From 89d8467f5c30ce40bccd8a17fa99868c32682c5a Mon Sep 17 00:00:00 2001
From: Fabian Reinartz <fab.reinartz@gmail.com>
Date: Mon, 9 Jan 2017 18:07:45 +0100
Subject: [PATCH] Add missing lock

---
 head.go | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/head.go b/head.go
index a56f3ad2f..07724b965 100644
--- a/head.go
+++ b/head.go
@@ -406,6 +406,8 @@ func (h *HeadBlock) updateMapping() {
 	h.mapper.mtx.Lock()
 	defer h.mapper.mtx.Unlock()
 
+	h.mtx.RLock()
+
 	if h.mapper.sortable != nil && h.mapper.Len() == len(h.descs) {
 		return
 	}
@@ -413,6 +415,8 @@ func (h *HeadBlock) updateMapping() {
 	cds := make([]*chunkDesc, len(h.descs))
 	copy(cds, h.descs)
 
+	h.mtx.RUnlock()
+
 	s := slice.SortInterface(cds, func(i, j int) bool {
 		return labels.Compare(cds[i].lset, cds[j].lset) < 0
 	})