feat: add warning log for blocking futures

Cargo.toml

@@ -12,11 +12,12 @@ members = [
     "examples/updater",
     "examples/updater_threads",
     "examples/blocking_genserver",
+    "examples/busy_genserver_warning",
 ]
 
 [workspace.dependencies]
-spawned-rt = { path = "rt", version = "0.4.0"}
-spawned-concurrency = { path = "concurrency", version = "0.4.0"}
+spawned-rt = { path = "rt", version = "0.4.0" }
+spawned-concurrency = { path = "concurrency", version = "0.4.0" }
 tracing = { version = "0.1.41", features = ["log"] }
 tracing-subscriber = { version = "0.3.19", features = ["env-filter"] }
 

concurrency/Cargo.toml

@@ -10,6 +10,7 @@ spawned-rt = { workspace = true }
 tracing = { workspace = true }
 futures = "0.3.1"
 thiserror = "2.0.12"
+pin-project-lite = "0.2"
 
 [dev-dependencies]
 # This tokio imports are only used in tests, we should not use them in the library code.
@@ -18,3 +19,7 @@ tokio = { version = "1", features = ["full"] }
 
 [lib]
 path = "./src/lib.rs"
+
+[features]
+# Enable this to log warnings when non-blocking GenServers block the runtime for too much time
+warn-on-block = []

concurrency/src/tasks/gen_server.rs

@@ -1,13 +1,12 @@
 //! GenServer trait and structs to create an abstraction similar to Erlang gen_server.
 //! See examples/name_server for a usage example.
-use futures::future::FutureExt as _;
-use spawned_rt::tasks::{self as rt, mpsc, oneshot, timeout, CancellationToken};
-use std::{fmt::Debug, future::Future, panic::AssertUnwindSafe, time::Duration};
-
 use crate::{
     error::GenServerError,
     tasks::InitResult::{NoSuccess, Success},
 };
+use futures::future::FutureExt as _;
+use spawned_rt::tasks::{self as rt, mpsc, oneshot, timeout, CancellationToken};
+use std::{fmt::Debug, future::Future, panic::AssertUnwindSafe, time::Duration};
 
 const DEFAULT_CALL_TIMEOUT: Duration = Duration::from_secs(5);
 
@@ -36,12 +35,19 @@ impl<G: GenServer> GenServerHandle<G> {
             cancellation_token,
         };
         let handle_clone = handle.clone();
-        // Ignore the JoinHandle for now. Maybe we'll use it in the future
-        let _join_handle = rt::spawn(async move {
+        let inner_future = async move {
             if gen_server.run(&handle, &mut rx).await.is_err() {
                 tracing::trace!("GenServer crashed")
-            };
-        });
+            }
+        };
+
+        #[cfg(feature = "warn-on-block")]
+        // Optionally warn if the GenServer future blocks for too much time
+        let inner_future = warn_on_block::WarnOnBlocking::new(inner_future);
+
+        // Ignore the JoinHandle for now. Maybe we'll use it in the future
+        let _join_handle = rt::spawn(inner_future);
+
         handle_clone
     }
 
@@ -294,6 +300,46 @@ pub trait GenServer: Send + Sized {
     }
 }
 
+#[cfg(feature = "warn-on-block")]
+mod warn_on_block {
+    use super::*;
+
+    use std::time::Instant;
+    use tracing::warn;
+
+    pin_project_lite::pin_project! {
+        pub struct WarnOnBlocking<F: Future>{
+            #[pin]
+            inner: F
+        }
+    }
+
+    impl<F: Future> WarnOnBlocking<F> {
+        pub fn new(inner: F) -> Self {
+            Self { inner }
+        }
+    }
+
+    impl<F: Future> Future for WarnOnBlocking<F> {
+        type Output = F::Output;
+
+        fn poll(
+            self: std::pin::Pin<&mut Self>,
+            cx: &mut std::task::Context<'_>,
+        ) -> std::task::Poll<Self::Output> {
+            let type_id = std::any::type_name::<F>();
+            let this = self.project();
+            let now = Instant::now();
+            let res = this.inner.poll(cx);
+            let elapsed = now.elapsed();
+            if elapsed > Duration::from_millis(10) {
+                warn!(future = ?type_id, elapsed = ?elapsed, "Blocking operation detected");
+            }
+            res
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
 

examples/busy_genserver_warning/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "busy_genserver_warning"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+spawned-rt = { workspace = true }
+spawned-concurrency = { workspace = true }
+tracing = { workspace = true }
+
+[[bin]]
+name = "busy_genserver_warning"
+path = "main.rs"

examples/busy_genserver_warning/main.rs

@@ -0,0 +1,73 @@
+use spawned_rt::tasks as rt;
+use std::time::Duration;
+use std::{process::exit, thread};
+use tracing::info;
+
+use spawned_concurrency::tasks::{CallResponse, CastResponse, GenServer, GenServerHandle};
+
+// We test a scenario with a badly behaved task
+struct BusyWorker;
+
+impl BusyWorker {
+    pub fn new() -> Self {
+        BusyWorker
+    }
+}
+
+#[derive(Clone)]
+pub enum InMessage {
+    GetCount,
+    Stop,
+}
+
+#[derive(Clone)]
+pub enum OutMsg {
+    Count(u64),
+}
+
+impl GenServer for BusyWorker {
+    type CallMsg = InMessage;
+    type CastMsg = ();
+    type OutMsg = ();
+    type Error = ();
+
+    async fn handle_call(
+        &mut self,
+        _: Self::CallMsg,
+        _: &GenServerHandle<Self>,
+    ) -> CallResponse<Self> {
+        CallResponse::Stop(())
+    }
+
+    async fn handle_cast(
+        &mut self,
+        _: Self::CastMsg,
+        handle: &GenServerHandle<Self>,
+    ) -> CastResponse {
+        info!("sleeping");
+        thread::sleep(Duration::from_millis(542));
+        handle.clone().cast(()).await.unwrap();
+        // This sleep is needed to yield control to the runtime.
+        // If not, the future never returns and the warning isn't emitted.
+        rt::sleep(Duration::from_millis(0)).await;
+        CastResponse::NoReply
+    }
+}
+
+/// Example of a program with a semi-blocking [`GenServer`].
+/// As mentioned in the `blocking_genserver` example, tasks that block can block
+/// the entire runtime in cooperative multitasking models. This is easy to find
+/// in practice, since it appears as if the whole world stopped. However, most
+/// of the time, tasks simply take longer than expected, which can lead to
+/// service degradation and increased latency. To tackle this, we print a warning
+/// whenever we detect tasks that take too long to run.
+pub fn main() {
+    rt::run(async move {
+        // If we change BusyWorker to start_blocking instead, it won't print the warning
+        let mut badboy = BusyWorker::new().start();
+        let _ = badboy.cast(()).await;
+
+        rt::sleep(Duration::from_secs(5)).await;
+        exit(0);
+    })
+}