Nil pointer dereference caused in 6.2.0 when bufferVolumeMetrics is unset

[am9zZWY]

Describe the bug:
The error was encountered when upgrading from 6.1.0 to 6.2.0. In our testsuite we wait for the fluentbit DaemonSet which never starts. When investigating the Fluentd Pod, we got a nil pointer dereference in the logs.

Expected behaviour:
The DaemonSet should start without any errors.

Steps to reproduce the bug:
Don't set bufferVolumeMetrics in the FluentdConfig.

Environment details:

• kind: kind v0.30.0 go1.24.6 linux/amd64
• logging-operator version: 6.2.0
• Install method: helm with terraform module kostomization which lets us use helm charts with terraform
• Logs from the misbehaving component (and any other relevant logs):

{"level":"error","ts":"2025-11-26T15:16:00Z","msg":"Observed a panic","controller":"logging","controllerGroup":"logging.banzaicloud.io","controllerKind":"Logging","Logging":{"name":"cluster-logging"},"namespace":"","name":"cluster-logging","reconcileID":"2298ccc7-bf43-4e42-89a2-8ec44affc937","panic":"runtime error: invalid memory address or nil pointer dereference","panicGoValue":"\"invalid memory address or nil pointer dereference\"","stacktrace":"goroutine 785 [running]:\nk8s.io/apimachinery/pkg/util/runtime.logPanic({0x248fb18, 0xc001660fc0}, {0x1e0f780, 0x35fea80})\n\t/go/pkg/mod/k8s.io/apimachinery@v0.34.2/pkg/util/runtime/runtime.go:132 +0xbc\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Reconcile.func1()\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:198 +0x10e\npanic({0x1e0f780?, 0x35fea80?})\n\t/usr/local/go/src/runtime/panic.go:783 +0x132\ngithub.com/kube-logging/logging-operator/pkg/resources/fluentd.(*Reconciler).monitorBufferServiceMetrics(0xc0018076b0)\n\t/usr/local/src/logging-operator/pkg/resources/fluentd/service.go:184 +0x4c\ngithub.com/kube-logging/logging-operator/pkg/resources/fluentd.(*Reconciler).Reconcile(0xc0018076b0, {0x248fb18, 0xc001661f50})\n\t/usr/local/src/logging-operator/pkg/resources/fluentd/fluentd.go:258 +0x116e\ngithub.com/kube-logging/logging-operator/controllers/logging.(*LoggingReconciler).Reconcile(0xc00080b140, {0x248fb18, 0xc001660fc0}, {{{0x0?, 0xc000c64008?}, {0xc000f80f20?, 0x0?}}})\n\t/usr/local/src/logging-operator/controllers/logging/logging_controller.go:293 +0x1d8a\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Reconcile(0xc001660f30?, {0x248fb18?, 0xc001660fc0}, {{{0x0, 0x0?}, {0xc000f80f20?, 0x0?}}})\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:216 +0x165\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler(0x24b19e0, {0x248fb50, 0xc0004255e0}, {{{0x0, 0x0}, {0xc000f80f20, 0xf}}}, 0x0)\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:461 +0x3ad\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem(0x24b19e0, {0x248fb50, 0xc0004255e0})\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:421 +0x21b\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func1.1()\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:296 +0x85\ncreated by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func1 in goroutine 78\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:292 +0x26b\n","stacktrace":"k8s.io/apimachinery/pkg/util/runtime.logPanic\n\t/go/pkg/mod/k8s.io/apimachinery@v0.34.2/pkg/util/runtime/runtime.go:142\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Reconcile.func1\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:198\nruntime.gopanic\n\t/usr/local/go/src/runtime/panic.go:783\nruntime.panicmem\n\t/usr/local/go/src/runtime/panic.go:262\nruntime.sigpanic\n\t/usr/local/go/src/runtime/signal_unix.go:925\ngithub.com/kube-logging/logging-operator/pkg/resources/fluentd.(*Reconciler).monitorBufferServiceMetrics\n\t/usr/local/src/logging-operator/pkg/resources/fluentd/service.go:184\ngithub.com/kube-logging/logging-operator/pkg/resources/fluentd.(*Reconciler).Reconcile\n\t/usr/local/src/logging-operator/pkg/resources/fluentd/fluentd.go:258\ngithub.com/kube-logging/logging-operator/controllers/logging.(*LoggingReconciler).Reconcile\n\t/usr/local/src/logging-operator/controllers/logging/logging_controller.go:293\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Reconcile\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:216\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:461\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:421\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func1.1\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:296"}
{"level":"error","ts":"2025-11-26T15:16:00Z","msg":"Reconciler error","controller":"logging","controllerGroup":"logging.banzaicloud.io","controllerKind":"Logging","Logging":{"name":"cluster-logging"},"namespace":"","name":"cluster-logging","reconcileID":"2298ccc7-bf43-4e42-89a2-8ec44affc937","error":"panic: runtime error: invalid memory address or nil pointer dereference [recovered]","stacktrace":"sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).reconcileHandler\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:474\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).processNextWorkItem\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:421\nsigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller[...]).Start.func1.1\n\t/go/pkg/mod/sigs.k8s.io/controller-runtime@v0.22.4/pkg/internal/controller/controller.go:296"}
{"level":"info","ts":"2025-11-26T15:16:00Z","logger":"logging.validation","msg":"found detached fluentd aggregator, making association","logging":"cluster-logging","name":"cluster-logging"}

• Resource definition (possibly in YAML format) that caused the issue, without sensitive data:

apiVersion: logging.banzaicloud.io/v1beta1
kind: FluentdConfig
metadata:
  generation: 1
  name: cluster-logging
  namespace: logging
  resourceVersion: "1110"
spec:
  affinity:
    podAntiAffinity:
      requiredDuringSchedulingIgnoredDuringExecution:
      - labelSelector:
          matchLabels:
            scheduling.breuninger.com/placement-selector: cluster-logging
        matchLabelKeys:
        - pod-template-hash
        topologyKey: kubernetes.io/hostname
  bufferStorageVolume:
    pvc:
      spec:
        accessModes:
        - ReadWriteOnce
        resources:
          requests:
            storage: 30Gi
        storageClassName: standard
  configReloaderResources:
    limits:
      memory: 10Mi
    requests:
      cpu: 10m
      memory: 10Mi
  labels:
    scheduling.breuninger.com/placement-selector: cluster-logging
  metrics:
    prometheusRules: true
    prometheusRulesOverride:
    - alert: FluentdRecordsCountsHigh
      labels:
        severity: warning
    - alert: FluentdNodeDown
      labels:
        severity: warning
    - alert: FluentdQueueLength
      labels:
        severity: warning
    serviceMonitor: true
  pdb:
    minAvailable: 1
  resources:
    limits:
      memory: 512Mi
    requests:
      cpu: 50m
      memory: 512Mi
  scaling:
    replicas: 2
  security:
    securityContext:
      allowPrivilegeEscalation: false
      capabilities:
        drop:
        - ALL
      seccompProfile:
        type: RuntimeDefault
  topologySpreadConstraints:
  - labelSelector:
      matchLabels:
        scheduling.breuninger.com/placement-selector: cluster-logging
    matchLabelKeys:
    - pod-template-hash
    maxSkew: 1
    topologyKey: topology.kubernetes.io/zone
    whenUnsatisfiable: DoNotSchedule

Setting the bufferVolumeMetrics.enabled in the FluentdConfig explicitly to either true or false, solves this problem.

/kind bug

[aslafy-z]

I just opened #2145 that should address this issue

[csatib02]

6.2.1 release is ready, with fixes in place.

Many thanks @aslafy-z !