component-base/logs: add slog support

Integration of a slog-based package into a Kubernetes application is completely
transparent when that package uses the global slog default logger because
component-base will set that. When the package wants to be passed a logger
instance, then one can be constructed with slogr.NewSlogHandler.

Integration of a Kubernetes package into an application which uses a slog
Logger is a bit more work when configuring logging. The main binary must call
klog.SetLogger and needs logr/slogr to convert a slog.Handler to a logr.Logger.

staging/src/k8s.io/component-base/logs/api/v1/options.go

@@ -282,6 +282,7 @@ func apply(c *LoggingConfiguration, options *LoggingOptions, featureGate feature
 	if err := loggingFlags.Lookup("vmodule").Value.Set(VModuleConfigurationPflag(&c.VModule).String()); err != nil {
 		return fmt.Errorf("internal error while setting klog vmodule: %v", err)
 	}
+	setSlogDefaultLogger()
 	klog.StartFlushDaemon(c.FlushFrequency.Duration.Duration)
 	klog.EnableContextualLogging(p.ContextualLoggingEnabled)
 	return nil

staging/src/k8s.io/component-base/logs/api/v1/options_no_slog.go

@@ -0,0 +1,24 @@
+//go:build !go1.21
+// +build !go1.21
+
+/*
+Copyright 2023 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 v1
+
+func setSlogDefaultLogger() {
+	// Do nothing when build with Go < 1.21.
+}

staging/src/k8s.io/component-base/logs/api/v1/options_slog.go

@@ -0,0 +1,37 @@
+//go:build go1.21
+// +build go1.21
+
+/*
+Copyright 2023 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 v1
+
+import (
+	"log/slog"
+
+	"github.com/go-logr/logr"
+	"k8s.io/klog/v2"
+)
+
+// setSlogDefaultLogger sets the global slog default logger to the same default
+// that klog currently uses.
+func setSlogDefaultLogger() {
+	// klog.Background() always returns a valid logr.Logger, regardless of
+	// how logging was configured. We just need to turn it into a
+	// slog.Handler. SetDefault then needs a slog.Logger.
+	handler := logr.ToSlogHandler(klog.Background())
+	slog.SetDefault(slog.New(handler))
+}

staging/src/k8s.io/component-base/logs/example/README.md

@@ -5,6 +5,9 @@ component-base/logs and what effect the different command line options have.
 Like most Kubernetes components, `cmd` uses Cobra and pflags. `stdlib` uses
 just plain Go libraries. `test` contains a unit test with per-test output.
 
+`slog2k8s` shows how an application using `log/slog` from Go 1.21 can include
+packages from Kubernetes. `k8s2slog` is the other direction.
+
 Below we can see examples of how some features work.
 
 ## Default

staging/src/k8s.io/component-base/logs/example/k8s2slog/k8s2slog.go

@@ -0,0 +1,89 @@
+//go:build go1.21
+// +build go1.21
+
+/*
+Copyright 2018 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 main
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"os"
+	"strings"
+
+	"github.com/spf13/cobra"
+
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/util/runtime"
+	"k8s.io/component-base/cli"
+	"k8s.io/component-base/featuregate"
+	"k8s.io/component-base/logs"
+	logsapi "k8s.io/component-base/logs/api/v1"
+	"k8s.io/klog/v2"
+
+	_ "k8s.io/component-base/logs/json/register"
+)
+
+var featureGate = featuregate.NewFeatureGate()
+
+func main() {
+	runtime.Must(logsapi.AddFeatureGates(featureGate))
+	command := NewLoggerCommand()
+	code := cli.Run(command)
+	os.Exit(code)
+}
+
+func NewLoggerCommand() *cobra.Command {
+	c := logsapi.NewLoggingConfiguration()
+	cmd := &cobra.Command{
+		Run: func(cmd *cobra.Command, args []string) {
+			// This configures the global logger in klog *and* slog, if compiled
+			// with Go >= 1.21.
+			logs.InitLogs()
+			if err := logsapi.ValidateAndApply(c, featureGate); err != nil {
+				fmt.Fprintf(os.Stderr, "%v\n", err)
+				os.Exit(1)
+			}
+			if len(args) > 0 {
+				fmt.Fprintf(os.Stderr, "Unexpected additional command line arguments:\n    %s\n", strings.Join(args, "\n    "))
+				os.Exit(1)
+			}
+
+			// Produce some output. Special types used by Kubernetes work.
+			podRef := klog.KObj(&metav1.ObjectMeta{Name: "some-pod", Namespace: "some-namespace"})
+			podRefs := klog.KObjSlice([]interface{}{
+				&metav1.ObjectMeta{Name: "some-pod", Namespace: "some-namespace"},
+				nil,
+				&metav1.ObjectMeta{Name: "other-pod"},
+			})
+			slog.Info("slog.Info", "pod", podRef, "pods", podRefs)
+			klog.InfoS("klog.InfoS", "pod", podRef, "pods", podRefs)
+			klog.Background().Info("klog.Background+logr.Logger.Info")
+			klog.FromContext(context.Background()).Info("klog.FromContext+logr.Logger.Info")
+			slogLogger := slog.Default()
+			slogLogger.Info("slog.Default+slog.Logger.Info")
+		},
+	}
+	if err := logsapi.AddFeatureGates(featureGate); err != nil {
+		// Shouldn't happen.
+		panic(err)
+	}
+	featureGate.AddFlag(cmd.Flags())
+	logsapi.AddFlags(c, cmd.Flags())
+	return cmd
+}

staging/src/k8s.io/component-base/logs/example/slog2k8s/slog2k8s.go

@@ -0,0 +1,50 @@
+//go:build go1.21
+// +build go1.21
+
+/*
+Copyright 2023 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.
+*/
+
+// slog2k8s demonstrates how an application using log/slog for logging
+// can include Kubernetes packages.
+package main
+
+import (
+	"context"
+	"log/slog"
+	"os"
+
+	"k8s.io/klog/v2"
+)
+
+func main() {
+	options := slog.HandlerOptions{AddSource: true}
+	textHandler := slog.NewTextHandler(os.Stderr, &options)
+	textLogger := slog.New(textHandler)
+
+	// Use text output as default logger.
+	slog.SetDefault(textLogger)
+
+	// This also needs to be done through klog to ensure that all code
+	// using klog uses the text handler. klog.Background/TODO/FromContext
+	// will return a thin wrapper around the textHandler, so all that klog
+	// still does is manage the global default and retrieval from contexts.
+	klog.SetSlogLogger(textLogger)
+
+	textLogger.Info("slog.Logger.Info")
+	klog.InfoS("klog.InfoS")
+	klog.Background().Info("klog.Background+logr.Logger.Info")
+	klog.FromContext(context.Background()).Info("klog.FromContext+logr.Logger.Info")
+}