Fix race condition with Exemplar in Counter (#1146)

* Fix race condition with Exemplar in Counter Potential fix for #1145. Signed-off-by: Fabian Stäber <fabian@fstab.de> * Fix race condition with Exemplar in Counter Signed-off-by: Fabian Stäber <fabian@fstab.de> Signed-off-by: Fabian Stäber <fabian@fstab.de>
prometheus · Oct 17, 2022 · 10b0550 · 10b0550
1 parent dcea97e
commit 10b0550
Showing 1 changed file with 3 additions and 2 deletions.
diff --git a/prometheus/counter.go b/prometheus/counter.go
@@ -140,12 +140,13 @@ func (c *counter) get() float64 {
 }
 
 func (c *counter) Write(out *dto.Metric) error {
-	val := c.get()
-
+	// 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)
 	}
+	val := c.get()
 
 	return populateMetric(CounterValue, val, c.labelPairs, exemplar, out)
 }