Kubelet Startup - Log Initialization

import (
	"math/rand"
	"os"
	"time"

	"k8s.io/component-base/logs"
	_ "k8s.io/component-base/metrics/prometheus/restclient"
	_ "k8s.io/component-base/metrics/prometheus/version" // for version metric registration
	"k8s.io/kubernetes/cmd/kubelet/app"
)

func main() {
	rand.Seed(time.Now().UnixNano())

	command := app.NewKubeletCommand()
	logs.InitLogs()
	defer logs.FlushLogs()

	if err := command.Execute(); err != nil {
		os.Exit(1)
	}
}

// init sets up the defaults and runs flushDaemon.
func init() {
	logging.stderrThreshold = errorLog // Default stderrThreshold is ERROR.
	logging.setVState(0, nil, false)
	logging.logDir = ""
	logging.logFile = ""
	logging.logFileMaxSizeMB = 1800
	logging.toStderr = true
	logging.alsoToStderr = false
	logging.skipHeaders = false
	logging.addDirHeader = false
	logging.skipLogHeaders = false
	go logging.flushDaemon()
}

const flushInterval = 5 * time.Second

// flushDaemon periodically flushes the log file buffers.
func (l *loggingT) flushDaemon() {
	for range time.NewTicker(flushInterval).C {
		l.lockAndFlushAll()
	}
}

// lockAndFlushAll is like flushAll but locks l.mu first.
func (l *loggingT) lockAndFlushAll() {
	l.mu.Lock()
	l.flushAll()
	l.mu.Unlock()
}

// flushAll flushes all the logs and attempts to "sync" their data to disk.
// l.mu is held.
func (l *loggingT) flushAll() {
	// Flush from fatal down, in case there's trouble flushing.
	for s := fatalLog; s >= infoLog; s-- {
		file := l.file[s]
		if file != nil {
			file.Flush() // ignore error
			file.Sync()  // ignore error
		}
	}
}

// InitFlags is for explicitly initializing the flags.
func InitFlags(flagset *flag.FlagSet) {
	if flagset == nil {
		flagset = flag.CommandLine
	}

	flagset.StringVar(&logging.logDir, "log_dir", logging.logDir, "If non-empty, write log files in this directory")
	flagset.StringVar(&logging.logFile, "log_file", logging.logFile, "If non-empty, use this log file")
	flagset.Uint64Var(&logging.logFileMaxSizeMB, "log_file_max_size", logging.logFileMaxSizeMB,
		"Defines the maximum size a log file can grow to. Unit is megabytes. "+
			"If the value is 0, the maximum file size is unlimited.")
	flagset.BoolVar(&logging.toStderr, "logtostderr", logging.toStderr, "log to standard error instead of files")
	flagset.BoolVar(&logging.alsoToStderr, "alsologtostderr", logging.alsoToStderr, "log to standard error as well as files")
	flagset.Var(&logging.verbosity, "v", "number for the log level verbosity")
	flagset.BoolVar(&logging.skipHeaders, "add_dir_header", logging.addDirHeader, "If true, adds the file directory to the header")
	flagset.BoolVar(&logging.skipHeaders, "skip_headers", logging.skipHeaders, "If true, avoid header prefixes in the log messages")
	flagset.BoolVar(&logging.skipLogHeaders, "skip_log_headers", logging.skipLogHeaders, "If true, avoid headers when opening log files")
	flagset.Var(&logging.stderrThreshold, "stderrthreshold", "logs at or above this threshold go to stderr")
	flagset.Var(&logging.vmodule, "vmodule", "comma-separated list of pattern=N settings for file-filtered logging")
	flagset.Var(&logging.traceLocation, "log_backtrace_at", "when logging hits line file:N, emit a stack trace")
}

// Write implements the io.Writer interface.
func (writer KlogWriter) Write(data []byte) (n int, err error) {
	klog.InfoDepth(1, string(data))
	return len(data), nil
}

// InitLogs initializes logs the way we want for kubernetes.
func InitLogs() {
	// Compatibility with log for logging, but effectively using klog.
	log.SetOutput(KlogWriter{})
	// Set log output format to empty; log formatting is determined by klog
	log.SetFlags(0)
	// The default glog flush interval is 5 seconds.
	go wait.Forever(klog.Flush, *logFlushFreq)
}

// AddGlobalFlags explicitly registers flags that libraries (glog, verflag, etc.) register
// against the global flagsets from "flag" and "github.com/spf13/pflag".
// We do this in order to prevent unwanted flags from leaking into the Kubelet's flagset.
func AddGlobalFlags(fs *pflag.FlagSet) {
	addKlogFlags(fs)
	addCadvisorFlags(fs)
	addCredentialProviderFlags(fs)
	verflag.AddFlags(fs)
	logs.AddFlags(fs)
}

// addKlogFlags adds flags from k8s.io/klog
func addKlogFlags(fs *pflag.FlagSet) {
	local := flag.NewFlagSet(os.Args[0], flag.ExitOnError)
	klog.InitFlags(local)
	fs.AddGoFlagSet(local)
}