dcrwallet: Fix spv syncer loop panic

The SPVSyncer.Run() method would not previously error out when running
out of peers. Now it does, leading to an error when re-running it.

This was fixed in dcrwallet, but dcrlnd needs a copy of that code
(because spvLoop lives in the main package).

Author: matheusd

lnwallet/dcrwallet/spvsync.go

@@ -28,18 +28,19 @@ type SPVSyncer struct {
 	cfg *SPVSyncerConfig
 	wg  sync.WaitGroup
 
-	mtx sync.Mutex
-
-	// The following fields are protected by mtx.
-
-	cancel func()
+	// runCtx is canceled when Stop() is called.
+	runCtx    context.Context
+	runCancel func()
 }
 
 // NewSPVSyncer initializes a new syncer backed by the dcrd network in SPV
 // mode.
 func NewSPVSyncer(cfg *SPVSyncerConfig) (*SPVSyncer, error) {
+	ctx, cancel := context.WithCancel(context.Background())
 	return &SPVSyncer{
-		cfg: cfg,
+		cfg:       cfg,
+		runCtx:    ctx,
+		runCancel: cancel,
 	}, nil
 }
 
@@ -56,56 +57,45 @@ func (s *SPVSyncer) start(w *DcrWallet) error {
 	addr := &net.TCPAddr{IP: net.ParseIP("::1"), Port: 0}
 	amgrDir := filepath.Join(s.cfg.AppDataDir, s.cfg.Net.Name)
 	amgr := addrmgr.New(amgrDir, lookup)
-	lp := p2p.NewLocalPeer(s.cfg.Net, addr, amgr)
-	if s.cfg.DialFunc != nil {
-		lp.SetDialFunc(s.cfg.DialFunc)
-	}
-	lp.SetDisableRelayTx(s.cfg.DisableRelayTx)
-
-	syncer := spv.NewSyncer(w.wallet, lp)
-	if len(s.cfg.Peers) > 0 {
-		syncer.SetPersistentPeers(s.cfg.Peers)
-	}
-	w.wallet.SetNetworkBackend(syncer)
-
-	if disableDiscoverAccts {
-		syncer.DisableDiscoverAccounts()
-	}
-
-	syncer.SetNotifications(&spv.Notifications{
-		Synced: w.onSyncerSynced,
-	})
-
-	// This context will be canceled by `w` once its Stop() method is
-	// called.
-	ctx, cancel := context.WithCancel(context.Background())
-	s.mtx.Lock()
-	s.cancel = cancel
-	s.mtx.Unlock()
 
 	s.wg.Add(1)
 	go func() {
 		defer s.wg.Done()
 
 		for {
 			dcrwLog.Debugf("Starting SPV syncer")
+			lp := p2p.NewLocalPeer(s.cfg.Net, addr, amgr)
+			if s.cfg.DialFunc != nil {
+				lp.SetDialFunc(s.cfg.DialFunc)
+			}
+			lp.SetDisableRelayTx(s.cfg.DisableRelayTx)
+
+			syncer := spv.NewSyncer(w.wallet, lp)
 			if len(s.cfg.Peers) > 0 {
 				dcrwLog.Debugf("Forcing SPV to peers: %s", s.cfg.Peers)
+				syncer.SetPersistentPeers(s.cfg.Peers)
 			}
 
-			err := syncer.Run(ctx)
-			select {
-			case <-ctx.Done():
+			if disableDiscoverAccts {
+				syncer.DisableDiscoverAccounts()
+			}
+
+			syncer.SetNotifications(&spv.Notifications{
+				Synced: w.onSyncerSynced,
+			})
+
+			err := syncer.Run(s.runCtx)
+			if err == nil || s.runCtx.Err() != nil {
 				// stop() requested.
 				return
-			default:
-				w.rpcSyncerFinished()
-				dcrwLog.Errorf("SPV synchronization ended: %v", err)
 			}
 
+			w.rpcSyncerFinished()
+			dcrwLog.Errorf("SPV synchronization ended: %v", err)
+
 			// Backoff for 5 seconds.
 			select {
-			case <-ctx.Done():
+			case <-s.runCtx.Done():
 				// Graceful shutdown.
 				dcrwLog.Debugf("RPCsyncer shutting down")
 				return
@@ -119,12 +109,7 @@ func (s *SPVSyncer) start(w *DcrWallet) error {
 
 func (s *SPVSyncer) stop() {
 	dcrwLog.Debugf("SPVSyncer requested shutdown")
-	s.mtx.Lock()
-	if s.cancel != nil {
-		s.cancel()
-		s.cancel = nil
-	}
-	s.mtx.Unlock()
+	s.runCancel()
 }
 
 func (s *SPVSyncer) waitForShutdown() {