Add example for co-existence of klog v1 and v2

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

examples/coexist_klog_v1_and_v2/coexist_klog_v1_and_v2.go

@@ -0,0 +1,73 @@
+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
+// 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)")
+}

examples/coexist_klog_v1_and_v2/go.mod

@@ -0,0 +1,8 @@
+module k8s.io/klog/examples/coexist_klog_v1_and_v2
+
+go 1.13
+
+require (
+	k8s.io/klog v1.0.0
+	k8s.io/klog/v2 v2.0.0
+)