spv: Teardown syncer after all peers are lost

Recreate a new syncer in the SPV reconnect loop in main.  This will allow
initial sync logic to be reperformed if peers can be connected later.

This also adds a forced 5s backoff before peers connection attempts are made
again, matching the backoff behavior for RPC syncing.

dcrwallet.go

@@ -523,21 +523,26 @@ func spvLoop(ctx context.Context, w *wallet.Wallet) {
 	addr := &net.TCPAddr{IP: net.ParseIP("::1"), Port: 0}
 	amgrDir := filepath.Join(cfg.AppDataDir.Value, w.ChainParams().Name)
 	amgr := addrmgr.New(amgrDir, cfg.lookup)
-	lp := p2p.NewLocalPeer(w.ChainParams(), addr, amgr)
-	lp.SetDialFunc(cfg.dial)
-	lp.SetDisableRelayTx(cfg.SPVDisableRelayTx)
-	syncer := spv.NewSyncer(w, lp)
-	if len(cfg.SPVConnect) > 0 {
-		syncer.SetPersistentPeers(cfg.SPVConnect)
-	}
-	w.SetNetworkBackend(syncer)
 	for {
+		lp := p2p.NewLocalPeer(w.ChainParams(), addr, amgr)
+		lp.SetDialFunc(cfg.dial)
+		lp.SetDisableRelayTx(cfg.SPVDisableRelayTx)
+		syncer := spv.NewSyncer(w, lp)
+		if len(cfg.SPVConnect) > 0 {
+			syncer.SetPersistentPeers(cfg.SPVConnect)
+		}
+		w.SetNetworkBackend(syncer)
 		err := syncer.Run(ctx)
-		if done(ctx) {
+		if err == nil || done(ctx) {
 			loggers.SyncLog.Infof("SPV synchronization stopped")
 			return
 		}
 		loggers.SyncLog.Errorf("SPV synchronization stopped: %v", err)
+		select {
+		case <-ctx.Done():
+			return
+		case <-time.After(5 * time.Second):
+		}
 	}
 }
 

spv/sync.go

@@ -92,9 +92,10 @@ type Syncer struct {
 	mempool     sync.Map // k=chainhash.Hash v=*wire.MsgTx
 	mempoolAdds chan *chainhash.Hash
 
-	done   chan struct{}
-	err    error
-	doneMu sync.Mutex
+	teardown func()
+	done     chan struct{}
+	err      error
+	doneMu   sync.Mutex
 }
 
 // Notifications struct to contain all of the upcoming callbacks that will
@@ -176,10 +177,12 @@ func (s *Syncer) synced() {
 // unsynced checks the atomic that controls wallet syncness and if previously
 // synced, updates to unsynced and notifies the callback, if set.
 func (s *Syncer) unsynced() {
-	if s.atomicWalletSynced.CompareAndSwap(1, 0) &&
-		s.notifications != nil &&
-		s.notifications.Synced != nil {
-		s.notifications.Synced(false)
+	if s.atomicWalletSynced.CompareAndSwap(1, 0) {
+		if s.notifications != nil &&
+			s.notifications.Synced != nil {
+			s.notifications.Synced(false)
+		}
+		s.teardown()
 	}
 }
 
@@ -368,6 +371,11 @@ func (s *Syncer) Run(ctx context.Context) (err error) {
 
 	// Start background handlers to read received messages from remote peers
 	g, gctx := errgroup.WithContext(context.Background())
+	gctx, cancel := context.WithCancel(gctx)
+	s.teardown = func() {
+		err = errors.E(errors.NoPeers)
+		cancel()
+	}
 	g.Go(func() error { return s.receiveGetData(gctx) })
 	g.Go(func() error { return s.receiveInv(gctx) })
 	g.Go(func() error { return s.receiveHeadersAnnouncements(gctx) })
@@ -450,7 +458,10 @@ func (s *Syncer) Run(ctx context.Context) (err error) {
 	})
 
 	// Wait until cancellation or a handler errors.
-	return g.Wait()
+	if e := g.Wait(); err == nil {
+		err = e
+	}
+	return
 }
 
 // peerCandidate returns a peer address that we shall attempt to connect to.