PMM-3473 Fix panic.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fixed some function comments based on best practices from Effective Go #137, thx [@CodeLingoBot](https://github.com/CodeLingoBot).
+- [PMM-3473](https://jira.percona.com/browse/PMM-3473): Fixed panic and runtime error.
 
 ## [0.7.0]
 ### Changed

collector/mongod/metrics.go

@@ -407,10 +407,18 @@ type ReplStats struct {
 
 // Export exposes the replication stats.
 func (replStats *ReplStats) Export(ch chan<- prometheus.Metric) {
-	replStats.Apply.Export(ch)
-	replStats.Buffer.Export(ch)
-	replStats.Network.Export(ch)
-	replStats.PreloadStats.Export(ch)
+	if replStats.Apply != nil {
+		replStats.Apply.Export(ch)
+	}
+	if replStats.Buffer != nil {
+		replStats.Buffer.Export(ch)
+	}
+	if replStats.Network != nil {
+		replStats.Network.Export(ch)
+	}
+	if replStats.PreloadStats != nil {
+		replStats.PreloadStats.Export(ch)
+	}
 	// 3.0+ only
 	if replStats.Executor != nil {
 		replStats.Executor.Export(ch)
@@ -425,11 +433,15 @@ type PreloadStats struct {
 
 // Export exposes the preload stats.
 func (preloadStats *PreloadStats) Export(ch chan<- prometheus.Metric) {
-	metricsReplPreloadDocsNumTotal.Set(preloadStats.Docs.Num)
-	metricsReplPreloadDocsTotalMilliseconds.Set(preloadStats.Docs.TotalMillis)
+	if preloadStats.Docs != nil {
+		metricsReplPreloadDocsNumTotal.Set(preloadStats.Docs.Num)
+		metricsReplPreloadDocsTotalMilliseconds.Set(preloadStats.Docs.TotalMillis)
+	}
 
-	metricsReplPreloadIndexesNumTotal.Set(preloadStats.Indexes.Num)
-	metricsReplPreloadIndexesTotalMilliseconds.Set(preloadStats.Indexes.TotalMillis)
+	if preloadStats.Indexes != nil {
+		metricsReplPreloadIndexesNumTotal.Set(preloadStats.Indexes.Num)
+		metricsReplPreloadIndexesTotalMilliseconds.Set(preloadStats.Indexes.TotalMillis)
+	}
 }
 
 // StorageStats are the stats associated with the storage.

collector/mongod/metrics_test.go

@@ -0,0 +1,24 @@
+package mongod
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestReplStatsExportShouldNotPanic(t *testing.T) {
+	rs := &ReplStats{}
+	ch := make(chan prometheus.Metric)
+	f := func() { rs.Export(ch) }
+
+	assert.NotPanics(t, f, "nil pointer in ReplStats")
+}
+
+func TestPreloadStatsExportShouldNotPanic(t *testing.T) {
+	ps := &PreloadStats{}
+	ch := make(chan prometheus.Metric)
+	f := func() { ps.Export(ch) }
+
+	assert.NotPanics(t, f, "nil pointer in PreloadStats")
+}