ethstats: avoid creating subscriptions on background goroutine (#22587)

This fixes an issue where the ethstats service could crash if geth was
started and then immediately stopped due to an internal error. The
cause of the crash was a nil subscription being returned by the backend,
because the background goroutine creating them was scheduled after
the backend had already shut down.

Moving the creation of subscriptions into the Start method, which runs
synchronously during startup of the node, means the returned subscriptions
can never be 'nil'.

Co-authored-by: Felix Lange <fjl@twurst.com>
Co-authored-by: Martin Holst Swende <martin@swende.se>

ethstats/ethstats.go

@@ -95,6 +95,8 @@ type Service struct {
 	pongCh chan struct{} // Pong notifications are fed into this channel
 	histCh chan []uint64 // History request block numbers are fed into this channel
 
+	headSub event.Subscription
+	txSub   event.Subscription
 }
 
 // connWrapper is a wrapper to prevent concurrent-write or concurrent-read on the
@@ -167,7 +169,12 @@ func New(node *node.Node, backend backend, engine consensus.Engine, url string)
 
 // Start implements node.Lifecycle, starting up the monitoring and reporting daemon.
 func (s *Service) Start() error {
-	go s.loop()
+	// Subscribe to chain events to execute updates on
+	chainHeadCh := make(chan core.ChainHeadEvent, chainHeadChanSize)
+	s.headSub = s.backend.SubscribeChainHeadEvent(chainHeadCh)
+	txEventCh := make(chan core.NewTxsEvent, txChanSize)
+	s.txSub = s.backend.SubscribeNewTxsEvent(txEventCh)
+	go s.loop(chainHeadCh, txEventCh)
 
 	log.Info("Stats daemon started")
 	return nil
@@ -175,22 +182,15 @@ func (s *Service) Start() error {
 
 // Stop implements node.Lifecycle, terminating the monitoring and reporting daemon.
 func (s *Service) Stop() error {
+	s.headSub.Unsubscribe()
+	s.txSub.Unsubscribe()
 	log.Info("Stats daemon stopped")
 	return nil
 }
 
 // loop keeps trying to connect to the netstats server, reporting chain events
 // until termination.
-func (s *Service) loop() {
+func (s *Service) loop(chainHeadCh chan core.ChainHeadEvent, txEventCh chan core.NewTxsEvent) {
-	// Subscribe to chain events to execute updates on
-	chainHeadCh := make(chan core.ChainHeadEvent, chainHeadChanSize)
-	headSub := s.backend.SubscribeChainHeadEvent(chainHeadCh)
-	defer headSub.Unsubscribe()
-
-	txEventCh := make(chan core.NewTxsEvent, txChanSize)
-	txSub := s.backend.SubscribeNewTxsEvent(txEventCh)
-	defer txSub.Unsubscribe()
-
 	// Start a goroutine that exhausts the subscriptions to avoid events piling up
 	var (
 		quitCh = make(chan struct{})
@@ -223,9 +223,9 @@ func (s *Service) loop() {
 				}
 
 			// node stopped
-			case <-txSub.Err():
+			case <-s.txSub.Err():
 				break HandleLoop
-			case <-headSub.Err():
+			case <-s.headSub.Err():
 				break HandleLoop
 			}
 		}