chainntnfs: support caching confirm/spend hints for scripts

In this commit, we refactor the HeightHintCache and its underlying
interfaces to be able to manipulate hints for ConfRequests and
SpendRequests. By doing so, we'll be able to manipulate hints for
scripts if the request includes either a zero hash or a zero outpoint.

chainntnfs/txnotifier.go

@@ -1,6 +1,7 @@
 package chainntnfs
 
 import (
+	"bytes"
 	"errors"
 	"fmt"
 	"sync"
@@ -8,6 +9,7 @@ import (
 	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/btcsuite/btcutil"
+	"github.com/lightningnetwork/lnd/channeldb"
 )
 
 const (
@@ -165,6 +167,21 @@ func (r ConfRequest) String() string {
 	return fmt.Sprintf("script=%v", r.PkScript)
 }
 
+// ConfHintKey returns the key that will be used to index the confirmation
+// request's hint within the height hint cache.
+func (r ConfRequest) ConfHintKey() ([]byte, error) {
+	if r.TxID == ZeroHash {
+		return r.PkScript.Script(), nil
+	}
+
+	var txid bytes.Buffer
+	if err := channeldb.WriteElement(&txid, r.TxID); err != nil {
+		return nil, err
+	}
+
+	return txid.Bytes(), nil
+}
+
 // ConfNtfn represents a notifier client's request to receive a notification
 // once the target transaction gets sufficient confirmations. The client is
 // asynchronously notified via the ConfirmationEvent channels.
@@ -265,6 +282,22 @@ func (r SpendRequest) String() string {
 	return fmt.Sprintf("script=%v", r.PkScript)
 }
 
+// SpendHintKey returns the key that will be used to index the spend request's
+// hint within the height hint cache.
+func (r SpendRequest) SpendHintKey() ([]byte, error) {
+	if r.OutPoint == ZeroOutPoint {
+		return r.PkScript.Script(), nil
+	}
+
+	var outpoint bytes.Buffer
+	err := channeldb.WriteElement(&outpoint, r.OutPoint)
+	if err != nil {
+		return nil, err
+	}
+
+	return outpoint.Bytes(), nil
+}
+
 // SpendNtfn represents a client's request to receive a notification once an
 // outpoint has been spent on-chain. The client is asynchronously notified via
 // the SpendEvent channels.