[release-v2.0] spv: Fix request of new blocks after initial sync

This fixes an issue where SPV wallets would only request blocks from
peers after a new block was mined.

The issue could be triggered if the number of peers ever dropped to
zero, causing the SPV syncer to consider itself unsynced, and if blocks
were mined in the intervening period until a new peer was connected.

The root cause was the addition of the unsynced status on the
refactoring made in commit 6cde2cc. That made the sync status atomic
decoupled to the state of the initialSyncDone signalling channel, which
meant that if the atomic was switched to zero after the initial sync was
done, new headers would fail to be requested from newly connected peers.

Upon further review, the check is ultimately unnecessary and is removed
in this commit ensuring that, after initial sync is completed, the
headers are asked of every peer.

Backport of 794fff4.

spv/sync.go

@@ -1820,11 +1820,6 @@ 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.atomicWalletSynced.Load() == 1
-
 	// Only continue with peer startup after the initial sync process
 	// has completed.
 	select {
@@ -1860,16 +1855,14 @@ func (s *Syncer) peerStartup(ctx context.Context, rp *p2p.RemotePeer) error {
 		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
-		}
+	// Request any updated headers from the peer.
+	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