Fix panic: assignment to entry in nil map

Closing the store did not properly return an error for in-flight
writes because the closing channel was set to nil when closed.  A
nil channel is not selectable so writes continue on past the guard
checks and trigger panics.

tsdb/store.go

@@ -28,6 +28,7 @@ func NewStore(path string) *Store {
 
 var (
 	ErrShardNotFound = fmt.Errorf("shard not found")
+	ErrStoreClosed   = fmt.Errorf("store is closed")
 )
 
 const (
@@ -46,6 +47,7 @@ type Store struct {
 
 	closing chan struct{}
 	wg      sync.WaitGroup
+	opened  bool
 }
 
 // Path returns the store's root path.
@@ -78,7 +80,7 @@ func (s *Store) CreateShard(database, retentionPolicy string, shardID uint64) er
 
 	select {
 	case <-s.closing:
-		return fmt.Errorf("closing")
+		return ErrStoreClosed
 	default:
 	}
 
@@ -369,6 +371,7 @@ func (s *Store) Open() error {
 	}
 
 	go s.periodicMaintenance()
+	s.opened = true
 
 	return nil
 }
@@ -376,6 +379,13 @@ func (s *Store) Open() error {
 func (s *Store) WriteToShard(shardID uint64, points []models.Point) error {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
+
+	select {
+	case <-s.closing:
+		return ErrStoreClosed
+	default:
+	}
+
 	sh, ok := s.shards[shardID]
 	if !ok {
 		return ErrShardNotFound
@@ -410,9 +420,9 @@ func (s *Store) CreateMapper(shardID uint64, stmt influxql.Statement, chunkSize
 func (s *Store) Close() error {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	if s.closing != nil {
+
+	if s.opened {
 		close(s.closing)
-		s.closing = nil
 	}
 	s.wg.Wait()
 
@@ -421,10 +431,7 @@ func (s *Store) Close() error {
 			return err
 		}
 	}
-	if s.closing != nil {
-		close(s.closing)
-	}
-	s.closing = nil
+	s.opened = false
 	s.shards = nil
 	s.databaseIndexes = nil