Wait for IterStart and IterEnd events to be processed

The browser module uses the IterStart and IterEnd events for browser
initialization and shutdown, so we need to wait for them to complete.
There is some concern that this might add some delay to the overall
iteration duration, and not just to the iterations where browsers
processes are managed, but it should be negligible overall. We should
run benchmarks after this change to confirm this.

js/runner.go

@@ -772,7 +772,13 @@ func (u *ActiveVU) RunOnce() error {
 		VUID:         u.ID,
 		ScenarioName: u.scenarioName,
 	}
-	u.moduleVUImpl.events.local.Emit(&event.Event{Type: event.IterStart, Data: eventIterData})
+
+	waitEventCtx, waitEventCancel := context.WithTimeout(u.RunContext, 30*time.Minute)
+	defer waitEventCancel()
+	waitEventDone := u.moduleVUImpl.events.local.Emit(&event.Event{Type: event.IterStart, Data: eventIterData})
+	if err := waitEventDone(waitEventCtx); err != nil {
+		panic(fmt.Sprintf("error waiting for '%s' event processing to complete: %s", event.IterStart, err))
+	}
 
 	// Call the exported function.
 	_, isFullIteration, totalTime, err := u.runFn(ctx, true, fn, cancel, u.setupData)
@@ -786,7 +792,12 @@ func (u *ActiveVU) RunOnce() error {
 		}
 		eventIterData.Error = err
 	}
-	u.moduleVUImpl.events.local.Emit(&event.Event{Type: event.IterEnd, Data: eventIterData})
+	waitEventCtx, waitEventCancel = context.WithTimeout(u.RunContext, 30*time.Minute)
+	defer waitEventCancel()
+	waitEventDone = u.moduleVUImpl.events.local.Emit(&event.Event{Type: event.IterEnd, Data: eventIterData})
+	if err = waitEventDone(waitEventCtx); err != nil {
+		panic(fmt.Sprintf("error waiting for '%s' event processing to complete: %s", event.IterEnd, err))
+	}
 
 	// If MinIterationDuration is specified and the iteration wasn't canceled
 	// and was less than it, sleep for the remainder