feat(p2p): allow inbound-only peers without underlay addresses

Relax the non-empty underlay validation in bzz.ParseAddress/MarshalJSON so
browser/wasm and strict-NAT peers that cannot be dialed back can still
handshake and use protocols over existing connections. Guard the Connect
addressbook.Put so such peers are not stored for reconnection (v2.8 already
guards handleIncoming and the reacher). Port of pr-5326; peer registry
rename already landed upstream via ethersphere#5337.

Note: this is a deliberate protocol relaxation vs upstream v2.8.

Author: v1rtl

pkg/bzz/address.go

@@ -114,10 +114,10 @@ func ParseAddress(underlay, overlay, signature, nonce []byte, timestamp int64, n
 		return nil, fmt.Errorf("deserialize underlays: %w: %w", ErrInvalidAddress, err)
 	}
 
-	if len(multiUnderlays) == 0 {
-		// no underlays sent
-		return nil, ErrInvalidAddress
-	}
+	// Empty underlays are allowed for inbound-only peers (e.g. browsers/wasm,
+	// strict NAT) that cannot be dialed back. These peers can still use protocols
+	// over existing connections but won't participate in Kademlia topology or hive
+	// gossip (see the addressbook/reacher guards in pkg/p2p/libp2p).
 
 	ethAddress, err := crypto.NewEthereumAddress(*recoveredPK)
 	if err != nil {
@@ -198,9 +198,7 @@ func AreUnderlaysEqual(a, b []ma.Multiaddr) bool {
 }
 
 func (a *Address) MarshalJSON() ([]byte, error) {
-	if len(a.Underlays) == 0 {
-		return nil, fmt.Errorf("no underlays for %s", a.Overlay)
-	}
+	// Empty underlays are allowed for inbound-only peers that cannot be dialed back.
 
 	chequebook := ""
 	if a.ChequebookAddress != (common.Address{}) {

pkg/p2p/libp2p/libp2p.go

@@ -813,7 +813,10 @@ func (s *Service) Connect(ctx context.Context, addrs []ma.Multiaddr) (address *b
 		return nil, p2p.ErrPeerNotFound
 	}
 
-	if i.FullNode {
+	// Only persist peers with underlays to the addressbook. Inbound-only peers
+	// (empty underlays, e.g. browsers/wasm) cannot be dialed back, so there is no
+	// point storing them for reconnection.
+	if i.FullNode && len(i.BzzAddress.Underlays) > 0 {
 		if err := s.putHandshakeAddress(i.BzzAddress); err != nil {
 			_ = s.Disconnect(overlay, "failed storing peer in addressbook")
 			return nil, fmt.Errorf("storing bzz address: %w", err)