cmd/syncthing: Send Ping events to the disk events API

The Ping event is important, as it means that requests complete within
a sensible time. The disk events API didn't have the Ping event, so
if there were no disk events, the request would keep taking forever.
Unless, of course, there's a reverse proxy which times the request out
after a suitably large interval (or something else aborts it), in which
case Syncthing isn't very happy.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3929

cmd/syncthing/main.go

@@ -584,7 +584,7 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 	// events. The LocalChangeDetected event might overwhelm the event
 	// receiver in some situations so we will not subscribe to it here.
 	apiSub := events.NewBufferedSubscription(events.Default.Subscribe(events.AllEvents&^events.LocalChangeDetected&^events.RemoteChangeDetected), 1000)
-	diskSub := events.NewBufferedSubscription(events.Default.Subscribe(events.LocalChangeDetected|events.RemoteChangeDetected), 1000)
+	diskSub := events.NewBufferedSubscription(events.Default.Subscribe(events.LocalChangeDetected|events.RemoteChangeDetected|events.Ping), 1000)
 
 	if len(os.Getenv("GOMAXPROCS")) == 0 {
 		runtime.GOMAXPROCS(runtime.NumCPU())