spv: Request headers from peer after initial sync

This modifies the peer setup procedure to request any updated headers
from a newly connected peer if the connection is made after the initial
sync is completed.

This speeds up re-syncing after initial sync when the local client has
been offline for some time (for example, due to a flicking network
connection) by ensuring any and all updated headers are requested
immediately, instead of waiting until a new block is announced to begin
the catchup process.

spv/sync.go

@@ -1618,6 +1618,11 @@ func (s *Syncer) initialSyncRescan(ctx context.Context) error {
 // peerStartup performs initial startup operations with a recently connected
 // peer.
 func (s *Syncer) peerStartup(ctx context.Context, rp *p2p.RemotePeer) error {
+	// If the initial sync process has already completed, then immediately
+	// request any new headers from the peer at the end of the peer setup
+	// process.
+	requestHeaders := s.Synced()
+
 	// Only continue with peer startup after the initial sync process
 	// has completed.
 	select {
@@ -1649,7 +1654,23 @@ func (s *Syncer) peerStartup(ctx context.Context, rp *p2p.RemotePeer) error {
 	}
 
 	// Ask peer to send any new headers.
-	return rp.SendHeaders(ctx)
+	if err := rp.SendHeaders(ctx); err != nil {
+		return err
+	}
+
+	// If needed, request any updated headers from the peer.
+	if requestHeaders {
+		log.Debugf("Requesting updated headers from peer %v", rp)
+		locators, _, err := s.wallet.BlockLocators(ctx, nil)
+		if err != nil {
+			return err
+		}
+		if err := rp.HeadersAsync(ctx, locators, &hashStop); err != nil {
+			return err
+		}
+	}
+
+	return nil
 }
 
 // handleMempool handles eviction from the local mempool of non-wallet-backed