tapchannel: add 2nd level information to tapscriptSweepDesc

In this commit, we add two fields to the tapscriptSweepDesc struct
related to 2nd level HTLCs. First, we add the second level sig index,
which is needed to be able to know where to insert our signature after
we generate it. We also add an optional lnwallet.AuxSigDesc, which
contains the remote party's signature, and also the sighash needed for
it and a sign desc to generate it with.

Author: Roasbeef

tapchannel/aux_sweeper.go

@@ -382,26 +382,37 @@ func (a *AuxSweeper) createAndSignSweepVpackets(
 
 // tapscriptSweepDesc is a helper struct that contains the tapscript tree and
 // the control block needed to generate a valid spend.
-//
-// TODO(roasbeef): only needs the merkle root?
 type tapscriptSweepDesc struct {
+	auxSigInfo lfn.Option[lnwallet.AuxSigDesc]
+
 	scriptTree input.TapscriptDescriptor
 
 	ctrlBlockBytes []byte
 
-	relativeDelay fn.Option[uint64]
+	relativeDelay lfn.Option[uint64]
+
+	absoluteDelay lfn.Option[uint64]
+
+	secondLevelSigIndex lfn.Option[uint32]
+}
+
+// tapscriptSweepDescs contains the sweep decs for the first and second level.
+// Most outputs only go to the first level, but HTLCs on our local commitment
+// transaction go to the second level.
+type tapscriptSweepDescs struct {
+	firstLevel tapscriptSweepDesc
 
-	absoluteDelay fn.Option[uint64] //nolint:unused
+	secondLevel lfn.Option[tapscriptSweepDesc]
 }
 
 // commitNoDelaySweepDesc creates a sweep desc for a commitment output that
 // resides on the remote party's commitment transaction. This output is a
 // non-delay output, so we don't need to worry about the CSV delay when
 // sweeping it.
 func commitNoDelaySweepDesc(keyRing *lnwallet.CommitmentKeyRing,
-	csvDelay uint32) lfn.Result[tapscriptSweepDesc] {
+	csvDelay uint32) lfn.Result[tapscriptSweepDescs] {
 
-	type returnType = tapscriptSweepDesc
+	type returnType = tapscriptSweepDescs
 
 	// We'll make the script tree for the to remote script (we're remote as
 	// this is their commitment transaction). We don't have an auxLeaf here
@@ -410,8 +421,8 @@ func commitNoDelaySweepDesc(keyRing *lnwallet.CommitmentKeyRing,
 		keyRing.ToRemoteKey, input.NoneTapLeaf(),
 	)
 	if err != nil {
-		return lfn.Errf[returnType]("unable to make remote script "+
-			"tree: %w", err)
+		return lfn.Errf[returnType]("unable to make remote "+
+			"script tree: %w", err)
 	}
 
 	// Now that we have the script tree, we'll make the control block
@@ -420,29 +431,29 @@ func commitNoDelaySweepDesc(keyRing *lnwallet.CommitmentKeyRing,
 		input.ScriptPathSuccess,
 	)
 	if err != nil {
-		return lfn.Errf[returnType]("unable to make ctrl block: %w",
-			err)
 	}
 	ctrlBlockBytes, err := ctrlBlock.ToBytes()
 	if err != nil {
-		return lfn.Errf[returnType]("unable to encode ctrl block: %w",
-			err)
+		return lfn.Errf[returnType]("unable to encode ctrl "+
+			"block: %w", err)
 	}
 
-	return lfn.Ok(tapscriptSweepDesc{
-		scriptTree:     toRemoteScriptTree,
-		relativeDelay:  fn.Some(uint64(csvDelay)),
-		ctrlBlockBytes: ctrlBlockBytes,
+	return lfn.Ok(tapscriptSweepDescs{
+		firstLevel: tapscriptSweepDesc{
+			scriptTree:     toRemoteScriptTree,
+			relativeDelay:  lfn.Some(uint64(csvDelay)),
+			ctrlBlockBytes: ctrlBlockBytes,
+		},
 	})
 }
 
 // commitDelaySweepDesc creates a sweep desc for a commitment output that
 // resides on our local commitment transaction. This output is a delay output,
 // so we need to mind the CSV delay when sweeping it.
 func commitDelaySweepDesc(keyRing *lnwallet.CommitmentKeyRing,
-	csvDelay uint32) lfn.Result[tapscriptSweepDesc] {
+	csvDelay uint32) lfn.Result[tapscriptSweepDescs] {
 
