feat: allow pluggable tower layers in connector service stack

Cargo.toml

@@ -10,7 +10,7 @@ authors = ["Sean McArthur <sean@seanmonstar.com>"]
 readme = "README.md"
 license = "MIT OR Apache-2.0"
 edition = "2021"
-rust-version = "1.63.0"
+rust-version = "1.64.0"
 autotests = true
 
 [package.metadata.docs.rs]
@@ -105,6 +105,7 @@ url = "2.4"
 bytes = "1.0"
 serde = "1.0"
 serde_urlencoded = "0.7.1"
+tower = { version = "0.5.2", default-features = false, features = ["timeout", "util"] }
 tower-service = "0.3"
 futures-core = { version = "0.3.28", default-features = false }
 futures-util = { version = "0.3.28", default-features = false }
@@ -169,7 +170,6 @@ quinn = { version = "0.11.1", default-features = false, features = ["rustls", "r
 slab = { version = "0.4.9", optional = true } # just to get minimal versions working with quinn
 futures-channel = { version = "0.3", optional = true }
 
-
 [target.'cfg(not(target_arch = "wasm32"))'.dev-dependencies]
 env_logger = "0.10"
 hyper = { version = "1.1.0", default-features = false, features = ["http1", "http2", "client", "server"] }
@@ -222,6 +222,11 @@ features = [
 wasm-bindgen = { version = "0.2.89", features = ["serde-serialize"] }
 wasm-bindgen-test = "0.3"
 
+[dev-dependencies]
+tower = { version = "0.5.1", default-features = false, features = ["limit"] }
+num_cpus = "1.0"
+libc = "0"
+
 [lints.rust]
 unexpected_cfgs = { level = "warn", check-cfg = ['cfg(reqwest_unstable)'] }
 
@@ -253,6 +258,10 @@ path = "examples/form.rs"
 name = "simple"
 path = "examples/simple.rs"
 
+[[example]]
+name = "connect_via_lower_priority_tokio_runtime"
+path = "examples/connect_via_lower_priority_tokio_runtime.rs"
+
 [[test]]
 name = "blocking"
 path = "tests/blocking.rs"

