source/system: implement FeatureSource

Separate feature discovery and creation of feature labels in the system
source.

Also, change the implementation of the nodeName custom rule to utilize
the FeatureSource interface of the system source.

Also, add minimalist unit test.

source/custom/rules/nodename_rule.go

@@ -17,14 +17,13 @@ limitations under the License.
 package rules
 
 import (
-	"os"
+	"fmt"
 	"regexp"
 
 	"k8s.io/klog/v2"
-)
 
-var (
-	nodeName = os.Getenv("NODE_NAME")
+	"sigs.k8s.io/node-feature-discovery/source"
+	"sigs.k8s.io/node-feature-discovery/source/system"
 )
 
 // NodenameRule matches on nodenames configured in a ConfigMap
@@ -34,6 +33,11 @@ type NodenameRule []string
 var _ Rule = NodenameRule{}
 
 func (n NodenameRule) Match() (bool, error) {
+	nodeName, ok := source.GetFeatureSource("system").GetFeatures().Values[system.NameFeature].Elements["nodename"]
+	if !ok {
+		return false, fmt.Errorf("node name not available")
+	}
+
 	for _, nodenamePattern := range n {
 		klog.V(1).Infof("matchNodename %s", nodenamePattern)
 		match, err := regexp.MatchString(nodenamePattern, nodeName)

source/system/system.go

@@ -24,23 +24,35 @@ import (
 
 	"k8s.io/klog/v2"
 
+	"sigs.k8s.io/node-feature-discovery/pkg/api/feature"
+	"sigs.k8s.io/node-feature-discovery/pkg/utils"
 	"sigs.k8s.io/node-feature-discovery/source"
 )
 
 var osReleaseFields = [...]string{
 	"ID",
 	"VERSION_ID",
+	"VERSION_ID.major",
+	"VERSION_ID.minor",
 }
 
 const Name = "system"
 
-// systemSource implements the LabelSource interface.
-type systemSource struct{}
+const (
+	OsReleaseFeature = "osrelease"
+	NameFeature      = "name"
+)
+
+// systemSource implements the FeatureSource and LabelSource interfaces.
+type systemSource struct {
+	features *feature.DomainFeatures
+}
 
 // Singleton source instance
 var (
 	src systemSource
-	_   source.LabelSource = &src
+	_   source.FeatureSource = &src
+	_   source.LabelSource   = &src
 )
 
 func (s *systemSource) Name() string { return Name }
@@ -50,29 +62,54 @@ func (s *systemSource) Priority() int { return 0 }
 
 // GetLabels method of the LabelSource interface
 func (s *systemSource) GetLabels() (source.FeatureLabels, error) {
-	features := source.FeatureLabels{}
+	labels := source.FeatureLabels{}
+	features := s.GetFeatures()
 
+	for _, key := range osReleaseFields {
+		if value, exists := features.Values[OsReleaseFeature].Elements[key]; exists {
+			feature := "os_release." + key
+			labels[feature] = value
+		}
+	}
+	return labels, nil
+}
+
+// Discover method of the FeatureSource interface
+func (s *systemSource) Discover() error {
+	s.features = feature.NewDomainFeatures()
+
+	// Get node name
+	s.features.Values[NameFeature] = feature.NewValueFeatures(nil)
+	s.features.Values[NameFeature].Elements["nodename"] = os.Getenv("NODE_NAME")
+
+	// Get os-release information
 	release, err := parseOSRelease()
 	if err != nil {
 		klog.Errorf("failed to get os-release: %s", err)
 	} else {
-		for _, key := range osReleaseFields {
-			if value, exists := release[key]; exists {
-				feature := "os_release." + key
-				features[feature] = value
+		s.features.Values[OsReleaseFeature] = feature.NewValueFeatures(release)
 
-				if key == "VERSION_ID" {
-					versionComponents := splitVersion(value)
-					for subKey, subValue := range versionComponents {
-						if subValue != "" {
-							features[feature+"."+subKey] = subValue
-						}
-					}
+		if v, ok := release["VERSION_ID"]; ok {
+			versionComponents := splitVersion(v)
+			for subKey, subValue := range versionComponents {
+				if subValue != "" {
+					s.features.Values[OsReleaseFeature].Elements["VERSION_ID."+subKey] = subValue
 				}
 			}
 		}
 	}
-	return features, nil
+
+	utils.KlogDump(3, "discovered system features:", "  ", s.features)
+
+	return nil
+}
+
+// GetFeatures method of the FeatureSource Interface
+func (s *systemSource) GetFeatures() *feature.DomainFeatures {
+	if s.features == nil {
+		s.features = feature.NewDomainFeatures()
+	}
+	return s.features
 }
 
 // Read and parse os-release file

source/system/system_test.go

@@ -0,0 +1,36 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package system
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"sigs.k8s.io/node-feature-discovery/pkg/api/feature"
+)
+
+func TestSystemSource(t *testing.T) {
+	assert.Equal(t, src.Name(), Name)
+
+	// Check that GetLabels works with empty features
+	src.features = feature.NewDomainFeatures()
+	l, err := src.GetLabels()
+
+	assert.Nil(t, err, err)
+	assert.Empty(t, l)
+
+}