-	type returnType = tapscriptSweepDesc
+	type returnType = tapscriptSweepDescs
 
 	// We'll make the script tree for the to remote script (we're remote as
 	// this is their commitment transaction). We don't have an auxLeaf here
@@ -461,27 +472,28 @@ func commitDelaySweepDesc(keyRing *lnwallet.CommitmentKeyRing,
 		input.ScriptPathSuccess,
 	)
 	if err != nil {
-		return lfn.Err[returnType](err)
 	}
 	ctrlBlockBytes, err := ctrlBlock.ToBytes()
 	if err != nil {
 		return lfn.Err[returnType](err)
 	}
 
-	return lfn.Ok(tapscriptSweepDesc{
-		scriptTree:     toLocalScriptTree,
-		relativeDelay:  fn.Some(uint64(csvDelay)),
-		ctrlBlockBytes: ctrlBlockBytes,
+	return lfn.Ok(tapscriptSweepDescs{
+		firstLevel: tapscriptSweepDesc{
+			scriptTree:     toLocalScriptTree,
+			relativeDelay:  lfn.Some(uint64(csvDelay)),
+			ctrlBlockBytes: ctrlBlockBytes,
+		},
 	})
 }
 
 // commitRevokeSweepDesc creates a sweep desc for a commitment output that is
 // the local output on the remote party's commitment transaction. We can seep
 // this in the case of a revoked commitment.
 func commitRevokeSweepDesc(keyRing *lnwallet.CommitmentKeyRing,
-	csvDelay uint32) lfn.Result[tapscriptSweepDesc] {
+	csvDelay uint32) lfn.Result[tapscriptSweepDescs] {
 
-	type returnType = tapscriptSweepDesc
+	type returnType = tapscriptSweepDescs
 
 	// To sweep their revoked output, we'll make the script tree for the
 	// local tree of their commitment transaction, which is actually their
@@ -504,12 +516,14 @@ func commitRevokeSweepDesc(keyRing *lnwallet.CommitmentKeyRing,
 	}
 	ctrlBlockBytes, err := ctrlBlock.ToBytes()
 	if err != nil {
-		return lfn.Err[returnType](err)
+		return lfn.Err[tapscriptSweepDescs](err)
 	}
 
-	return lfn.Ok(tapscriptSweepDesc{
-		scriptTree:     toLocalScriptTree,
-		ctrlBlockBytes: ctrlBlockBytes,
+	return lfn.Ok(tapscriptSweepDescs{
+		firstLevel: tapscriptSweepDesc{
+			scriptTree:     toLocalScriptTree,
+			ctrlBlockBytes: ctrlBlockBytes,
+		},
 	})
 }
 
@@ -1159,7 +1173,7 @@ func (a *AuxSweeper) resolveContract(
 	}
 
 	var (
-		sweepDesc    lfn.Result[tapscriptSweepDesc]
+		sweepDesc    lfn.Result[tapscriptSweepDescs]
 		assetOutputs []*cmsg.AssetOutput
 	)
 
@@ -1192,13 +1206,13 @@ func (a *AuxSweeper) resolveContract(
 	// The remote party has breached the channel. We'll sweep the revoked
 	// key that we learned in the past.
 	case input.TaprootCommitmentRevoke:
-		// In this case, we'll be sweeping the remote party's asset
 		// outputs, as they broadcast a revoked commitment. For the
+		// In this case, we'll be sweeping the remote party's asset
 		// remote party, this is actually their local output.
 		assetOutputs = commitState.LocalAssets.Val.Outputs
 
-		// As we have multiple outputs to sweep above, we'll also have
-		// two sweep descs.
+		// Next, we'll make a sweep desk capable of sweeping the remote
+		// party's local output.
 		sweepDesc = commitRevokeSweepDesc(req.KeyRing, req.CsvDelay)
 
 	default:
@@ -1216,10 +1230,17 @@ func (a *AuxSweeper) resolveContract(
 	log.Infof("Sweeping %v asset outputs: %v", len(assetOutputs),
 		limitSpewer.Sdump(assetOutputs))
 
+	firstLevelSweepDesc := lfn.AndThen(
+		sweepDesc,
+		func(sweepDesc tapscriptSweepDescs) lfn.Result[tapscriptSweepDesc] { //nolint:lll
+			return lfn.Ok(sweepDesc.firstLevel)
+		},
+	)
+
 	// With the sweep desc constructed above, we'll create vPackets for
 	// each of the local assets, then sign them all.
 	sPkts := a.createAndSignSweepVpackets(
-		assetOutputs, req.SignDesc, sweepDesc,
+		assetOutputs, req.SignDesc, firstLevelSweepDesc,
 	)
 
 	// With the vPackets fully generated and signed above, we'll serialize