Add warning log callback in client-go loading rules (#117233)

* Add warning log callback in client-go loading rules

This provides a way to consumers use their own custom warning
mechanisms instead default klog warning.

* Use typed error instead plain string

* Fix interface change in unit test

staging/src/k8s.io/client-go/tools/clientcmd/loader.go

@@ -128,6 +128,28 @@ type ClientConfigLoadingRules struct {
 	// WarnIfAllMissing indicates whether the configuration files pointed by KUBECONFIG environment variable are present or not.
 	// In case of missing files, it warns the user about the missing files.
 	WarnIfAllMissing bool
+
+	// Warner is the warning log callback to use in case of missing files.
+	Warner WarningHandler
+}
+
+// WarningHandler allows to set the logging function to use
+type WarningHandler func(error)
+
+func (handler WarningHandler) Warn(err error) {
+	if handler == nil {
+		klog.V(1).Info(err)
+	} else {
+		handler(err)
+	}
+}
+
+type MissingConfigError struct {
+	Missing []string
+}
+
+func (c MissingConfigError) Error() string {
+	return fmt.Sprintf("Config not found: %s", strings.Join(c.Missing, ", "))
 }
 
 // ClientConfigLoadingRules implements the ClientConfigLoader interface.
@@ -219,7 +241,7 @@ func (rules *ClientConfigLoadingRules) Load() (*clientcmdapi.Config, error) {
 	}
 
 	if rules.WarnIfAllMissing && len(missingList) > 0 && len(kubeconfigs) == 0 {
-		klog.Warningf("Config not found: %s", strings.Join(missingList, ", "))
+		rules.Warner.Warn(MissingConfigError{Missing: missingList})
 	}
 
 	// first merge all of our maps

staging/src/k8s.io/client-go/tools/clientcmd/loader_test.go

@@ -18,6 +18,7 @@ package clientcmd
 
 import (
 	"bytes"
+	"flag"
 	"fmt"
 	"os"
 	"path"
@@ -32,6 +33,7 @@ import (
 	"k8s.io/apimachinery/pkg/runtime"
 	clientcmdapi "k8s.io/client-go/tools/clientcmd/api"
 	clientcmdlatest "k8s.io/client-go/tools/clientcmd/api/latest"
+	"k8s.io/klog/v2"
 )
 
 var (
@@ -120,14 +122,77 @@ func TestNonExistentCommandLineFile(t *testing.T) {
 }
 
 func TestToleratingMissingFiles(t *testing.T) {
+	envVarValue := "bogus"
 	loadingRules := ClientConfigLoadingRules{
 		Precedence:       []string{"bogus1", "bogus2", "bogus3"},
+		WarnIfAllMissing: true,
+		Warner:           func(err error) { klog.Warning(err) },
 	}
 
+	buffer := &bytes.Buffer{}
+
+	klog.LogToStderr(false)
+	klog.SetOutput(buffer)
+
 	_, err := loadingRules.Load()
 	if err != nil {
 		t.Fatalf("Unexpected error: %v", err)
 	}
+	klog.Flush()
+	expectedLog := fmt.Sprintf("Config not found: %s", envVarValue)
+	if !strings.Contains(buffer.String(), expectedLog) {
+		t.Fatalf("expected log: \"%s\"", expectedLog)
+	}
+}
+
+func TestWarningMissingFiles(t *testing.T) {
+	envVarValue := "bogus"
+	os.Setenv(RecommendedConfigPathEnvVar, envVarValue)
+	loadingRules := NewDefaultClientConfigLoadingRules()
+
+	buffer := &bytes.Buffer{}
+
+	flags := &flag.FlagSet{}
+	klog.InitFlags(flags)
+	flags.Set("v", "1")
+	klog.LogToStderr(false)
+	klog.SetOutput(buffer)
+
+	_, err := loadingRules.Load()
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	klog.Flush()
+
+	expectedLog := fmt.Sprintf("Config not found: %s", envVarValue)
+	if !strings.Contains(buffer.String(), expectedLog) {
+		t.Fatalf("expected log: \"%s\"", expectedLog)
+	}
+}
+
+func TestNoWarningMissingFiles(t *testing.T) {
+	envVarValue := "bogus"
+	os.Setenv(RecommendedConfigPathEnvVar, envVarValue)
+	loadingRules := NewDefaultClientConfigLoadingRules()
+
+	buffer := &bytes.Buffer{}
+
+	flags := &flag.FlagSet{}
+	klog.InitFlags(flags)
+	flags.Set("v", "0")
+	klog.LogToStderr(false)
+	klog.SetOutput(buffer)
+
+	_, err := loadingRules.Load()
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	klog.Flush()
+
+	logNotExpected := fmt.Sprintf("Config not found: %s", envVarValue)
+	if strings.Contains(buffer.String(), logNotExpected) {
+		t.Fatalf("log not expected: \"%s\"", logNotExpected)
+	}
 }
 
 func TestErrorReadingFile(t *testing.T) {