Hold the event source when there are no listeners (#15725)

* Hold the event source when there are no listeners

The event source does not need to run when there are no listeners. Therefore
pause it when there are none.

* add some more logging

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/eventsource/manager.go

@@ -13,6 +13,7 @@ type Manager struct {
 	mutex sync.Mutex
 
 	messengers map[int64]*Messenger
+	connection chan struct{}
 }
 
 var manager *Manager
@@ -20,6 +21,7 @@ var manager *Manager
 func init() {
 	manager = &Manager{
 		messengers: make(map[int64]*Messenger),
+		connection: make(chan struct{}, 1),
 	}
 }
 
@@ -36,6 +38,10 @@ func (m *Manager) Register(uid int64) <-chan *Event {
 		messenger = NewMessenger(uid)
 		m.messengers[uid] = messenger
 	}
+	select {
+	case m.connection <- struct{}{}:
+	default:
+	}
 	m.mutex.Unlock()
 	return messenger.Register()
 }