domain: close slow query channel after closing session pool (pingcap#…

…7847)

domain/domain.go

@@ -337,6 +337,12 @@ func (do *Domain) Reload() error {
 
 // LogSlowQuery keeps topN recent slow queries in domain.
 func (do *Domain) LogSlowQuery(query *SlowQueryInfo) {
+	do.slowQuery.mu.RLock()
+	defer do.slowQuery.mu.RUnlock()
+	if do.slowQuery.mu.closed {
+		return
+	}
+
 	select {
 	case do.slowQuery.ch <- query:
 	default:
@@ -459,8 +465,8 @@ func (do *Domain) Close() {
 	if do.etcdClient != nil {
 		terror.Log(errors.Trace(do.etcdClient.Close()))
 	}
-	do.slowQuery.Close()
 	do.sysSessionPool.Close()
+	do.slowQuery.Close()
 	do.wg.Wait()
 	log.Info("[domain] close")
 }

domain/topn_slow_query.go

@@ -115,6 +115,11 @@ type topNSlowQueries struct {
 	period   time.Duration
 	ch       chan *SlowQueryInfo
 	msgCh    chan *showSlowMessage
+
+	mu struct {
+		sync.RWMutex
+		closed bool
+	}
 }
 
 func newTopNSlowQueries(topN int, period time.Duration, queueSize int) *topNSlowQueries {
@@ -196,6 +201,10 @@ func (q *topNSlowQueries) QueryTop(count int, kind ast.ShowSlowKind) []*SlowQuer
 }
 
 func (q *topNSlowQueries) Close() {
+	q.mu.Lock()
+	q.mu.closed = true
+	q.mu.Unlock()
+
 	close(q.ch)
 }