Attempt to notify systemd of service readiness on linux (#929)

cmd/nebula/main.go

@@ -65,6 +65,7 @@ func main() {
 
 	if !*configTest {
 		ctrl.Start()
+		notifyReady(l)
 		ctrl.ShutdownBlock()
 	}
 

cmd/nebula/notify_linux.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"net"
+	"os"
+	"time"
+
+	"github.com/sirupsen/logrus"
+)
+
+// SdNotifyReady tells systemd the service is ready and dependent services can now be started
+// https://www.freedesktop.org/software/systemd/man/sd_notify.html
+// https://www.freedesktop.org/software/systemd/man/systemd.service.html
+const SdNotifyReady = "READY=1"
+
+func notifyReady(l *logrus.Logger) {
+	sockName := os.Getenv("NOTIFY_SOCKET")
+	if sockName == "" {
+		l.Debugln("NOTIFY_SOCKET systemd env var not set, not sending ready signal")
+		return
+	}
+
+	conn, err := net.DialTimeout("unixgram", sockName, time.Second)
+	if err != nil {
+		l.WithError(err).Error("failed to connect to systemd notification socket")
+		return
+	}
+	defer conn.Close()
+
+	err = conn.SetWriteDeadline(time.Now().Add(time.Second))
+	if err != nil {
+		l.WithError(err).Error("failed to set the write deadline for the systemd notification socket")
+		return
+	}
+
+	if _, err = conn.Write([]byte(SdNotifyReady)); err != nil {
+		l.WithError(err).Error("failed to signal the systemd notification socket")
+		return
+	}
+
+	l.Debugln("notified systemd the service is ready")
+}

cmd/nebula/notify_notlinux.go

@@ -0,0 +1,10 @@
+//go:build !linux
+// +build !linux
+
+package main
+
+import "github.com/sirupsen/logrus"
+
+func notifyReady(_ *logrus.Logger) {
+	// No init service to notify
+}