Return nop iterator for invalid chunk references

chunks/chunk.go

@@ -69,6 +69,17 @@ type Iterator interface {
 	Next() bool
 }
 
+// NewNopIterator returns a new chunk iterator that does not hold any data.
+func NewNopIterator() Iterator {
+	return nopIterator{}
+}
+
+type nopIterator struct{}
+
+func (nopIterator) At() (int64, float64) { return 0, 0 }
+func (nopIterator) Next() bool           { return false }
+func (nopIterator) Err() error           { return nil }
+
 type Pool interface {
 	Put(Chunk) error
 	Get(e Encoding, b []byte) (Chunk, error)

head.go

@@ -1270,6 +1270,12 @@ func computeChunkEndTime(start, cur, max int64) int64 {
 
 func (s *memSeries) iterator(id int) chunks.Iterator {
 	c := s.chunk(id)
+	// TODO(fabxc): Work around! A querier may have retrieved a pointer to a series' chunk,
+	// which got then garbage collected before it got accessed.
+	// We must ensure to not garbage collect as long as any readers still hold a reference.
+	if c == nil {
+		return chunks.NewNopIterator()
+	}
 
 	if id-s.firstChunkID < len(s.chunks)-1 {
 		return c.chunk.Iterator()