Use sdnotify for systemd (#768)

* Use sdnotify for systemd

This is useful if a configuration is long to load.
Systemd will continue dependency chain only when server have finish to start.

https://www.freedesktop.org/software/systemd/man/systemd.service.html#Type=

* Extract the waiting behavior from Start()

contrib/systemd/traefik.service

@@ -2,6 +2,7 @@
 Description=Traefik
 
 [Service]
+Type=notify
 ExecStart=/usr/bin/traefik --configFile=/etc/traefik.toml
 Restart=on-failure
 

glide.yaml

@@ -102,3 +102,7 @@ import:
 - package: github.com/docker/leadership
 - package: github.com/satori/go.uuid
   version: ^1.1.0
+- package: github.com/coreos/go-systemd
+  version: v12
+  subpackages:
+  - daemon

server.go

@@ -91,7 +91,7 @@ func NewServer(globalConfiguration GlobalConfiguration) *Server {
 	return server
 }
 
-// Start starts the server and blocks until server is shutted down.
+// Start starts the server.
 func (server *Server) Start() {
 	server.startHTTPServers()
 	server.startLeadership()
@@ -104,6 +104,10 @@ func (server *Server) Start() {
 	server.configureProviders()
 	server.startProviders()
 	go server.listenSignals()
+}
+
+// Wait blocks until server is shutted down.
+func (server *Server) Wait() {
 	<-server.stopChan
 }
 

traefik.go

@@ -24,6 +24,8 @@ import (
 	"github.com/containous/traefik/version"
 	"github.com/docker/libkv/store"
 	"github.com/satori/go.uuid"
+
+	"github.com/coreos/go-systemd/daemon"
 )
 
 var versionTemplate = `Version:      {{.Version}}
@@ -268,6 +270,11 @@ func run(traefikConfiguration *TraefikConfiguration) {
 	server := NewServer(globalConfiguration)
 	server.Start()
 	defer server.Close()
+	sent, err := daemon.SdNotify("READY=1")
+	if !sent && err != nil {
+		log.Error("Fail to notify", err)
+	}
+	server.Wait()
 	log.Info("Shutting down")
 }