wai/wai-rs/src/energy_burst.rs

@@ -1121,10 +1121,26 @@ mod tests {
                 std::thread::sleep(std::time::Duration::from_millis(2));
             },
             move || {
-                // Called as the window opens: the workers may have begun the
-                // first unit, but none has completed one.
+                // Called as the window opens. How many units have *begun* by
+                // this instant is a race this test does not control, and it
+                // used to be asserted: `begun <= 3`, one per worker.
+                //
+                // Nothing in `burst_from_rest` entails that. Each worker, once
+                // the barrier releases, increments this counter and sleeps 2 ms;
+                // the caller then calls this closure. For `begun` to exceed the
+                // worker count, the caller need only be descheduled for longer
+                // than that 2 ms at exactly this point — which a contended host
+                // will do, and did: the assertion failed on the linux-arm64
+                // runner (a VM on a box at load 173) while passing on every
+                // other platform. It is a 2 ms race, not a property.
+                //
+                // Recorded, not asserted. What the test establishes about
+                // starting from rest is asserted below and does not depend on
+                // this ordering: the window opened `FromRest`, every worker was
+                // in flight, and completions never exceed starts by more than
+                // the worker count.
                 let begun = at_open.load(Ordering::SeqCst);
-                assert!(begun <= 3, "no more than one unit per worker can have begun");
+                eprintln!("from-rest burst: {begun} unit(s) had begun as the window opened");
                 std::thread::sleep(std::time::Duration::from_millis(40));
                 vec![1.0, 2.0]
             },
@@ -1133,9 +1149,18 @@ mod tests {
         assert_eq!(count.in_flight, 3);
         assert!(count.completions > 0 && window_s > 0.0);
         // Every completion started inside the window: completions can never
-        // exceed starts, and starts exceed completions by at most N.
+        // exceed starts, and starts exceed completions by at most N — the
+        // in-flight units — plus the N a worker may begin between the window
+        // closing and this read. This is the test's real claim, and unlike the
+        // count at window-open it holds however the host schedules the threads:
+        // it bounds a difference by the worker count rather than asserting that
+        // a particular instant arrived before a 2 ms sleep finished.
         let starts = started.load(Ordering::SeqCst);
-        assert!(count.completions <= starts && starts <= count.completions + 3 + 3);
+        assert!(
+            count.completions <= starts && starts <= count.completions + 3 + 3,
+            "{} completions against {starts} starts with 3 workers",
+            count.completions
+        );
     }
 
     #[test]