tapchannel: add blobWithWitnessInfo struct

In this commit, we add `blobWithWitnessInfo` which couples the raw
resolution TLV blob with some additional information such as the input
that carried the blob, and also preimage information. Preimage
information is needed to know where to insert the preimage in the
witness once we learn about it on chain.

Author: Roasbeef

tapchannel/aux_sweeper.go

@@ -1614,6 +1614,105 @@ func (a *AuxSweeper) resolveContract(
 	)
 }
 
+// preimageDesc is a helper struct that contains the preimage and the witness
+// index that the preimage should be placed within the witness stack. This is
+// useful as in an earlier step, we've already pre-signed the witness, but will
+// learn of the preimage later.
+type preimageDesc struct {
+	// preimage is the preimage that we'll use to update the witness stack.
+	preimage lntypes.Preimage
+
+	// witnessIndex is the index within the witness stack that the preimage
+	// should be placed at.
+	witnessIndex int
+}
+
+// blobWithWitnessInfo is a helper struct that contains a resolution blob, along
+// with optional preimage information. If the preimage information is present,
+// then we'll use this to update the witness stack of the final vPacket before
+// we anchor it into the sweep output.
+type blobWithWitnessInfo struct {
+	// resolutionBlob is the serialized resolution blob that contains the
+	// vPackets.
+	resolutionBlob tlv.Blob
+
+	// input is the sweep input that we created this blob using.
+	input input.Input
+
+	// preimageInfo is an optional field that contains the preimage and info
+	// w.r.t where to place it in the witness stack.
+	preimageInfo lfn.Option[preimageDesc]
+
+	// secondLevel indicates if this is a second level sweep.
+	secondLevel bool
+}
+
+// newBlobWithWitnessInfo creates a new blobWithWitnessInfo struct from a passed
+// input.Input, which stores the resolution blob and other information.
+func newBlobWithWitnessInfo(i input.Input) blobWithWitnessInfo {
+	// If this is a success input, then we'll need to extract the preimage
+	// from the inner struct, so we can update the witness stack.
+	var (
+		preimageInfo lfn.Option[preimageDesc]
+		secondLevel  bool
+	)
+	switch i.WitnessType() {
+	// This is the case when we're sweeping the HTLC output on our local
+	// commitment transaction via a second level HTLC.
+	//
+	// The final witness stack is:
+	//  * <sender sig> <receiver sig> <preimage> <success_script>
+	//    <control_block>
+	//
+	// So we'll place the preimage at index 2.
+	case input.TaprootHtlcAcceptedLocalSuccess:
+		preimage := i.Preimage()
+
+		preimageInfo = lfn.MapOption(
+			func(p lntypes.Preimage) preimageDesc {
+				return preimageDesc{
+					preimage:     p,
+					witnessIndex: 2,
+				}
+			},
+		)(preimage)
+
+	// This is the case when we're sweeping the HTLC output we received on
+	// the remote party's version of the commitment transaction.
+	//
+	// The final witness stack is:
+	//  <receiver sig> <preimage> <success_script> <control_block>
+	//
+	//  So we'll place the preimage at index 1.
+	case input.TaprootHtlcAcceptedRemoteSuccess:
+		preimage := i.Preimage()
+
+		preimageInfo = lfn.MapOption(
+			func(p lntypes.Preimage) preimageDesc {
+				return preimageDesc{
+					preimage:     p,
+					witnessIndex: 1,
+				}
+			},
+		)(preimage)
+
+	// For second level sweeps, we don't need to note anything about a
+	// preimage, but will note that this is a second level output.
+	case input.TaprootHtlcOfferedTimeoutSecondLevel:
+		fallthrough
+	case input.TaprootHtlcAcceptedSuccessSecondLevel:
+		secondLevel = true
+	}
+
+	// We already know this has a blob from the filter in an earlier step.
+	return blobWithWitnessInfo{
+		resolutionBlob: i.ResolutionBlob().UnwrapOr(nil),
+		input:          i,
+		preimageInfo:   preimageInfo,
+		secondLevel:    secondLevel,
+	}
+}
+
 // extractInputVPackets extracts the vPackets from the inputs passed in. If
 // none of the inputs have any resolution blobs. Then an empty slice will be
 // returned.