fleetd: prevent to run more than one fleetd deamon

Add /var/run/fleet.pid to prevent running more than
one fleetd deamon

Fixes coreos#1220

fleetd/fleet.go

@@ -18,8 +18,10 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"os/signal"
+	"strconv"
 	"strings"
 	"syscall"
 
@@ -35,6 +37,7 @@ import (
 
 const (
 	DefaultConfigFile = "/etc/fleet/fleet.conf"
+	PidFile           = "/var/run/fleet.pid"
 )
 
 func main() {
@@ -53,6 +56,11 @@ func main() {
 		os.Exit(0)
 	}
 
+	if err := createPidFile(PidFile); err != nil {
+		fmt.Println(err)
+		os.Exit(1)
+	}
+
 	log.Infof("Starting fleetd version %v", version.Version)
 
 	cfgset := flag.NewFlagSet("fleet", flag.ExitOnError)
@@ -105,6 +113,7 @@ func main() {
 		log.Infof("Gracefully shutting down")
 		srv.Stop()
 		srv.Purge()
+		removePidFile(PidFile)
 		os.Exit(0)
 	}
 
@@ -213,6 +222,33 @@ func listenForSignals(sigmap map[os.Signal]func()) {
 	}
 }
 
+func createPidFile(pidfile string) error {
+	if pidstring, err := ioutil.ReadFile(pidfile); err == nil {
+		pid, err := strconv.Atoi(string(pidstring))
+		if err == nil {
+			if _, err := os.Stat(fmt.Sprintf("/proc/%d/", pid)); err == nil {
+				return fmt.Errorf("pid file found, ensure fleetd is not running or delete %s", pidfile)
+			}
+		}
+	}
+
+	file, err := os.Create(pidfile)
+	if err != nil {
+		return err
+	}
+
+	defer file.Close()
+
+	_, err = fmt.Fprintf(file, "%d", os.Getpid())
+	return err
+}
+
+func removePidFile(pidfile string) {
+	if err := os.Remove(pidfile); err != nil {
+		log.Error("Error removing %s: %s", pidfile, err)
+	}
+}
+
 type stringSlice []string
 
 func (f *stringSlice) Set(value string) error {