lnwallet/btcwallet: concretely implement the new TransactionSubscription interface

This commit returns the BtcWallet wrapper struct back to conformance to
the WalletController interface by adding support for
SubscribeTransactions.

The implementation of the lnwallet.TransactionSubscription consists
simply of a proxy goroutine which forwards notifications from the
wallet’s internal NotificaitonServer.

lnwallet/btcwallet/btcwallet.go

@@ -492,3 +492,121 @@ func (b *BtcWallet) ListTransactionDetails() ([]*lnwallet.TransactionDetail, err
 
 	return txDetails, nil
 }
+
+// txSubscriptionClient encapsulates the transaction notification client from
+// the base wallet. Notifications received from the client will be proxied over
+// two distinct channels.
+type txSubscriptionClient struct {
+	txClient base.TransactionNotificationsClient
+
+	confirmed   chan *lnwallet.TransactionDetail
+	unconfirmed chan *lnwallet.TransactionDetail
+
+	w *base.Wallet
+
+	wg   sync.WaitGroup
+	quit chan struct{}
+}
+
+// ConfirmedTransactions returns a channel which will be sent on as new
+// relevant transactions are confirmed.
+//
+// This is part of the TransactionSubscription interface.
+func (t *txSubscriptionClient) ConfirmedTransactions() chan *lnwallet.TransactionDetail {
+	return t.confirmed
+}
+
+// UnconfirmedTransactions returns a channel which will be sent on as
+// new relevant transactions are seen within the network.
+//
+// This is part of the TransactionSubscription interface.
+func (t *txSubscriptionClient) UnconfirmedTransactions() chan *lnwallet.TransactionDetail {
+	return t.unconfirmed
+}
+
+// Cancel finalizes the subscription, cleaning up any resources allocated.
+//
+// This is part of the TransactionSubscription interface.
+func (t *txSubscriptionClient) Cancel() {
+	close(t.quit)
+	t.wg.Wait()
+
+	t.txClient.Done()
+}
+
+// notificationProxier proxies the notifications received by the underlying
+// wallet's notification client to a higher-level TransactionSubscription
+// client.
+func (t *txSubscriptionClient) notificationProxier() {
+out:
+	for {
+		select {
+		case txNtfn := <-t.txClient.C:
+			// TODO(roasbeef): handle detached blocks
+			currentHeight := t.w.Manager.SyncedTo().Height
+
+			// Launch a goroutine to re-package and send
+			// notifications for any newly confirmed transactions.
+			go func() {
+				for _, block := range txNtfn.AttachedBlocks {
+					details, err := minedTransactionsToDetails(currentHeight, block)
+					if err != nil {
+						continue
+					}
+
+					for _, d := range details {
+						select {
+						case t.confirmed <- d:
+						case <-t.quit:
+							return
+						}
+					}
+				}
+
+			}()
+
+			// Launch a goroutine to re-package and send
+			// notifications for any newly unconfirmed transactions.
+			go func() {
+				for _, tx := range txNtfn.UnminedTransactions {
+					detail, err := unminedTransactionsToDetail(tx)
+					if err != nil {
+						continue
+					}
+
+					select {
+					case t.unconfirmed <- detail:
+					case <-t.quit:
+						return
+					}
+				}
+			}()
+		case <-t.quit:
+			break out
+		}
+	}
+
+	t.wg.Done()
+}
+
+// SubscribeTransactions returns a TransactionSubscription client which
+// is capable of receiving async notifications as new transactions
+// related to the wallet are seen within the network, or found in
+// blocks.
+//
+// This is a part of the WalletController interface.
+func (b *BtcWallet) SubscribeTransactions() (lnwallet.TransactionSubscription, error) {
+	walletClient := b.wallet.NtfnServer.TransactionNotifications()
+
+	txClient := &txSubscriptionClient{
+		txClient:    walletClient,
+		confirmed:   make(chan *lnwallet.TransactionDetail),
+		unconfirmed: make(chan *lnwallet.TransactionDetail),
+		w:           b.wallet,
+		quit:        make(chan struct{}),
+	}
+	txClient.wg.Add(1)
+	go txClient.notificationProxier()
+
+	return txClient, nil
+}