linkerd · cratelyn · Nov 17, 2025 · Nov 14, 2025 · Nov 14, 2025 · Nov 14, 2025
@@ -1850,9 +1850,12 @@ dependencies = [
  "futures",
  "http",
  "http-body",
+ "http-body-util",
  "linkerd-error",
+ "linkerd-http-body-eos",
  "linkerd-http-box",
  "linkerd-metrics",
+ "linkerd-mock-http-body",
  "linkerd-stack",
  "pin-project",
  "prometheus-client",

@@ -17,13 +17,20 @@ bytes = { workspace = true }
 futures = { version = "0.3", default-features = false }
 http = { workspace = true }
 http-body = { workspace = true }
+http-body-util = { workspace = true }
 pin-project = "1"
 prometheus-client = { workspace = true }
 thiserror = "2"
 tokio = { version = "1", features = ["time"] }
 tonic = { workspace = true }
 
 linkerd-error = { path = "../../error" }
+linkerd-http-body-eos = { path = "../body-eos" }
 linkerd-http-box = { path = "../box" }
 linkerd-metrics = { path = "../../metrics" }
 linkerd-stack = { path = "../../stack" }
+
+[dev-dependencies]
+futures = { version = "0.3", default-features = false, features = ["executor"] }
+
+linkerd-mock-http-body = { path = "../../mock/http-body" }
@@ -4,4 +4,5 @@
 pub mod body_data;
 pub mod count_reqs;
 pub mod record_response;
+pub mod status;
 pub mod stream_label;
@@ -32,7 +32,7 @@ pub struct Params<L: MkStreamLabel, M> {
 
 #[derive(Clone, Debug, thiserror::Error)]
 #[error("request was cancelled before completion")]
-pub struct RequestCancelled(());
+pub struct RequestCancelled;
 
 /// Instruments an `N`-typed [`svc::NewService<T>`] with metrics.
 ///
@@ -252,7 +252,7 @@ where
     fn drop(self: Pin<&mut Self>) {
         let this = self.project();
         if this.state.is_some() {
-            end_stream(this.state, Err(&RequestCancelled(()).into()));
+            end_stream(this.state, Err(&RequestCancelled.into()));
         }
     }
 }

