prevent nil panic due to throttler closing early

Signed-off-by: Tim Vaillancourt <tim@timvaillancourt.com>

go/vt/throttler/throttler_test.go

@@ -19,6 +19,7 @@ package throttler
 import (
 	"context"
 	"runtime"
+	"sync"
 	"testing"
 	"time"
 
@@ -409,17 +410,14 @@ func TestThrottlerMaxLag(t *testing.T) {
 	throttler, _ := newThrottlerWithClock("test", "queries", 1, 1, ReplicationLagModuleDisabled, fc.now)
 	defer throttler.Close()
 
-	var err error
-	ratesHistory := newFakeRatesHistory()
-	fc.setNow(1 * time.Second)
-	config := NewMaxReplicationLagModuleConfig(10)
-	throttler.maxReplicationLagModule, err = NewMaxReplicationLagModule(config, ratesHistory.aggregatedIntervalHistory, fc.now)
-	require.NoError(t, err)
-
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
 
-	go func(ctx context.Context, throttler *Throttler) {
+	var wg sync.WaitGroup
+
+	go func(wg *sync.WaitGroup, ctx context.Context, throttler *Throttler) {
+		wg.Add(1)
+		defer wg.Done()
 		for {
 			select {
 			case <-ctx.Done():
@@ -433,9 +431,11 @@ func TestThrottlerMaxLag(t *testing.T) {
 				}
 			}
 		}
-	}(ctx, throttler)
+	}(&wg, ctx, throttler)
 
-	go func(ctx context.Context, throttler *Throttler) {
+	go func(wg *sync.WaitGroup, ctx context.Context, throttler *Throttler) {
+		wg.Add(1)
+		defer wg.Done()
 		for {
 			select {
 			case <-ctx.Done():
@@ -464,7 +464,9 @@ func TestThrottlerMaxLag(t *testing.T) {
 				}
 			}
 		}
-	}(ctx, throttler)
+	}(&wg, ctx, throttler)
 
 	time.Sleep(time.Second)
+	cancel()
+	wg.Wait()
 }