tapchannel: fix coop close asset distribution issue

This commit fixes the actual issue. It makes sure that the way the asset
pieces of different asset IDs (for example in grouped asset channels)
are distributed matches exactly the last commitment transaction.
See comment above call to CommitmentToPackets in this commit for more
details.

Author: guggero

tapchannel/aux_closer.go

@@ -402,13 +402,43 @@ func (a *AuxChanCloser) AuxCloseOutputs(
 		closeAllocs[idx].OutputIndex = uint32(idx)
 	}
 
-	// Now that we have the complete set of allocations, we'll distribute
-	// them to create the vPackets we'll need to anchor everything.
-	vPackets, err := tapsend.DistributeCoins(
-		inputProofs, closeAllocs, a.cfg.ChainParams, true, tappsbt.V1,
+	// Now we know the deterministic ordering of the local/remote asset/btc
+	// outputs, we can extract the output indexes for the allocations.
+	var (
+		localOutputIndex, remoteOutputIndex uint32
+	)
+	if localAlloc != nil {
+		localOutputIndex = localAlloc.OutputIndex
+	}
+	if remoteAlloc != nil {
+		remoteOutputIndex = remoteAlloc.OutputIndex
+	}
+
+	// We don't use the normal allocation code here. This requires a bit of
+	// a lengthy explanation: When we close a channel, the output of the
+	// `lncli closedchannels` command will show the last commitment state of
+	// the channel as the closing asset balance. Which is correct in terms
+	// of balances. But if there are multiple different asset IDs (e.g., in
+	// a grouped asset channel), then _how_ those pieces are distributed
+	// within the commitment transaction depends on the order of the
+	// allocations. And the order of the allocations is dependent on the
+	// BTC amount and the pkScript of the BTC-level output. Both of which
+	// are different in the coop close output (we set the asset-level output
+	// BTC amount to the dummy amount, and the pkScript will be a newly
+	// derived internal key with no sibling script path).
+	// So, long story short: If we used the tapsend.DistributeCoins method
+	// here, it could happen that the actual asset output distribution shown
+	// in the `lncli closedchannels` command would be different from the
+	// actual distribution in the co-op close transaction.
+	// This could mostly be seen as an UX-only issue, but was actually
+	// discovered while attempting to assert the final closing balance of
+	// grouped asset channels in the litd integration test.
+	vPackets, err := CommitmentToPackets(
+		commitState, inputProofs, a.cfg.ChainParams, localShutdown,
+		remoteShutdown, localOutputIndex, remoteOutputIndex, tappsbt.V1,
 	)
 	if err != nil {
-		return none, fmt.Errorf("unable to distribute coins: %w", err)
+		return none, fmt.Errorf("unable to create vPackets: %w", err)
 	}
 
 	// We can now add the witness for the OP_TRUE spend of the commitment