@@ -0,0 +1,312 @@
+//! A tower middleware for counting response status codes.
+
+use crate::{
+    record_response::RequestCancelled,
+    stream_label::{LabelSet, MkStreamLabel, StreamLabel},
+};
+use http::{Request, Response};
+use http_body::Body;
+use linkerd_error::Error;
+use linkerd_http_body_eos::{BodyWithEosFn, EosRef};
+use linkerd_metrics::prom::{Counter, Family, Registry};
+use linkerd_stack::{self as svc, layer::Layer, ExtractParam, NewService, Service};
+use pin_project::pin_project;
+use std::{
+    future::Future,
+    hash::Hash,
+    marker::PhantomData,
+    pin::Pin,
+    task::{Context, Poll},
+};
+
+#[cfg(test)]
+mod tests;
+
+/// Wraps a [`NewService<T>`] with response status code metrics.
+pub struct NewRecordStatusCode<N, X, ML, L> {
+    /// The inner wrapped service generator.
+    mk_svc: N,
+    /// Extracts an `ML`-typed [`MkStreamLabel`] from targets.
+    extract: X,
+    /// Marker indicating the `ML`-typed [`MkStreamLabel`] extracted from targets.
+    labeler: PhantomData<ML>,
+    /// Marker indicating the labels for the [`Family<L>`] extracted from targets.
+    label: PhantomData<L>,
+}
+
+/// Wraps a [`Service<T>`] with response status code metrics.
+#[derive(Clone)]
+pub struct RecordStatusCode<S, ML, L> {
+    /// The inner wrapped service.
+    svc: S,
+    /// A [`MkStreamLabel`] implementation to label the service's traffic.
+    ///
+    /// This generates an `L`-typed label set to acquire a [`Counter`] from the metrics family.
+    mk_stream_label: ML,
+    /// The [`Family`] of Prometheus [`Counter`]s tracking status codes.
+    metrics: StatusMetrics<L>,
+}
+
+/// A [`Future`] returned by a [`RecordStatusCode<S>`] service.
+#[pin_project(project = RecordStatusFutureProj)]
+pub enum RecordStatusFuture<F, SL, L> {
+    /// A transparent [`RecordStatusFuture`].
+    ///
+    /// This means that the [`MkStreamLabel`] did not emit a [`StreamLabel`], and that the
+    /// request/response pair did not need to be recorded.
+    Passthru(#[pin] F),
+    /// An instrumented [`RecordStatusFuture`].
+    ///
+    /// This will use the `SL`-typed [`StreamLabel`] type to label the traffic.
+    Instrumented {
+        /// The inner wrapped future.
+        #[pin]
+        fut: F,
+        stream_label: Option<SL>,
+        /// A [`Family`] of labeled counters.
+        metrics: StatusMetrics<L>,
+    },
+}
+
+/// Parameters for [`NewRecordStatusCode<S, ML, L>`] services.
+pub struct Params<ML, L> {
+    /// A [`MkStreamLabel`] implementation to label the service's traffic.
+    ///
+    /// This generates an `L`-typed label set to acquire a [`Counter`] from the metrics family.
+    pub mk_stream_label: ML,
+    /// The [`Family`] of Prometheus [`Counter`]s tracking status codes.
+    pub metrics: StatusMetrics<L>,
+}
+
+/// Prometheus metrics for [`NewRecordStatusCode<N, X, ML, L>`].
+#[derive(Clone, Debug)]
+pub struct StatusMetrics<L> {
+    counters: CounterFamily<L>,
+}
+
+/// A [`Family`] of labeled counters.
+type CounterFamily<L> = Family<L, Counter>;
+
+/// A [`Body`] returned by [`RecordStatusFuture`].
+pub type RecordStatusBody<B> = http_body_util::Either<InstrumentedBody<B>, B>;
+
+/// A [`Body`] that will invoke a closure upon reaching the end of the stream.
+type InstrumentedBody<B> = BodyWithEosFn<B, EosCallback>;
+
+/// A boxed callback used by [`InstrumentedBody<B>`] to inspect the end of the stream.
+type EosCallback = Box<dyn FnOnce(EosRef<'_, Error>) + Send>;
+
+// === impl NewRecordStatusCode ===
+
+impl<N, X, ML, L> NewRecordStatusCode<N, X, ML, L>
+where
+    X: Clone,
+{
+    /// Returns a [`Layer`] that can be applied to an inner [`NewService<T>`].
+    pub fn layer_via(extract: X) -> impl Layer<N, Service = Self> {
+        svc::layer::mk(move |inner| Self {
+            mk_svc: inner,
+            extract: extract.clone(),
+            labeler: PhantomData,
+            label: PhantomData,
+        })
+    }
+
+    /// A helper to confirm that this type can be used as a [`NewService<T>`].
+    ///
+    /// This helps provide friendlier error messages.
+    pub fn check_new_service<T>(self) -> Self
+    where
+        N: NewService<T>,
+        X: ExtractParam<Params<ML, L>, T>,
+    {
+        self
+    }
+}
+
+impl<N, T, L, ML, X> NewService<T> for NewRecordStatusCode<N, X, ML, L>
+where
+    N: NewService<T>,
+    X: ExtractParam<Params<ML, L>, T>,
+{
+    type Service = RecordStatusCode<N::Service, ML, L>;
+
+    fn new_service(&self, target: T) -> Self::Service {
+        let Self {
+            mk_svc,
+            extract,
+            labeler: _,
+            label: _,
+        } = self;
+
+        // Extract a stream labeler and a family of counters.
+        let Params {
+            mk_stream_label,
+            metrics,
+        } = extract.extract_param(&target);
+        let svc = mk_svc.new_service(target);
+
+        RecordStatusCode {
+            svc,
+            mk_stream_label,
+            metrics,
+        }
+    }
+}
+
+// === impl RecordStatusCode ===
+
