prometheus-adapter-config only configure rules and externalRules, Crane will crash

[aheizi]

Describe the bug

When using Crane to dynamically load prometheus-adapter-config, if you only configure rules and externalRules, Crane will crash.

Configuration information:

rules:
- metricsQuery: sum(rate(<<.Series>>{<<.LabelMatchers>>}[30s])) by (<<.GroupBy>>)
  name:
    as: ${1}_qps
    matches: (.*)_total
  resources:
    overrides:
      namespace:
        resource: namespace
      pod:
        resource: pod
  seriesQuery: '{__name__=~"^http_requests.*_total$",container!="POD",namespace!="",pod!=""}'
externalRules:
- metricsQuery: avg(rate(<<.Series>>{<<.LabelMatchers>>}[30s])) by (name)
  resources:
    namespaced: false
  seriesQuery: 'http_requests_total'

Error message:

I0629 22:30:41.116099       1 config_fetcher.go:42] Got prometheus adapter configmap crane-system/prometheus-adapter
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x161a4d9]

goroutine 474 [running]:
github.com/gocrane/crane/pkg/prometheus-adapter.ParsingResourceRules(...)
    /go/src/github.com/gocrane/crane/pkg/prometheus-adapter/expression.go:65
github.com/gocrane/crane/pkg/prometheus-adapter.FlushRules({{0xc000cc8800, 0x1, 0x1}, 0x0, {0xc00000c960, 0x4, 0x4}}, {0x2361640, 0xc0000c6ff0})
    /go/src/github.com/gocrane/crane/pkg/prometheus-adapter/config_fetcher.go:138 +0xb9
github.com/gocrane/crane/pkg/prometheus-adapter.(*PrometheusAdapterConfigFetcher).Reconcile(0xc0007d4000, {0x2339878, 0xc000cdb8c0}, {{{0xc000430510, 0xc}, {0xc000b7c7e0, 0x12}}})
    /go/src/github.com/gocrane/crane/pkg/prometheus-adapter/config_fetcher.go:60 +0x525
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Reconcile(0xc0002246e0, {0x2339878, 0xc000cdb860}, {{{0xc000430510, 0x1e5b0a0}, {0xc000b7c7e0, 0xc0009c9a40}}})
    /go/pkg/mod/sigs.k8s.io/controller-runtime@v0.10.2/pkg/internal/controller/controller.go:114 +0x222
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler(0xc0002246e0, {0x23397d0, 0xc000922000}, {0x1dc7f80, 0xc000d0a600})
    /go/pkg/mod/sigs.k8s.io/controller-runtime@v0.10.2/pkg/internal/controller/controller.go:311 +0x2f2
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem(0xc0002246e0, {0x23397d0, 0xc000922000})
    /go/pkg/mod/sigs.k8s.io/controller-runtime@v0.10.2/pkg/internal/controller/controller.go:266 +0x205
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2.2()
    /go/pkg/mod/sigs.k8s.io/controller-runtime@v0.10.2/pkg/internal/controller/controller.go:227 +0x85
created by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2
    /go/pkg/mod/sigs.k8s.io/controller-runtime@v0.10.2/pkg/internal/controller/controller.go:223 +0x354

Reproduce steps

1. Prometheus adapter config

apiVersion: v1
kind: ConfigMap
metadata:
  name: prometheus-adapter
  namespace: crane-system
  labels:
    helm.sh/chart: prometheus-adapter-4.2.0
    app.kubernetes.io/managed-by: Helm
    app.kubernetes.io/component: metrics
    app.kubernetes.io/part-of: prometheus-adapter
    app.kubernetes.io/name: prometheus-adapter
    app.kubernetes.io/instance: prometheus-adapter
    app.kubernetes.io/version: "v0.10.0"
data:
  config.yaml: |
    rules:
    - metricsQuery: sum(rate(<<.Series>>{<<.LabelMatchers>>}[30s])) by (<<.GroupBy>>)
      name:
        as: ${1}_qps
        matches: (.*)_total
      resources:
        overrides:
          namespace:
            resource: namespace
          pod:
            resource: pod
      seriesQuery: '{__name__=~"^http_requests.*_total$",container!="POD",namespace!="",pod!=""}'
    externalRules:
    - metricsQuery: avg(rate(<<.Series>>{<<.LabelMatchers>>}[30s])) by (name)
      resources:
        namespaced: false
      seriesQuery: 'http_requests_total'

2. Crane start args

- --prometheus-adapter-configmap-namespace=crane-system
- --prometheus-adapter-configmap-name=prometheus-adapter
- --prometheus-adapter-configmap-key=config.yaml
- --prometheus-adapter-extension-labels=region="cn1"

Expected behavior

When only rules and externalRules are configured, metrics rules can be refreshed normally.

Screenshots

Environment (please complete the following information):

• K8S Version: [e.g. 1.19]
• Crane Version: [e.g. 0.1.0]
• Browser [e.g. chrome, safari]

[aheizi]

this issue has already solved by #726