From 7c2fd4767db2dfe1e7eeb7bb69ba35e8aec22928 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Fabian=20St=C3=A4ber?= <fabian@fstab.de>
Date: Mon, 17 Oct 2022 15:42:03 +0200
Subject: [PATCH] Fix race condition with Exemplar in Counter

---
 prometheus/counter.go | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/prometheus/counter.go b/prometheus/counter.go
index 40d1b24f3..3668a16b3 100644
--- a/prometheus/counter.go
+++ b/prometheus/counter.go
@@ -140,7 +140,8 @@ func (c *counter) get() float64 {
 }
 
 func (c *counter) Write(out *dto.Metric) error {
-
+	// Read the Exemplar first and the value second. This is to avoid a race condition
+	// where users see an exemplar for a not-yet-existing observation.
 	var exemplar *dto.Exemplar
 	if e := c.exemplar.Load(); e != nil {
 		exemplar = e.(*dto.Exemplar)