+impl<S, ML, L, ReqB, RspB> Service<Request<ReqB>> for RecordStatusCode<S, ML, L>
+where
+    S: Service<Request<ReqB>, Response = Response<RspB>, Error = Error>,
+    S::Error: Into<Error>,
+    ML: MkStreamLabel<StatusLabels = L>,
+    RspB: Body<Error = Error>,
+    L: Clone + Hash + Eq + Send + Sync + 'static,
+{
+    type Response = Response<RecordStatusBody<RspB>>;
+    type Error = Error;
+    type Future = RecordStatusFuture<S::Future, ML::StreamLabel, ML::StatusLabels>;
+
+    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
+        let Self {
+            svc,
+            mk_stream_label: _,
+            metrics: _,
+        } = self;
+
+        svc.poll_ready(cx).map_err(Into::into)
+    }
+
+    fn call(&mut self, req: http::Request<ReqB>) -> Self::Future {
+        let Self {
+            svc,
+            mk_stream_label,
+            metrics,
+        } = self;
+
+        if let stream_label @ Some(_) = mk_stream_label.mk_stream_labeler(&req) {
+            // If this request should be recorded, return an instrumented future.
+            let fut = svc.call(req);
+            let metrics = metrics.clone();
+            RecordStatusFuture::Instrumented {
+                fut,
+                stream_label,
+                metrics,
+            }
+        } else {
+            let fut = svc.call(req);
+            RecordStatusFuture::Passthru(fut)
+        }
+    }
+}
+
+// === impl RecordStatusFuture ===
+
+impl<F, B, SL, L> Future for RecordStatusFuture<F, SL, L>
+where
+    F: Future<Output = Result<Response<B>, Error>>,
+    B: Body<Error = Error>,
+    SL: StreamLabel<StatusLabels = L>,
+    L: Clone + Hash + Eq + Send + Sync + 'static,
+{
+    type Output = Result<Response<RecordStatusBody<B>>, Error>;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        use http_body_util::Either::{Left as Enabled, Right as Disabled};
+
+        match self.project() {
+            // If this is a transparent future, poll the future and return the response.
+            RecordStatusFutureProj::Passthru(fut) => fut.poll(cx).map_ok(|rsp| rsp.map(Disabled)),
+            // If this reponse should be recorded, instrument the response when it is ready.
+            RecordStatusFutureProj::Instrumented {
+                fut,
+                stream_label,
+                metrics,
+            } => {
+                // Wait for the inner future. Once ready, take the labeler and metric family.
+                let rsp = futures::ready!(fut.poll(cx));
+                let metrics = metrics.clone();
+                let mut stream_label = stream_label.take().expect("futures only yield once");
+
+                let rsp = match rsp {
+                    Ok(rsp) => {
+                        // Observe the start of the response, and then instrument the response
+                        // body with a callback that will record the outcome once finished.
+                        stream_label.init_response(&rsp);
+                        let on_eos =
+                            move |eos: EosRef<'_>| Self::on_eos(eos, stream_label, metrics);
+                        let instrument = |body| InstrumentedBody::new(body, Box::new(on_eos));
+                        Ok(rsp.map(instrument).map(Enabled))
+                    }
+                    Err(err) => {
+                        // Record an error if there is no response.
+                        Self::on_eos(EosRef::Error(&err), stream_label, metrics);
+                        Err(err)
+                    }
+                };
+
+                Poll::Ready(rsp)
+            }
+        }
+    }
+}
+
+impl<F, SL, L> RecordStatusFuture<F, SL, L>
+where
+    SL: StreamLabel<StatusLabels = L>,
+    L: Clone + Hash + Eq + Send + Sync + 'static,
+{
+    fn on_eos(eos: EosRef<'_, Error>, mut stream_label: SL, metrics: StatusMetrics<L>) {
+        let ugh = RequestCancelled.into(); // XXX(kate)
+
+        stream_label.end_response(match eos {
+            EosRef::None => Ok(None),
+            EosRef::Trailers(trls) => Ok(Some(trls)),
+            EosRef::Error(error) => Err(error),
+            EosRef::Cancelled => Err(&ugh),
+        });
+
+        let labels = stream_label.status_labels();
+        let counter = metrics.metric(&labels);
+        counter.inc();
+    }
+}
+
+// === impl StatusMetrics ===
+
+impl<L> Default for StatusMetrics<L>
+where
+    L: LabelSet,
+{
+    fn default() -> Self {
+        Self {
+            counters: Default::default(),
+        }
+    }
+}
+
+impl<L> StatusMetrics<L>
+where
+    L: LabelSet,
+{
+    /// Registers a new [`StatusMetrics<L>`] with the given metrics registry.
+    pub fn register(registry: &mut Registry, help: &'static str) -> Self {
+        let counters = Family::default();
+
+        registry.register("statuses", help, counters.clone());
+
+        Self { counters }
+    }
+}
+
+impl<L> StatusMetrics<L>
+where
+    L: Clone + Hash + Eq,
+{
+    pub fn metric(&self, labels: &L) -> Counter {
+        let Self { counters } = self;
+
+        counters.get_or_create(labels).to_owned()
+    }
+}