Fix tag dereferencing panic.

Clones series tags under lock during var ref iterator creation.

CHANGELOG.md

@@ -13,6 +13,7 @@
 - [#7905](https://github.com/influxdata/influxdb/issues/7905): Fix ORDER BY time DESC with ordering series keys.
 - [#7966](https://github.com/influxdata/influxdb/pull/7966): Prevent a panic when aggregates are used in an inner query with a raw query.
 - [#8001](https://github.com/influxdata/influxdb/issues/8001): Map types correctly when using a regex and one of the measurements is empty.
+- [#8011](https://github.com/influxdata/influxdb/issues/8011): Fix tag dereferencing panic.
 
 ## v1.2.0 [2017-01-24]
 

tsdb/meta.go

@@ -272,7 +272,7 @@ func (d *DatabaseIndex) TagsForSeries(key string) models.Tags {
 	if ss == nil {
 		return nil
 	}
-	return ss.Tags
+	return ss.CloneTags()
 }
 
 // MeasurementsByExpr takes an expression containing only tags and returns a
@@ -1629,6 +1629,13 @@ func (s *Series) ForEachTag(fn func(models.Tag)) {
 	}
 }
 
+// CloneTags returns a copy of the series tags under lock.
+func (s *Series) CloneTags() models.Tags {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.Tags.Clone()
+}
+
 // Dereference removes references to a byte slice.
 func (s *Series) Dereference(b []byte) {
 	s.mu.Lock()