tapchannel: no passive assets in funding proofs

Previously, anchorVPackets included all "TypeSimple" outputs in the
funding proofs sent to a responder. Passive assets (viz. those located
in the same UTXO, but not selected for funding) also have default output
type TypeSimple, which caused them to be included incorrectly.

The fix is to also check that AnchorOutputIndex == 0, since passive
assets are always assigned to anchor outputs at index >= 1.

Author: jtobin

tapchannel/aux_funding_controller.go

@@ -86,6 +86,10 @@ const (
 	// message is 64k bytes, we leave some breathing room for the chunk
 	// metadata.
 	proofChunkSize = 60_000
+
+	// FundingOutputIndex is the anchor output index for channel funding.
+	// Funding outputs are always at index 0; passive assets use index >= 1.
+	FundingOutputIndex uint32 = 0
 )
 
 // ErrorReporter is used to report an error back to the caller and/or peer that
@@ -1114,6 +1118,13 @@ func (f *FundingController) signAllVPackets(ctx context.Context,
 	return allPackets, activePackets, passivePkts, nil
 }
 
+// isFundingOutput returns true if the output should be included in channel
+// funding proofs.
+func isFundingOutput(vOut *tappsbt.VOutput) bool {
+	return vOut.Type == tappsbt.TypeSimple &&
+		vOut.AnchorOutputIndex == FundingOutputIndex
+}
+
 // anchorVPackets anchors the vPackets to the funding PSBT, creating a
 // complete, but unsigned PSBT packet that can be used to create out asset
 // channel.
@@ -1178,13 +1189,10 @@ func (f *FundingController) anchorVPackets(fundedPkt *tapsend.FundedPsbt,
 
 			vPkt.Outputs[vOutIdx].ProofSuffix = proofSuffix
 
-			// Any output that isn't a split root output is a
-			// channel funding output, so we'll store the proofs
-			// for those outputs. If there is change, that will be
-			// the split root output. And if there is no change,
-			// there is no split root output, as it's an interactive
-			// transfer.
-			if vPkt.Outputs[vOutIdx].Type == tappsbt.TypeSimple {
+			// Only include outputs destined for the funding output
+			// (index 0). Passive assets go to separate anchor
+			// outputs and should not be included in funding proofs.
+			if isFundingOutput(vPkt.Outputs[vOutIdx]) {
 				fundingProofs = append(
 					fundingProofs, proofSuffix,
 				)