Add func to get next n active addresses.

This adds a new function, ExtendActiveAddresses, to extend the number
of active addresses, generated by the address chain, by any arbitrary
positive number.  This will be used for recovering wallet addresses
from a wallet backup, as well as getting the next as many addresses as
required to keep a watching wallet in sync with an original wallet
with the private keys, or sync an original wallet with new addresses
generated by the watching wallet.

wallet/wallet.go

@@ -1376,6 +1376,33 @@ func (w *Wallet) ActiveAddresses() map[btcutil.Address]*AddressInfo {
 	return addrs
 }
 
+// ExtendActiveAddresses gets or creates the next n addresses from the
+// address chain and marks each as active.  This is used to recover
+// deterministic (not imported) addresses from a wallet backup, or to
+// keep the active addresses in sync between an encrypted wallet with
+// private keys and an exported watching wallet without.
+//
+// A slice is returned with the btcutil.Address of each new address.
+// The blockchain must be rescanned for these addresses.
+func (w *Wallet) ExtendActiveAddresses(n int, keypoolSize uint) ([]btcutil.Address, error) {
+	if n <= 0 {
+		return nil, errors.New("n is not positive")
+	}
+
+	last := w.addrMap[*w.chainIdxMap[w.highestUsed]]
+	bs := &BlockStamp{Height: last.firstBlock}
+
+	addrs := make([]btcutil.Address, 0, n)
+	for i := 0; i < n; i++ {
+		addr, err := w.NextChainedAddress(bs, keypoolSize)
+		if err != nil {
+			return nil, err
+		}
+		addrs = append(addrs, addr)
+	}
+	return addrs, nil
+}
+
 type walletFlags struct {
 	useEncryption bool
 	watchingOnly  bool

wallet/wallet_test.go

@@ -579,6 +579,52 @@ func TestWatchingWalletExport(t *testing.T) {
 		}
 	}
 
+	// Test that ExtendActiveAddresses for the watching wallet match
+	// manually requested addresses of the original wallet.
+	newAddrs := make([]btcutil.Address, 0, keypoolSize)
+	for i := 0; i < keypoolSize; i++ {
+		addr, err := w.NextChainedAddress(createdAt, keypoolSize)
+		if err != nil {
+			t.Errorf("Cannot get next chained address for original wallet: %v", err)
+			return
+		}
+		newAddrs = append(newAddrs, addr)
+	}
+	newWWAddrs, err := ww.ExtendActiveAddresses(keypoolSize, keypoolSize)
+	if err != nil {
+		t.Errorf("Cannot extend active addresses for watching wallet: %v", err)
+		return
+	}
+	for i := range newAddrs {
+		if newAddrs[i].EncodeAddress() != newWWAddrs[i].EncodeAddress() {
+			t.Errorf("Extended active addresses do not match manually requested addresses.")
+			return
+		}
+	}
+
+	// Test ExtendActiveAddresses for the original wallet after manually
+	// requesting addresses for the watching wallet.
+	newWWAddrs = make([]btcutil.Address, 0, keypoolSize)
+	for i := 0; i < keypoolSize; i++ {
+		addr, err := ww.NextChainedAddress(createdAt, keypoolSize)
+		if err != nil {
+			t.Errorf("Cannot get next chained address for watching wallet: %v", err)
+			return
+		}
+		newWWAddrs = append(newWWAddrs, addr)
+	}
+	newAddrs, err = w.ExtendActiveAddresses(keypoolSize, keypoolSize)
+	if err != nil {
+		t.Errorf("Cannot extend active addresses for original wallet: %v", err)
+		return
+	}
+	for i := range newAddrs {
+		if newAddrs[i].EncodeAddress() != newWWAddrs[i].EncodeAddress() {
+			t.Errorf("Extended active addresses do not match manually requested addresses.")
+			return
+		}
+	}
+
 	// Test (de)serialization of watching wallet.
 	buf := new(bytes.Buffer)
 	_, err = ww.WriteTo(buf)