examples/connect_via_lower_priority_tokio_runtime.rs

@@ -0,0 +1,272 @@
+#![deny(warnings)]
+// This example demonstrates how to delegate the connect calls, which contain TLS handshakes,
+// to a secondary tokio runtime of lower OS thread priority using a custom tower layer.
+// This helps to ensure that long-running futures during handshake crypto operations don't block other I/O futures.
+//
+// This does introduce overhead of additional threads, channels, extra vtables, etc,
+// so it is best suited to services with large numbers of incoming connections or that
+// are otherwise very sensitive to any blocking futures.  Or, you might want fewer threads
+// and/or to use the current_thread runtime.
+//
+// This is using the `tokio` runtime and certain other dependencies:
+//
+// `tokio = { version = "1", features = ["full"] }`
+// `num_cpus = "1.0"`
+// `libc = "0"`
+// `pin-project-lite = "0.2"`
+// `tower = { version = "0.5", default-features = false}`
+
+#[cfg(not(target_arch = "wasm32"))]
+#[tokio::main]
+async fn main() -> Result<(), reqwest::Error> {
+    background_threadpool::init_background_runtime();
+    tokio::time::sleep(std::time::Duration::from_millis(10)).await;
+
+    let client = reqwest::Client::builder()
+        .connector_layer(background_threadpool::BackgroundProcessorLayer::<http::Uri>::new())
+        .build()
+        .expect("should be able to build reqwest client");
+
+    let url = if let Some(url) = std::env::args().nth(1) {
+        url
+    } else {
+        println!("No CLI URL provided, using default.");
+        "https://hyper.rs".into()
+    };
+
+    eprintln!("Fetching {url:?}...");
+
+    let res = client.get(url).send().await?;
+
+    eprintln!("Response: {:?} {}", res.version(), res.status());
+    eprintln!("Headers: {:#?}\n", res.headers());
+
+    let body = res.text().await?;
+
+    println!("{body}");
+
+    Ok(())
+}
+
+// separating out for convenience to avoid a million #[cfg(not(target_arch = "wasm32"))]
+#[cfg(not(target_arch = "wasm32"))]
+mod background_threadpool {
+    use std::{
+        future::Future,
+        marker::PhantomData,
+        pin::Pin,
+        sync::OnceLock,
+        task::{Context, Poll},
+    };
+
+    use futures_util::TryFutureExt;
+    use pin_project_lite::pin_project;
+    use tokio::{runtime::Handle, select, sync::mpsc::error::TrySendError};
+    use tower::{BoxError, Layer, Service};
+
+    static CPU_HEAVY_THREAD_POOL: OnceLock<
+        tokio::sync::mpsc::Sender<Pin<Box<dyn Future<Output = ()> + Send + 'static>>>,
+    > = OnceLock::new();
+
+    pub(crate) fn init_background_runtime() {
+        std::thread::Builder::new()
+            .name("cpu-heavy-background-threadpool".to_string())
+            .spawn(move || {
+                let rt = tokio::runtime::Builder::new_multi_thread()
+                    .thread_name("cpu-heavy-background-pool-thread")
+                    .worker_threads(num_cpus::get() as usize)
+                    // ref: https://github.com/tokio-rs/tokio/issues/4941
+                    // consider uncommenting if seeing heavy task contention
+                    // .disable_lifo_slot()
+                    .on_thread_start(move || unsafe {
+                        // Reduce thread pool thread niceness, so they are lower priority
+                        // than the foreground executor and don't interfere with I/O tasks
+                        #[cfg(target_os = "linux")]
+                        {
+                            *libc::__errno_location() = 0;
+                            if libc::nice(10) == -1 && *libc::__errno_location() != 0 {
+                                let error = std::io::Error::last_os_error();
+                                log::error!("failed to set threadpool niceness: {}", error);
+                            }
+                        }
+                    })
+                    .enable_all()
+                    .build()
+                    .unwrap_or_else(|e| panic!("cpu heavy runtime failed_to_initialize: {}", e));
+                rt.block_on(async {
+                    log::debug!("starting background cpu-heavy work");
+                    process_cpu_work().await;
+                });
+            })
+            .unwrap_or_else(|e| panic!("cpu heavy thread failed_to_initialize: {}", e));
+    }
+
+    #[cfg(not(target_arch = "wasm32"))]
+    async fn process_cpu_work() {
+        // we only use this channel for routing work, it should move pretty quick, it can be small
+        let (tx, mut rx) = tokio::sync::mpsc::channel(10);
+        // share the handle to the background channel globally
+        CPU_HEAVY_THREAD_POOL.set(tx).unwrap();
+
+        while let Some(work) = rx.recv().await {
+            tokio::task::spawn(work);
+        }
+    }
+
+    // retrieve the sender to the background channel, and send the future over to it for execution
+    fn send_to_background_runtime(future: impl Future<Output = ()> + Send + 'static) {
+        let tx = CPU_HEAVY_THREAD_POOL.get().expect(
+            "start up the secondary tokio runtime before sending to `CPU_HEAVY_THREAD_POOL`",
+        );
+
+        match tx.try_send(Box::pin(future)) {
+            Ok(_) => (),
+            Err(TrySendError::Closed(_)) => {
