test: ensure unbounded hotspot resets load

Author: bootjp

distribution/engine.go

@@ -132,16 +132,22 @@ func (e *Engine) routeIndex(key []byte) int {
 func (e *Engine) splitRange(idx int) {
 	r := e.routes[idx]
 	if r.End == nil {
-		// cannot split unbounded range
+		// cannot split unbounded range; reset load to avoid repeated attempts
+		e.routes[idx].Load = 0
 		return
 	}
 	mid := midpoint(r.Start, r.End)
 	if mid == nil {
+		// cannot determine midpoint; reset load to avoid repeated attempts
+		e.routes[idx].Load = 0
 		return
 	}
 	left := Route{Start: r.Start, End: mid, GroupID: r.GroupID}
 	right := Route{Start: mid, End: r.End, GroupID: r.GroupID}
-	e.routes = append(append(e.routes[:idx], left, right), e.routes[idx+1:]...)
+	// replace the range at idx with left and right in an idiomatic manner
+	e.routes = append(e.routes[:idx+1], e.routes[idx:]...)
+	e.routes[idx] = left
+	e.routes[idx+1] = right
 }
 
 func cloneBytes(b []byte) []byte {

distribution/engine_test.go

@@ -2,6 +2,7 @@ package distribution
 
 import (
 	"bytes"
+	"sync"
 	"testing"
 )
 
@@ -89,6 +90,42 @@ func TestEngineSplitOnHotspot(t *testing.T) {
 	}
 }
 
+func TestEngineHotspotUnboundedResetsLoad(t *testing.T) {
+	e := NewEngineWithThreshold(2)
+	e.UpdateRoute([]byte("a"), nil, 1)
+	e.RecordAccess([]byte("b"))
+	e.RecordAccess([]byte("b"))
+	stats := e.Stats()
+	if len(stats) != 1 {
+		t.Fatalf("expected 1 route, got %d", len(stats))
+	}
+	if stats[0].Load != 0 {
+		t.Fatalf("expected load reset to 0, got %d", stats[0].Load)
+	}
+}
+
+func TestEngineRecordAccessConcurrent(t *testing.T) {
+	t.Parallel()
+	e := NewEngineWithThreshold(6)
+	e.UpdateRoute([]byte("a"), []byte("c"), 1)
+
+	var wg sync.WaitGroup
+	const workers = 10
+	for i := 0; i < workers; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			e.RecordAccess([]byte("b"))
+		}()
+	}
+	wg.Wait()
+
+	stats := e.Stats()
+	if len(stats) != 2 {
+		t.Fatalf("expected 2 routes after concurrent split, got %d", len(stats))
+	}
+}
+
 func assertRange(t *testing.T, r Route, start, end []byte) {
 	t.Helper()
 	if !bytes.Equal(r.Start, start) || !bytes.Equal(r.End, end) {