fix: harden high-fanout delivery under load

loadtest/im-pressure/callback_queue_test.go

@@ -0,0 +1,47 @@
+package main
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+func TestCallbackQueueMetricsTrackWaitingAndProcessing(t *testing.T) {
+	metrics := &callbackQueueMetrics{}
+	metrics.enqueue()
+	metrics.enqueue()
+	metrics.startCallback(time.Now().Add(-10 * time.Millisecond))
+
+	snapshot := metrics.snapshot()
+	if snapshot.Current != 1 || snapshot.Processing != 1 || snapshot.Maximum != 2 || snapshot.Enqueued != 2 {
+		t.Fatalf("unexpected active queue snapshot: %+v", snapshot)
+	}
+	if snapshot.WaitSecondsTotal <= 0 || snapshot.WaitSecondsMax <= 0 || metrics.isDrained() {
+		t.Fatalf("wait or drain state not tracked: %+v", snapshot)
+	}
+
+	metrics.finishCallback()
+	metrics.startCallback(time.Now())
+	metrics.finishCallback()
+	if snapshot = metrics.snapshot(); snapshot.Current != 0 || snapshot.Processing != 0 || snapshot.Completed != 2 || !metrics.isDrained() {
+		t.Fatalf("unexpected drained queue snapshot: %+v", snapshot)
+	}
+}
+
+func TestWaitForCallbackQueueWaitsForProcessingCallback(t *testing.T) {
+	metrics := &callbackQueueMetrics{}
+	metrics.enqueue()
+	metrics.startCallback(time.Now())
+	go func() {
+		time.Sleep(20 * time.Millisecond)
+		metrics.finishCallback()
+	}()
+
+	started := time.Now()
+	if err := waitForCallbackQueue(context.Background(), metrics, time.Second); err != nil {
+		t.Fatal(err)
+	}
+	if time.Since(started) < 15*time.Millisecond {
+		t.Fatal("wait returned before the callback finished")
+	}
+}