = |headers: &mut HeaderMap| {
for header in &[
Expand Down Expand Up @@ -342,6 +344,11 @@ where
#[derive(Clone)]
pub struct RequestUri(pub Uri);

/// Response [`Extensions`][http::Extensions] value that contains the redirect [`Policy`] that
/// was run before the last request of the redirect chain by a [`FollowRedirect`] middleware.
#[derive(Clone)]
pub struct FollowedPolicy<P>(pub P);

#[derive(Debug)]
enum BodyRepr<B> {
Some(B),
Expand Down Expand Up @@ -423,6 +430,12 @@ mod tests {
res.extensions().get::<RequestUri>().unwrap().0,
"http://example.com/0"
);
assert!(res
.extensions()
.get::<FollowedPolicy<Action>>()
.unwrap()
.0
.is_follow());
}

#[tokio::test]
Expand All @@ -441,6 +454,12 @@ mod tests {
res.extensions().get::<RequestUri>().unwrap().0,
"http://example.com/42"
);
assert!(res
.extensions()
.get::<FollowedPolicy<Action>>()
.unwrap()
.0
.is_stop());
}

#[tokio::test]
Expand All @@ -459,6 +478,14 @@ mod tests {
res.extensions().get::<RequestUri>().unwrap().0,
"http://example.com/32"
);
assert_eq!(
res.extensions()
.get::<FollowedPolicy<Limited>>()
.unwrap()
.0
.remaining,
0
);
}

/// A server with an endpoint `GET /{n}` which redirects to `/{n-1}` unless `n` equals zero,
Expand Down
2 changes: 1 addition & 1 deletion tower-http/src/follow_redirect/policy/limited.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@ use super::{Action, Attempt, Policy};
/// A redirection [`Policy`] that limits the number of successive redirections.
#[derive(Clone, Copy, Debug)]
pub struct Limited {
remaining: usize,
pub(crate) remaining: usize,
}

impl Limited {
Expand Down
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