+                panic!("background cpu heavy runtime channel is closed")
+            }
+            Err(TrySendError::Full(msg)) => {
+                log::warn!(
+                    "background cpu heavy runtime channel is full, task spawning loop delayed"
+                );
+                let tx = tx.clone();
+                Handle::current().spawn(async move {
+                    tx.send(msg)
+                        .await
+                        .expect("background cpu heavy runtime channel is closed")
+                });
+            }
+        }
+    }
+
+    // This tower layer injects futures with a oneshot channel, and then sends them to the background runtime for processing.
+    // We don't use the Buffer service because that is intended to process sequentially on a single task, whereas we want to
+    // spawn a new task per call.
+    pub struct BackgroundProcessorLayer<Request> {
+        _p: PhantomData<fn(Request)>,
+    }
+    impl<Request> BackgroundProcessorLayer<Request> {
+        pub fn new() -> Self {
+            Self { _p: PhantomData }
+        }
+    }
+    impl<S, Request> Layer<S> for BackgroundProcessorLayer<Request> {
+        type Service = BackgroundProcessor<S>;
+        fn layer(&self, service: S) -> Self::Service {
+            BackgroundProcessor::new(service)
+        }
+    }
+
+    impl<Request> std::fmt::Debug for BackgroundProcessorLayer<Request> {
+        fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+            f.debug_struct("BackgroundProcessorLayer").finish()
+        }
+    }
+
+    impl<Request> Clone for BackgroundProcessorLayer<Request> {
+        fn clone(&self) -> Self {
+            Self { _p: PhantomData }
+        }
+    }
+
+    impl<Request> Copy for BackgroundProcessorLayer<Request> {}
+
+    // This tower service injects futures with a oneshot channel, and then sends them to the background runtime for processing.
+    #[derive(Debug, Clone)]
+    pub struct BackgroundProcessor<S> {
+        inner: S,
+    }
+
+    impl<S> BackgroundProcessor<S> {
+        pub fn new(inner: S) -> Self {
+            BackgroundProcessor { inner }
+        }
+    }
+
+    impl<S, Request> Service<Request> for BackgroundProcessor<S>
+    where
+        S: Service<Request>,
+        S::Response: Send + 'static,
+        S::Error: Into<BoxError> + Send,
+        S::Future: Send + 'static,
+    {
+        type Response = S::Response;
+
+        type Error = BoxError;
+
+        type Future = BackgroundResponseFuture<S::Response>;
+
+        fn poll_ready(
+            &mut self,
+            cx: &mut std::task::Context<'_>,
+        ) -> std::task::Poll<Result<(), Self::Error>> {
+            match self.inner.poll_ready(cx) {
+                Poll::Pending => Poll::Pending,
+                Poll::Ready(r) => Poll::Ready(r.map_err(Into::into)),
+            }
+        }
+
+        fn call(&mut self, req: Request) -> Self::Future {
+            let response = self.inner.call(req);
+
+            // wrap our inner service's future with a future that writes to this oneshot channel
+            let (mut tx, rx) = tokio::sync::oneshot::channel();
+            let future = async move {
+                select!(
+                    _ = tx.closed() => {
+                        // receiver already dropped, don't need to do anything
+                    }
+                    result = response.map_err(|err| Into::<BoxError>::into(err)) => {
+                        // if this fails, the receiver already dropped, so we don't need to do anything
+                        let _ = tx.send(result);
+                    }
+                )
+            };
+            // send the wrapped future to the background
+            send_to_background_runtime(future);
+
+            BackgroundResponseFuture::new(rx)
+        }
+    }
+
+    // `BackgroundProcessor` response future
+    pin_project! {
+        #[derive(Debug)]
+        pub struct BackgroundResponseFuture<S> {
+            #[pin]
+            rx: tokio::sync::oneshot::Receiver<Result<S, BoxError>>,
+        }
+    }
+
+    impl<S> BackgroundResponseFuture<S> {
+        pub(crate) fn new(rx: tokio::sync::oneshot::Receiver<Result<S, BoxError>>) -> Self {
+            BackgroundResponseFuture { rx }
+        }
+    }
+
+    impl<S> Future for BackgroundResponseFuture<S>
+    where
+        S: Send + 'static,
+    {
+        type Output = Result<S, BoxError>;
+
+        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+            let this = self.project();
+
+            // now poll on the receiver end of the oneshot to get the result
+            match this.rx.poll(cx) {
+                Poll::Ready(v) => match v {
+                    Ok(v) => Poll::Ready(v.map_err(Into::into)),
+                    Err(err) => Poll::Ready(Err(Box::new(err) as BoxError)),
+                },
+                Poll::Pending => Poll::Pending,
+            }
+        }
+    }
+}
+
+// The [cfg(not(target_arch = "wasm32"))] above prevent building the tokio::main function
+// for wasm32 target, because tokio isn't compatible with wasm32.
+// If you aren't building for wasm32, you don't need that line.
+// The two lines below avoid the "'main' function not found" error when building for wasm32 target.
+#[cfg(any(target_arch = "wasm32"))]
+fn main() {}