chainntfs: add checks for previously confirmed transactions and spends

Without these checks, “zombie” notification requests that would never
be dispatched could be registered. This would occur if notification
requests were made for events (transaction confirmation and output
spent) that had already been recorded on the blockchain.

chainntnfs/btcdnotify/btcd.go

@@ -2,6 +2,7 @@ package btcdnotify
 
 import (
 	"container/heap"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -436,6 +437,7 @@ type spendNotification struct {
 // outpoint has been detected, the details of the spending event will be sent
 // across the 'Spend' channel.
 func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint) (*chainntnfs.SpendEvent, error) {
+
 	if err := b.chainConn.NotifySpent([]*wire.OutPoint{outpoint}); err != nil {
 		return nil, err
 	}
@@ -447,6 +449,33 @@ func (b *BtcdNotifier) RegisterSpendNtfn(outpoint *wire.OutPoint) (*chainntnfs.S
 
 	b.notificationRegistry <- ntfn
 
+	// The following conditional checks to ensure that when a spend notification
+	// is registered, the output hasn't already been spent. If the output
+	// is no longer in the UTXO set, the chain will be rescanned from the point
+	// where the output was added. The rescan will dispatch the notification.
+	txout, err := b.chainConn.GetTxOut(&outpoint.Hash, outpoint.Index, true)
+	if err != nil {
+		return nil, err
+	}
+
+	if txout == nil {
+		transaction, err := b.chainConn.GetRawTransactionVerbose(&outpoint.Hash)
+		if err != nil {
+			return nil, err
+		}
+
+		blockhash, err := wire.NewShaHashFromStr(transaction.BlockHash)
+		if err != nil {
+			return nil, err
+		}
+
+		ops := []*wire.OutPoint{outpoint}
+		if err := b.chainConn.Rescan(blockhash, nil, ops); err != nil {
+			chainntnfs.Log.Errorf("Rescan for spend notification txout failed: %v", err)
+			return nil, err
+		}
+	}
+
 	return &chainntnfs.SpendEvent{ntfn.spendChan}, nil
 }
 
@@ -477,6 +506,19 @@ func (b *BtcdNotifier) RegisterConfirmationsNtfn(txid *wire.ShaHash,
 
 	b.notificationRegistry <- ntfn
 
+	// The following conditional checks transaction confirmation notification
+	// requests so that if the transaction has already been included in a block
+	// with the requested number of confirmations, the notification will be
+	// dispatched immediately.
+	tx, err := b.chainConn.GetRawTransactionVerbose(txid)
+	if err != nil {
+		if !strings.Contains(err.Error(), "No information") {
+			return nil, err
+		}
+	} else if uint32(tx.Confirmations) > numConfs {
+		ntfn.finConf <- int32(tx.Confirmations)
+	}
+
 	return &chainntnfs.ConfirmationEvent{
 		Confirmed:    ntfn.finConf,
 		NegativeConf: ntfn.negativeConf,