coexist_klog_v1_and_v2.go

package main

import (
	"flag"

	klogv1 "k8s.io/klog"
	klogv2 "k8s.io/klog/v2"
)

// OutputCallDepth is the stack depth where we can find the origin of this call
const OutputCallDepth = 6

// DefaultPrefixLength is the length of the log prefix that we have to strip out
const DefaultPrefixLength = 53

// klogWriter is used in SetOutputBySeverity call below to redirect
// any calls to klogv1 to end up in klogv2
type klogWriter struct{}

func (kw klogWriter) Write(p []byte) (n int, err error) {
	if len(p) < DefaultPrefixLength {
		klogv2.InfoDepth(OutputCallDepth, string(p))
		return len(p), nil
	}
	if p[0] == 'I' {
		klogv2.InfoDepth(OutputCallDepth, string(p[DefaultPrefixLength:]))
	} else if p[0] == 'W' {
		klogv2.WarningDepth(OutputCallDepth, string(p[DefaultPrefixLength:]))
	} else if p[0] == 'E' {
		klogv2.ErrorDepth(OutputCallDepth, string(p[DefaultPrefixLength:]))
	} else if p[0] == 'F' {
		klogv2.FatalDepth(OutputCallDepth, string(p[DefaultPrefixLength:]))
	} else {
		klogv2.InfoDepth(OutputCallDepth, string(p[DefaultPrefixLength:]))
	}
	return len(p), nil
}

func main() {
	// initialize klog/v2, can also bind to a local flagset if desired
	klogv2.InitFlags(nil)

	// In this example, we want to show you that all the lines logged
	// end up in the myfile.log. You do NOT need them in your application
	// as all these flags are set up from the command line typically
	flag.Set("logtostderr", "false")     // By default klog logs to stderr, switch that off
	flag.Set("alsologtostderr", "false") // false is default, but this is informative
	flag.Set("stderrthreshold", "FATAL") // stderrthreshold defaults to ERROR, we don't want anything in stderr
	flag.Set("log_file", "myfile.log")   // log to a file

	// parse klog/v2 flags
	flag.Parse()
	// make sure we flush before exiting
	defer klogv2.Flush()

	// BEGIN : hack to redirect klogv1 calls to klog v2
	// Tell klog NOT to log into STDERR. Otherwise, we risk
	// certain kinds of API errors getting logged into a directory not
	// available in a `FROM scratch` Docker container, causing us to abort
	var klogv1Flags flag.FlagSet
	klogv1.InitFlags(&klogv1Flags)
	klogv1Flags.Set("logtostderr", "false")     // By default klog v1 logs to stderr, switch that off
	klogv1Flags.Set("stderrthreshold", "FATAL") // stderrthreshold defaults to ERROR, use this if you
	// don't want anything in your stderr

	klogv1.SetOutputBySeverity("INFO", klogWriter{}) // tell klog v1 to use the writer
	// END : hack to redirect klogv1 calls to klog v2

	// Now you can mix klogv1 and v2 in the same code base
	klogv2.Info("hello from klog (v2)!")
	klogv1.Info("hello from klog (v1)!")
	klogv1.Warning("beware from klog (v1)!")
	klogv1.Error("error from klog (v1)!")
	klogv2.Info("nice to meet you (v2)")
}