Minor windows focused improvements (#443)

Co-authored-by: Wade Simmons <wadey@slack-corp.com>

cmd/nebula-service/logs_generic.go

@@ -0,0 +1,9 @@
+// +build !windows
+
+package main
+
+import "github.com/sirupsen/logrus"
+
+func HookLogger(l *logrus.Logger) {
+	// Do nothing, let the logs flow to stdout/stderr
+}

cmd/nebula-service/logs_windows.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/kardianos/service"
+	"github.com/sirupsen/logrus"
+)
+
+// HookLogger routes the logrus logs through the service logger so that they end up in the Windows Event Viewer
+// logrus output will be discarded
+func HookLogger(l *logrus.Logger) {
+	l.AddHook(newLogHook(logger))
+	l.SetOutput(ioutil.Discard)
+}
+
+type logHook struct {
+	sl service.Logger
+}
+
+func newLogHook(sl service.Logger) *logHook {
+	return &logHook{sl: sl}
+}
+
+func (h *logHook) Fire(entry *logrus.Entry) error {
+	line, err := entry.String()
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Unable to read entry, %v", err)
+		return err
+	}
+
+	switch entry.Level {
+	case logrus.PanicLevel:
+		return h.sl.Error(line)
+	case logrus.FatalLevel:
+		return h.sl.Error(line)
+	case logrus.ErrorLevel:
+		return h.sl.Error(line)
+	case logrus.WarnLevel:
+		return h.sl.Warning(line)
+	case logrus.InfoLevel:
+		return h.sl.Info(line)
+	case logrus.DebugLevel:
+		return h.sl.Info(line)
+	default:
+		return nil
+	}
+}
+
+func (h *logHook) Levels() []logrus.Level {
+	return logrus.AllLevels
+}

cmd/nebula-service/service.go

@@ -25,7 +25,7 @@ func (p *program) Start(s service.Service) error {
 	logger.Info("Nebula service starting.")
 
 	l := logrus.New()
-	l.Out = os.Stdout
+	HookLogger(l)
 
 	config := nebula.NewConfig(l)
 	err := config.Load(*p.configPath)
@@ -70,6 +70,10 @@ func doService(configPath *string, configTest *bool, build string, serviceFlag *
 		build:      build,
 	}
 
+	// Here are what the different loggers are doing:
+	// - `log` is the standard go log utility, meant to be used while the process is still attached to stdout/stderr
+	// - `logger` is the service log utility that may be attached to a special place depending on OS (Windows will have it attached to the event log)
+	// - above, in `Run` we create a `logrus.Logger` which is what nebula expects to use
 	s, err := service.New(prg, svcConfig)
 	if err != nil {
 		log.Fatal(err)
@@ -85,6 +89,7 @@ func doService(configPath *string, configTest *bool, build string, serviceFlag *
 		for {
 			err := <-errs
 			if err != nil {
+				// Route any errors from the system logger to stdout as a best effort to notice issues there
 				log.Print(err)
 			}
 		}
@@ -94,6 +99,7 @@ func doService(configPath *string, configTest *bool, build string, serviceFlag *
 	case "run":
 		err = s.Run()
 		if err != nil {
+			// Route any errors to the system logger
 			logger.Error(err)
 		}
 	default: