Fix race condition with Exemplar in Counter

prometheus · Oct 17, 2022 · 7c2fd47 · 7c2fd47
1 parent 9e13cd2
commit 7c2fd47
Showing 1 changed file with 2 additions and 1 deletion.
diff --git 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)