Merge pull request #62515 from resouer/fix-58805

Automatic merge from submit-queue (batch tested with PRs 62481, 62643, 61877, 62515). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add write-config-to to scheduler

**What this PR does / why we need it**:
Scheduler should be able to write its default configure to file. This actually applies to all components which claims options other than `--config` will be deprecated.

Otherwise, users will be super confused to find out how to write a proper config file to these components.

See: https://stackoverflow.com/questions/47966440/how-to-create-a-config-file-for-kube-scheduler-to-use-by-the-config-argument
ref: #52562

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #58805

Usage:

```bash
./_output/bin/kube-scheduler --write-config-to /tmp/kube-scheduler.yaml
```

**Special notes for your reviewer**:
This should have been fixed several releases ago, so lets include it in 1.11

**Release note**:

```release-note
Add write-config-to to scheduler
```

cmd/kube-scheduler/app/BUILD

@@ -35,6 +35,7 @@ go_library(
         "//vendor/k8s.io/apimachinery/pkg/apis/meta/v1:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/runtime:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/runtime/serializer:go_default_library",
+        "//vendor/k8s.io/apimachinery/pkg/runtime/serializer/json:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/util/runtime:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/util/uuid:go_default_library",
         "//vendor/k8s.io/apimachinery/pkg/util/wait:go_default_library",

cmd/kube-scheduler/app/server.go

@@ -33,6 +33,7 @@ import (
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
 	"k8s.io/apimachinery/pkg/runtime"
 	"k8s.io/apimachinery/pkg/runtime/serializer"
+	"k8s.io/apimachinery/pkg/runtime/serializer/json"
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
 	"k8s.io/apimachinery/pkg/util/uuid"
 	"k8s.io/apimachinery/pkg/util/wait"
@@ -80,6 +81,9 @@ type Options struct {
 	// ConfigFile is the location of the scheduler server's configuration file.
 	ConfigFile string
 
+	// WriteConfigTo is the path where the default configuration will be written.
+	WriteConfigTo string
+
 	// config is the scheduler server's configuration object.
 	config *componentconfig.KubeSchedulerConfiguration
 
@@ -106,6 +110,7 @@ type Options struct {
 // AddFlags adds flags for a specific SchedulerServer to the specified FlagSet
 func (o *Options) AddFlags(fs *pflag.FlagSet) {
 	fs.StringVar(&o.ConfigFile, "config", o.ConfigFile, "The path to the configuration file.")
+	fs.StringVar(&o.WriteConfigTo, "write-config-to", o.WriteConfigTo, "If set, write the configuration values to this file and exit.")
 
 	// All flags below here are deprecated and will eventually be removed.
 
@@ -167,13 +172,25 @@ func NewOptions() (*Options, error) {
 }
 
 func (o *Options) Complete() error {
-	if len(o.ConfigFile) == 0 {
-		glog.Warning("WARNING: all flags other than --config are deprecated. Please begin using a config file ASAP.")
+	if len(o.ConfigFile) == 0 && len(o.WriteConfigTo) == 0 {
+		glog.Warning("WARNING: all flags other than --config, --write-config-to, and --cleanup are deprecated. Please begin using a config file ASAP.")
 		o.applyDeprecatedHealthzAddressToConfig()
 		o.applyDeprecatedHealthzPortToConfig()
 		o.applyDeprecatedAlgorithmSourceOptionsToConfig()
 	}
 
+	if len(o.ConfigFile) > 0 {
+		if c, err := o.loadConfigFromFile(o.ConfigFile); err != nil {
+			return err
+		} else {
+			o.config = c
+		}
+	}
+
+	// Apply algorithms based on feature gates.
+	// TODO: make configurable?
+	algorithmprovider.ApplyFeatureGates()
+
 	return nil
 }
 
@@ -303,21 +320,12 @@ func (o *Options) ApplyDefaults(in *componentconfig.KubeSchedulerConfiguration)
 }
 
 func (o *Options) Run() error {
-	config := o.config
-
-	if len(o.ConfigFile) > 0 {
-		if c, err := o.loadConfigFromFile(o.ConfigFile); err != nil {
-			return err
-		} else {
-			config = c
-		}
+	// If user only want to generate a configure file.
+	if len(o.WriteConfigTo) > 0 {
+		return o.writeConfigFile()
 	}
 
-	// Apply algorithms based on feature gates.
-	// TODO: make configurable?
-	algorithmprovider.ApplyFeatureGates()
-
-	server, err := NewSchedulerServer(config, o.master)
+	server, err := NewSchedulerServer(o.config, o.master)
 	if err != nil {
 		return err
 	}
@@ -326,6 +334,36 @@ func (o *Options) Run() error {
 	return server.Run(stop)
 }
 
+func (o *Options) writeConfigFile() error {
+	var encoder runtime.Encoder
+	mediaTypes := o.codecs.SupportedMediaTypes()
+	for _, info := range mediaTypes {
+		if info.MediaType == "application/yaml" {
+			encoder = info.Serializer
+			break
+		}
+	}
+	if encoder == nil {
+		return errors.New("unable to locate yaml encoder")
+	}
+	encoder = json.NewYAMLSerializer(json.DefaultMetaFactory, o.scheme, o.scheme)
+	encoder = o.codecs.EncoderForVersion(encoder, componentconfigv1alpha1.SchemeGroupVersion)
+
+	configFile, err := os.Create(o.WriteConfigTo)
+	if err != nil {
+		return err
+	}
+	defer configFile.Close()
+
+	if err := encoder.Encode(o.config, configFile); err != nil {
+		return err
+	}
+
+	glog.Infof("Wrote configuration to: %s\n", o.WriteConfigTo)
+
+	return nil
+}
+
 // NewSchedulerCommand creates a *cobra.Command object with default parameters
 func NewSchedulerCommand() *cobra.Command {
 	opts, err := NewOptions()