Fix deadlock in Index.ForEachMeasurementTagKey

Index.ForEachMeasurementTagKey held an RLock while call the fn,
if the fn made another call into the index which acquired an RLock
and after another goroutine tried to acquire a Lock, it would deadlock.

tsdb/index/inmem/inmem.go

@@ -263,10 +263,13 @@ func (i *Index) MeasurementTagKeysByExpr(name []byte, expr influxql.Expr) (map[s
 
 // ForEachMeasurementTagKey iterates over all tag keys for a measurement.
 func (i *Index) ForEachMeasurementTagKey(name []byte, fn func(key []byte) error) error {
+	// Ensure we do not hold a lock on the index while fn executes in case fn tries
+	// to acquire a lock on the index again.  If another goroutine has Lock, this will
+	// deadlock.
 	i.mu.RLock()
-	defer i.mu.RUnlock()
-
 	mm := i.measurements[string(name)]
+	i.mu.RUnlock()
+
 	if mm == nil {
 		return nil
 	}
@@ -537,9 +540,9 @@ func (i *Index) DropSeries(key []byte) error {
 // ForEachMeasurementSeriesByExpr iterates over all series in a measurement filtered by an expression.
 func (i *Index) ForEachMeasurementSeriesByExpr(name []byte, expr influxql.Expr, fn func(tags models.Tags) error) error {
 	i.mu.RLock()
-	defer i.mu.RUnlock()
-
 	mm := i.measurements[string(name)]
+	i.mu.RUnlock()
+
 	if mm == nil {
 		return nil
 	}
@@ -731,7 +734,6 @@ type ShardIndex struct {
 
 // CreateSeriesListIfNotExists creates a list of series if they doesn't exist in bulk.
 func (idx *ShardIndex) CreateSeriesListIfNotExists(keys, names [][]byte, tagsSlice []models.Tags) error {
-
 	keys, names, tagsSlice = idx.assignExistingSeries(idx.id, keys, names, tagsSlice)
 	if len(keys) == 0 {
 		return nil

tsdb/meta.go

@@ -287,12 +287,9 @@ func (m *Measurement) ForEachSeriesByExpr(condition influxql.Expr, fn func(tags
 		return err
 	}
 
-	m.mu.RLock()
-	defer m.mu.RUnlock()
-
 	// Iterate over each series.
 	for _, id := range ids {
-		s := m.seriesByID[id]
+		s := m.SeriesByID(id)
 		if err := fn(s.Tags()); err != nil {
 			return err
 		}