spv: Drop syncer-global block locators

The locators are now only needed during the initial getHeaders sync
stage, therefore they are not needed in the global syncer struct.

Since they are accessed only from a single goroutine now, the mutex is
also unneeded.

spv/sync.go

@@ -75,10 +75,6 @@ type Syncer struct {
 	sidechains  wallet.SidechainForest
 	sidechainMu sync.Mutex
 
-	currentLocators   []*chainhash.Hash
-	locatorGeneration uint
-	locatorMu         sync.Mutex
-
 	// Holds all potential callbacks used to notify clients
 	notifications *Notifications
 
@@ -333,12 +329,6 @@ func (s *Syncer) Run(ctx context.Context) error {
 		log.Infof("Transactions synced through block %v height %d", &tipHash, tipHeight)
 	}
 
-	locators, err := s.wallet.BlockLocators(ctx, nil)
-	if err != nil {
-		return err
-	}
-	s.currentLocators = locators
-
 	s.lp.AddrManager().Start()
 	defer func() {
 		err := s.lp.AddrManager().Stop()
@@ -1299,13 +1289,6 @@ func (s *Syncer) handleBlockAnnouncements(ctx context.Context, rp *p2p.RemotePee
 		return err
 	}
 
-	if len(bestChain) != 0 {
-		s.locatorMu.Lock()
-		s.currentLocators = nil
-		s.locatorGeneration++
-		s.locatorMu.Unlock()
-	}
-
 	// Log connected blocks.
 	for _, n := range bestChain {
 		log.Infof("Connected block %v, height %d, %d wallet transaction(s)",
@@ -1358,22 +1341,6 @@ var hashStop chainhash.Hash
 // getHeaders fetches headers from peers until the wallet is up to date with
 // all connected peers.  This is part of the startup sync process.
 func (s *Syncer) getHeaders(ctx context.Context) error {
-	var locators []*chainhash.Hash
-	var generation uint
-	var err error
-	s.locatorMu.Lock()
-	locators = s.currentLocators
-	generation = s.locatorGeneration
-	if len(locators) == 0 {
-		locators, err = s.wallet.BlockLocators(ctx, nil)
-		if err != nil {
-			s.locatorMu.Unlock()
-			return err
-		}
-		s.currentLocators = locators
-		s.locatorGeneration++
-	}
-	s.locatorMu.Unlock()
 
 	cnet := s.wallet.ChainParams().Net
 
@@ -1396,6 +1363,11 @@ nextbatch:
 			return nil
 		}
 
+		// Request headers from the selected peer.
+		locators, err := s.wallet.BlockLocators(ctx, nil)
+		if err != nil {
+			return err
+		}
 		headers, err := rp.Headers(ctx, locators, &hashStop)
 		if err != nil {
 			continue nextbatch
@@ -1502,21 +1474,6 @@ nextbatch:
 		if added == 0 {
 			s.sidechainMu.Unlock()
 
-			s.locatorMu.Lock()
-			if s.locatorGeneration > generation {
-				locators = s.currentLocators
-			}
-			if len(locators) == 0 {
-				locators, err = s.wallet.BlockLocators(ctx, nil)
-				if err != nil {
-					s.locatorMu.Unlock()
-					return err
-				}
-				s.currentLocators = locators
-				s.locatorGeneration++
-				generation = s.locatorGeneration
-			}
-			s.locatorMu.Unlock()
 			continue nextbatch
 		}
 		s.fetchHeadersProgress(headers[len(headers)-1])
@@ -1531,7 +1488,7 @@ nextbatch:
 		}
 		if len(bestChain) == 0 {
 			s.sidechainMu.Unlock()
-			continue
+			continue nextbatch
 		}
 
 		prevChain, err := s.wallet.ChainSwitch(ctx, &s.sidechains, bestChain, nil)
@@ -1561,17 +1518,6 @@ nextbatch:
 		// Peers should not be significantly behind the new tip.
 		s.setRequiredHeight(int32(tip.Header.Height))
 		s.disconnectStragglers(int32(tip.Header.Height))
-
-		// Generate new locators
-		s.locatorMu.Lock()
-		locators, err = s.wallet.BlockLocators(ctx, nil)
-		if err != nil {
-			s.locatorMu.Unlock()
-			return err
-		}
-		s.currentLocators = locators
-		s.locatorGeneration++
-		s.locatorMu.Unlock()
 	}
 
 	return ctx.Err()