From ea817e169b7f7bc17206285e6bac27fea69f2f2d Mon Sep 17 00:00:00 2001
From: Fabian Reinartz <fab.reinartz@gmail.com>
Date: Fri, 20 Oct 2017 09:43:52 +0200
Subject: [PATCH] Return nop iterator for invalid chunk references

---
 chunks/chunk.go | 11 +++++++++++
 head.go         |  6 ++++++
 2 files changed, 17 insertions(+)

diff --git a/chunks/chunk.go b/chunks/chunk.go
index 181693fae..4d298041e 100644
--- a/chunks/chunk.go
+++ b/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)
diff --git a/head.go b/head.go
index 0aeaef5a6..8182fb4b5 100644
--- a/head.go
+++ b/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()