fleetd: protect access to global Server instance

There are three different paths in the main fleetd goroutine that can
access the global `srv` Server - reconfigurations, shutdowns and
statedumps. Right now there's nothing preventing racy access to this
instance, so introduce a mutex to protect it.

One potential issue with this is that it means that a reconfigure or
state dump can "block" a shutdown, but IMHO if this occurs it will
expose behaviour that is broken and needs to be fixed anyway.

fleetd/fleet.go

@@ -23,6 +23,7 @@ import (
 	"os"
 	"os/signal"
 	"strings"
+	"sync"
 	"syscall"
 
 	"github.com/coreos/fleet/Godeps/_workspace/src/github.com/rakyll/globalconf"
@@ -85,7 +86,11 @@ func main() {
 	}
 	srv.Run()
 
+	srvMutex := sync.Mutex{}
+
 	reconfigure := func() {
+		srvMutex.Lock()
+		defer srvMutex.Unlock()
 		log.Infof("Reloading configuration from %s", *cfgPath)
 
 		cfg, err := getConfig(cfgset, *cfgPath)
@@ -105,12 +110,16 @@ func main() {
 
 	shutdown := func() {
 		log.Infof("Gracefully shutting down")
+		srvMutex.Lock()
+		defer srvMutex.Unlock()
 		srv.Stop()
 		srv.Purge()
 		os.Exit(0)
 	}
 
 	writeState := func() {
+		srvMutex.Lock()
+		defer srvMutex.Unlock()
 		log.Infof("Dumping server state")
 
 		encoded, err := json.Marshal(srv)