Add a tracing warning when a thread blocks steps

src/sim.rs

@@ -2,7 +2,10 @@ use std::cell::RefCell;
 use std::future::Future;
 use std::net::IpAddr;
 use std::ops::DerefMut;
-use std::sync::Arc;
+use std::sync::mpsc::RecvTimeoutError;
+use std::sync::{Arc, mpsc};
+use std::sync::atomic::AtomicUsize;
+use std::sync::atomic::Ordering::Relaxed;
 use std::time::UNIX_EPOCH;
 
 use indexmap::IndexMap;
@@ -32,7 +35,7 @@ pub struct Sim<'a> {
     /// Simulation elapsed time
     elapsed: Duration,
 
-    steps: usize,
+    steps: Arc<AtomicUsize>,
 }
 
 impl<'a> Sim<'a> {
@@ -48,7 +51,7 @@ impl<'a> Sim<'a> {
             rts: IndexMap::new(),
             since_epoch,
             elapsed: Duration::ZERO,
-            steps: 1, // bumped after each step
+            steps: Arc::new(1.into()), // bumped after each step
         }
     }
 
@@ -315,6 +318,27 @@ impl<'a> Sim<'a> {
     /// Executes a simple event loop that calls [step](#method.step) each iteration,
     /// returning early if any host software errors.
     pub fn run(&mut self) -> Result {
+        let steps = self.steps.clone();
+        let (_tx, rx) = mpsc::channel::<()>();
+        std::thread::spawn(move || {
+            let mut blocked = false;
+            loop {
+                let prev = steps.load(std::sync::atomic::Ordering::Relaxed);
+                // Exit if main thread has.
+                match rx.recv_timeout(Duration::from_secs(10)) {
+                    Ok(_) | Err(RecvTimeoutError::Disconnected) => break,
+                    _ => {}
+                }
+                if steps.load(std::sync::atomic::Ordering::Relaxed) == prev {
+                    if !blocked {
+                        tracing::warn!("A task is blocking preventing simulation steps at step {}.", prev);
+                    }
+                    blocked = true;
+                } else {
+                    blocked = false;
+                }
+            }
+        });
         loop {
             let is_finished = self.step()?;
 
@@ -334,7 +358,7 @@ impl<'a> Sim<'a> {
     ///
     /// Returns whether or not all clients have completed.
     pub fn step(&mut self) -> Result<bool> {
-        tracing::trace!("step {}", self.steps);
+        tracing::debug!("step {}", self.steps.load(Relaxed));
 
         let tick = self.config.tick;
         let mut is_finished = true;
@@ -386,12 +410,12 @@ impl<'a> Sim<'a> {
         }
 
         self.elapsed += tick;
-        self.steps += 1;
+        let steps = self.steps.fetch_add(1, Relaxed) + 1;
 
         if self.elapsed > self.config.duration && !is_finished {
             return Err(format!(
                 "Ran for duration: {:?} steps: {} without completing",
-                self.config.duration, self.steps,
+                self.config.duration, steps,
             ))?;
         }