Really notify systemd, don't just say we do

Doh, don't know how this was missed until now.

serverutil/http.go

@@ -13,6 +13,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/coreos/go-systemd/daemon"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
@@ -82,7 +83,8 @@ func Serve(h http.Handler, config *ServerConfig, addr string) error {
 		return err
 	}
 
-	// Create the net.Listener, then notify systemd that we are ready to serve.
+	// Create the net.Listener first, so we can detect
+	// initialization-time errors safely.
 	l, err := net.Listen("tcp", addr)
 	if err != nil {
 		return err
@@ -113,6 +115,9 @@ func Serve(h http.Handler, config *ServerConfig, addr string) error {
 
 	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
 
+	// Notify systemd that we are ready to serve.
+	daemon.SdNotify(false, "READY=1")
+
 	err = srv.Serve(l)
 	if err != http.ErrServerClosed {
 		return err