multi: fix and test v1 output spend ntfns

Because Taproot key spend only spends don't allow us to re-construct the
spent pkScript from the witness alone, we cannot support registering
spend notifications for v1 pkScripts only. We instead require the
outpoint to be specified. This commit makes it possible to only match by
outpoint and also adds an itest for it.

chainntnfs/txnotifier.go

@@ -39,6 +39,19 @@ var (
 	// notifier to match _and_ dispatch upon detecting the spend of the
 	// script on-chain, rather than the outpoint.
 	ZeroOutPoint wire.OutPoint
+
+	// zeroV1KeyPush is a pkScript that pushes an all-zero 32-byte Taproot
+	// SegWit v1 key to the stack.
+	zeroV1KeyPush = [34]byte{
+		txscript.OP_1, txscript.OP_DATA_32, // 32 byte of zeroes here
+	}
+
+	// ZeroTaprootPkScript is the parsed txscript.PkScript of an empty
+	// Taproot SegWit v1 key being pushed to the stack. This allows the
+	// notifier to match _and_ dispatch upon detecting the spend of the
+	// outpoint on-chain, rather than the pkScript (which cannot be derived
+	// from the witness alone in the SegWit v1 case).
+	ZeroTaprootPkScript, _ = txscript.ParsePkScript(zeroV1KeyPush[:])
 )
 
 var (
@@ -322,6 +335,24 @@ func NewSpendRequest(op *wire.OutPoint, pkScript []byte) (SpendRequest, error) {
 	}
 	r.PkScript = outputScript
 
+	// For Taproot spends we have the main problem that for the key spend
+	// path we cannot derive the pkScript from only looking at the input's
+	// witness. So we need to rely on the outpoint information alone.
+	//
+	// TODO(guggero): For script path spends we can derive the pkScript from
+	// the witness, since we have the full control block and the spent
+	// script available.
+	if outputScript.Class() == txscript.WitnessV1TaprootTy {
+		if op == nil {
+			return r, fmt.Errorf("cannot register witness v1 " +
+				"spend request without outpoint")
+		}
+
+		// We have an outpoint, so we can set the pkScript to an all
+		// zero Taproot key that we'll compare this spend request to.
+		r.PkScript = ZeroTaprootPkScript
+	}
+
 	return r, nil
 }
 
@@ -1488,6 +1519,19 @@ func (n *TxNotifier) filterTx(tx *btcutil.Tx, blockHash *chainhash.Hash,
 			if _, ok := n.spendNotifications[spendRequest]; ok {
 				notifyDetails(spendRequest, prevOut, uint32(i))
 			}
+
+			// Now try with an empty taproot key pkScript, since we
+			// cannot derive the spent pkScript directly from the
+			// witness. But we have the outpoint, which should be
+			// enough.
+			spendRequest.PkScript = ZeroTaprootPkScript
+			if _, ok := n.spendNotifications[spendRequest]; ok {
+				notifyDetails(spendRequest, prevOut, uint32(i))
+			}
+
+			// Restore the pkScript but try with a zero outpoint
+			// instead (won't be possible for Taproot).
+			spendRequest.PkScript = pkScript
 			spendRequest.OutPoint = ZeroOutPoint
 			if _, ok := n.spendNotifications[spendRequest]; ok {
 				notifyDetails(spendRequest, prevOut, uint32(i))