chanbackup: add backup version for TapscriptRoot

Previous to this change taproot assets channels and simple taproot channels were
considered the same in the context of chanbackup package, since they stored the
same data. In the following commits we are adding the data needed to produce a
signed commitment transaction from a SCB file and in order to do that we need to
add more fields and a custom channel gets one additional field (TapscriptRoot)
compared to a simple taproot channel. So now we have to distinguish these kinds
of channels in chanbackup package.

See PR https://github.com/lightningnetwork/lnd/pull/8183 for more details.

chanbackup/single.go

@@ -52,6 +52,10 @@ const (
 	// SimpleTaprootVersion is a version that denotes this channel is using
 	// the musig2 based taproot commitment format.
 	SimpleTaprootVersion = 5
+
+	// TapscriptRootVersion is a version that denotes this is a MuSig2
+	// channel with a top level tapscript commitment.
+	TapscriptRootVersion = 6
 )
 
 // Single is a static description of an existing channel that can be used for
@@ -218,7 +222,11 @@ func NewSingle(channel *channeldb.OpenChannel,
 
 	switch {
 	case channel.ChanType.IsTaproot():
-		single.Version = SimpleTaprootVersion
+		if channel.ChanType.HasTapscriptRoot() {
+			single.Version = TapscriptRootVersion
+		} else {
+			single.Version = SimpleTaprootVersion
+		}
 
 	case channel.ChanType.HasLeaseExpiration():
 		single.Version = ScriptEnforcedLeaseVersion
@@ -252,6 +260,7 @@ func (s *Single) Serialize(w io.Writer) error {
 	case AnchorsZeroFeeHtlcTxCommitVersion:
 	case ScriptEnforcedLeaseVersion:
 	case SimpleTaprootVersion:
+	case TapscriptRootVersion:
 	default:
 		return fmt.Errorf("unable to serialize w/ unknown "+
 			"version: %v", s.Version)
@@ -429,6 +438,7 @@ func (s *Single) Deserialize(r io.Reader) error {
 	case AnchorsZeroFeeHtlcTxCommitVersion:
 	case ScriptEnforcedLeaseVersion:
 	case SimpleTaprootVersion:
+	case TapscriptRootVersion:
 	default:
 		return fmt.Errorf("unable to de-serialize w/ unknown "+
 			"version: %v", s.Version)

chanbackup/single_test.go

@@ -250,13 +250,20 @@ func TestSinglePackUnpack(t *testing.T) {
 			valid:   true,
 		},
 
-		// The new taproot channel lease version should
+		// The new taproot channel version should
 		// pack/unpack with no problem.
 		{
 			version: SimpleTaprootVersion,
 			valid:   true,
 		},
 
+		// The new tapscript root channel version should pack/unpack
+		// with no problem.
+		{
+			version: TapscriptRootVersion,
+			valid:   true,
+		},
+
 		// A non-default version, atm this should result in a failure.
 		{
 			version: 99,