chainntnfs/txnotifier: watch for spends at tip

In this commit, we add support to allow the TxNotifier to properly
determine whether a new block extending the chain contains a transaction
that spends a registered outpoint. In the event that it does, spend
notifications will be dispatched to all active registered clients for
such outpoint.

chainntnfs/txnotifier.go

@@ -851,13 +851,22 @@ func (n *TxNotifier) dispatchSpendDetails(ntfn *SpendNtfn, details *SpendDetail)
 	return nil
 }
 
-// ConnectTip handles a new block extending the current chain. This checks each
-// transaction in the block to see if any watched transactions are included.
-// Also, if any watched transactions now have the required number of
-// confirmations as a result of this block being connected, this dispatches
-// notifications.
-func (n *TxNotifier) ConnectTip(blockHash *chainhash.Hash,
-	blockHeight uint32, txns []*btcutil.Tx) error {
+// ConnectTip handles a new block extending the current chain. It will go
+// through every transaction and determine if it is relevant to any of its
+// clients. A transaction can be relevant in either of the following two ways:
+//
+//   1. One of the inputs in the transaction spends an outpoint for which we
+//   currently have an active spend registration for.
+//
+//   2. The transaction is a transaction for which we currently have an active
+//   confirmation registration for.
+//
+// In the event that the transaction is relevant, a confirmation/spend
+// notification will be dispatched to the relevant clients. Confirmation
+// notifications will only be dispatched for transactions that have met the
+// required number of confirmations required by the client.
+func (n *TxNotifier) ConnectTip(blockHash *chainhash.Hash, blockHeight uint32,
+	txns []*btcutil.Tx) error {
 
 	select {
 	case <-n.quit:
@@ -876,14 +885,58 @@ func (n *TxNotifier) ConnectTip(blockHash *chainhash.Hash,
 	n.currentHeight++
 	n.reorgDepth = 0
 
-	// Record any newly confirmed transactions by their confirmed height so
-	// that notifications get dispatched when the transactions reach their
-	// required number of confirmations. We'll also watch these transactions
-	// at the height they were included in the chain so reorgs can be
-	// handled correctly.
+	// First, we'll iterate over all the transactions found in this block to
+	// determine if it includes any relevant transactions to the TxNotifier.
 	for _, tx := range txns {
 		txHash := tx.Hash()
 
+		// In order to determine if this transaction is relevant to the
+		// notifier, we'll check its inputs for any outstanding spend
+		// notifications.
+		for i, txIn := range tx.MsgTx().TxIn {
+			prevOut := txIn.PreviousOutPoint
+			spendSet, ok := n.spendNotifications[prevOut]
+			if !ok {
+				continue
+			}
+
+			// If we have any, we'll record its spend height so that
+			// notifications get dispatched to the respective
+			// clients.
+			spendDetails := &SpendDetail{
+				SpentOutPoint:     &prevOut,
+				SpenderTxHash:     txHash,
+				SpendingTx:        tx.MsgTx(),
+				SpenderInputIndex: uint32(i),
+				SpendingHeight:    int32(blockHeight),
+			}
+
+			// TODO(wilmer): cancel pending historical rescans if any?
+			spendSet.rescanStatus = rescanComplete
+			spendSet.details = spendDetails
+			for _, ntfn := range spendSet.ntfns {
+				// In the event that this notification was aware
+				// that the spending transaction of its outpoint
+				// was reorged out of the chain, we'll consume
+				// the reorg notification if it hasn't been
+				// done yet already.
+				select {
+				case <-ntfn.Event.Reorg:
+				default:
+				}
+			}
+
+			// We'll note the outpoints spending height in order to
+			// correctly handle dispatching notifications when the
+			// spending transactions gets reorged out of the chain.
+			opSet, exists := n.opsBySpendHeight[blockHeight]
+			if !exists {
+				opSet = make(map[wire.OutPoint]struct{})
+				n.opsBySpendHeight[blockHeight] = opSet
+			}
+			opSet[prevOut] = struct{}{}
+		}
+
 		// Check if we have any pending notifications for this txid. If
 		// none are found, we can proceed to the next transaction.
 		confSet, ok := n.confNotifications[*txHash]
@@ -903,6 +956,7 @@ func (n *TxNotifier) ConnectTip(blockHash *chainhash.Hash,
 			TxIndex:     uint32(tx.Index()),
 		}
 
+		// TODO(wilmer): cancel pending historical rescans if any?
 		confSet.rescanStatus = rescanComplete
 		confSet.details = details
 		for _, ntfn := range confSet.ntfns {
@@ -1031,17 +1085,33 @@ out:
 			return ErrTxNotifierExiting
 		}
 	}
-	delete(n.ntfnsByConfirmHeight, n.currentHeight)
+	delete(n.ntfnsByConfirmHeight, blockHeight)
 
-	// Clear entries from confNotifications and confTxsByInitialHeight. We
-	// assume that reorgs deeper than the reorg safety limit do not happen,
-	// so we can clear out entries for the block that is now mature.
-	if n.currentHeight >= n.reorgSafetyLimit {
-		matureBlockHeight := n.currentHeight - n.reorgSafetyLimit
-		for txHash := range n.txsByInitialHeight[matureBlockHeight] {
-			delete(n.confNotifications, txHash)
+	// We'll also dispatch spend notifications for all the outpoints that
+	// were spent at this new block height.
+	for op := range n.opsBySpendHeight[blockHeight] {
+		spendSet := n.spendNotifications[op]
+		for _, ntfn := range spendSet.ntfns {
+			err := n.dispatchSpendDetails(ntfn, spendSet.details)
+			if err != nil {
+				return err
+			}
+		}
+	}
+
+	// Finally, we'll clear the entries from our set of notifications for
+	// transactions and outpoints that are no longer under the risk of being
+	// reorged out of the chain.
+	if blockHeight >= n.reorgSafetyLimit {
+		matureBlockHeight := blockHeight - n.reorgSafetyLimit
+		for tx := range n.txsByInitialHeight[matureBlockHeight] {
+			delete(n.confNotifications, tx)
 		}
 		delete(n.txsByInitialHeight, matureBlockHeight)
+		for op := range n.opsBySpendHeight[matureBlockHeight] {
+			delete(n.spendNotifications, op)
+		}
+		delete(n.opsBySpendHeight, matureBlockHeight)
 	}
 
 	return nil