fix(tsdb): Fix race calling filterShards outside a lock

Move filterShards inside the lock, as it enumerates the shards map,
which can result in data race when the map is written concurrently.

tsdb/store.go

@@ -660,6 +660,11 @@ func (s *Store) DeleteShard(shardID uint64) error {
 	}
 	delete(s.shards, shardID)
 	s.pendingShardDeletes[shardID] = struct{}{}
+
+	db := sh.Database()
+	// Determine if the shard contained any series that are not present in any
+	// other shards in the database.
+	shards := s.filterShards(byDatabase(db))
 	s.mu.Unlock()
 
 	// Ensure the pending deletion flag is cleared on exit.
@@ -677,12 +682,6 @@ func (s *Store) DeleteShard(shardID uint64) error {
 
 	ss := index.SeriesIDSet()
 
-	db := sh.Database()
-
-	// Determine if the shard contained any series that are not present in any
-	// other shards in the database.
-	shards := s.filterShards(byDatabase(db))
-
 	s.walkShards(shards, func(sh *Shard) error {
 		index, err := sh.Index()
 		if err != nil {
@@ -880,6 +879,7 @@ func (s *Store) DeleteMeasurement(database, name string) error {
 
 // filterShards returns a slice of shards where fn returns true
 // for the shard. If the provided predicate is nil then all shards are returned.
+// filterShards should be called under a lock.
 func (s *Store) filterShards(fn func(sh *Shard) bool) []*Shard {
 	var shards []*Shard
 	if fn == nil {