Add logic to notify service manager when start up is completed

cmd/yggdrasil/main.go

@@ -272,6 +272,10 @@ func main() {
 		}
 	}
 
+	if _, err = notifyStartupCompleted(); err != nil {
+		log.Warnln("Error while sending start up notification:", err)
+	}
+
 	// Block until we are told to shut down.
 	<-ctx.Done()
 

cmd/yggdrasil/notify_startup_linux.go

@@ -0,0 +1,50 @@
+//go:build linux
+// +build linux
+
+package main
+
+import (
+	"net"
+	"os"
+)
+
+// Notify systemd daemon when start up is completed.
+// Required to ensure that dependent services are started only after TUN interface is ready.
+//
+// One of the following is returned:
+// (false, nil) - notification not supported (i.e. `notifySocketEnv` is unset)
+// (false, err) - notification supported, but failure happened (e.g. error connecting to `notifySocketEnv` or while sending data)
+// (true, nil) - notification supported, data has been sent
+//
+// Based on `SdNotify` from [`coreos/go-systemd`](https://github.com/coreos/go-systemd/blob/7d375ecc2b092916968b5601f74cca28a8de45dd/daemon/sdnotify.go#L56)
+func notifyStartupCompleted() (bool, error) {
+	const (
+		notifyReady     = "READY=1"
+		notifySocketEnv = "NOTIFY_SOCKET"
+	)
+
+	socketAddr := &net.UnixAddr{
+		Name: os.Getenv(notifySocketEnv),
+		Net:  "unixgram",
+	}
+
+	// `notifySocketEnv` not set
+	if socketAddr.Name == "" {
+		return false, nil
+	}
+
+	os.Unsetenv(notifySocketEnv)
+
+	conn, err := net.DialUnix(socketAddr.Net, nil, socketAddr)
+	// Error connecting to `notifySocketEnv`
+	if err != nil {
+		return false, err
+	}
+	defer conn.Close()
+
+	if _, err = conn.Write([]byte(notifyReady)); err != nil {
+		return false, err
+	}
+
+	return true, nil
+}

cmd/yggdrasil/notify_startup_other.go

@@ -0,0 +1,8 @@
+//go:build !linux
+// +build !linux
+
+package main
+
+func notifyStartupCompleted() (bool, error) {
+	return false, nil
+}

contrib/systemd/yggdrasil.service

@@ -6,6 +6,10 @@ After=network-online.target
 After=yggdrasil-default-config.service
 
 [Service]
+Type=notify
+# Allow forked off processes to send notifications.
+# Uncomment if e.g. yggdrasil is started by a script.
+#NotifyAccess=all
 Group=yggdrasil
 ProtectHome=true
 ProtectSystem=true

contrib/systemd/yggdrasil.service.debian

@@ -6,6 +6,7 @@ After=network-online.target
 After=yggdrasil-default-config.service
 
 [Service]
+Type=notify
 Group=yggdrasil
 ProtectHome=true
 ProtectSystem=strict