Fix race condition with Exemplar in Counter (#1146)

fstab · bwplotka · commit ddd7f0edcd31 · 2022-11-01T16:22:27.000Z
* 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>
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)
 }

Original file line number	Diff line number	Diff line change
`@@ -140,12 +140,13 @@ func (c *counter) get() float64 {`
`140`	`140`	`}`
`141`	`141`
`142`	`142`	`func (c counter) Write(out dto.Metric) error {`
`143`		`- val := c.get()`
`144`		`-`
	`143`	`+ // Read the Exemplar first and the value second. This is to avoid a race condition`
	`144`	`+ // where users see an exemplar for a not-yet-existing observation.`
`145`	`145`	`var exemplar *dto.Exemplar`
`146`	`146`	`if e := c.exemplar.Load(); e != nil {`
`147`	`147`	`exemplar = e.(*dto.Exemplar)`
`148`	`148`	`}`
	`149`	`+ val := c.get()`
`149`	`150`
`150`	`151`	`return populateMetric(CounterValue, val, c.labelPairs, exemplar, out)`
`151`	`152`	`}`