spv: Ask for sendheaders after initial sync

This makes the remote peers configure to send block announcements via
sendheaders only after the initial sync is completed. This ensures the
initial sync process can fetch all the needed headers from any and all
peers before they are configured to send header annoucements.

spv/sync.go

@@ -6,6 +6,7 @@ package spv
 
 import (
 	"context"
+	"fmt"
 	"runtime"
 	"sync"
 	"sync/atomic"
@@ -444,8 +445,18 @@ func (s *Syncer) Run(ctx context.Context) error {
 		}
 		s.rescanFinished()
 
+		log.Infof("Initial sync completed. Switching to header announcements sync.")
 		s.synced()
 
+		// Perform peer startup (publish txs, sendheaders, etc) on
+		// every connected peer.
+		err = s.forRemotes(func(rp *p2p.RemotePeer) error {
+			return s.startupPeerSync(ctx, rp)
+		})
+		if err != nil {
+			return err
+		}
+
 		// Rescan done.
 		return nil
 	})
@@ -547,16 +558,19 @@ func (s *Syncer) connectAndRunPeer(ctx context.Context, raddr string) {
 		s.peerDisconnected(n, raddr)
 	}()
 
-	// Perform peer startup.
-	err = s.startupSync(ctx, rp)
-	if err != nil {
-		if !errors.Is(err, context.Canceled) {
-			log.Warnf("Unable to complete startup sync with peer %v: %v", raddr, err)
-		} else {
-			log.Infof("Lost peer %v", raddr)
+	// Perform peer sync if the initial sync process has completed. This
+	// configures the peer to send new headers.
+	if s.Synced() {
+		err = s.startupPeerSync(ctx, rp)
+		if err != nil {
+			if !errors.Is(err, context.Canceled) {
+				log.Warnf("Unable to complete startup sync with peer %v: %v", raddr, err)
+			} else {
+				log.Infof("Lost peer %v", raddr)
+			}
+			rp.Disconnect(err)
+			return
 		}
-		rp.Disconnect(err)
-		return
 	}
 
 	// Finally, block until the peer disconnects.
@@ -1548,7 +1562,9 @@ nextbatch:
 	}
 }
 
-func (s *Syncer) startupSync(ctx context.Context, rp *p2p.RemotePeer) error {
+// startupPeerSync performs peer startup procedures after connection. This should
+// only be called after the wallet has had its initial sync complete.
+func (s *Syncer) startupPeerSync(ctx context.Context, rp *p2p.RemotePeer) error {
 	var err error
 
 	if rp.Pver() >= wire.InitStateVersion {
@@ -1558,6 +1574,11 @@ func (s *Syncer) startupSync(ctx context.Context, rp *p2p.RemotePeer) error {
 		}
 	}
 
+	err = rp.SendHeaders(ctx)
+	if err != nil {
+		return fmt.Errorf("unable ask for sendheaders from peer: %w", err)
+	}
+
 	unminedTxs, err := s.wallet.UnminedTransactions(ctx)
 	if err != nil {
 		log.Errorf("Cannot load unmined transactions for resending: %v", err)