LND fee-estimation problem when sweeping taproot assets

[ziggie1984]

Inital issue is here: lightninglabs/taproot-assets#1770

There is a problem of Fee-Estimation in LND when using Overlay channels which need to be fixed probably on the LND side however I open it here because this is the program which suffers from it.

Weight estimation in fee bumper doesn't account for extra change output, causing weight discrepancy and potentially not publishing the sweep transacstion

Description

The weight estimation in the fee bumper (sweep/fee_bumper.go) is missing the extra change output when calculating transaction weight

Steps to Reproduce

1. Create a sweep transaction that requires an extra change output
2. Use the fee bumper to estimate transaction weight
3. Compare estimated weight with actual transaction weight

Expected Behavior

The weight estimator should include all outputs, including the extra change output, to provide accurate size estimation.

Actual Behavior

The weight estimator only includes the main outputs but misses the extra change output, resulting in an underestimated transaction size.

Code Location

File: sweep/fee_bumper.go
Lines: 1704-1708

Something like this needs to be added on the LND side but also tested here on the LITD side. I cannot understand why we didn't run into this in our itests.

extraChangeOut.WhenSome(func(o SweepOutput) {
    log.Infof("Adding extra change output %v to weight estimator",
        o.Value)
    estimator.addOutput(&o.TxOut)
})

However just adding this isn't enough because the witnesssize of the localTaprootCommitSpend seems to differ with overlay channels, compared a taproot channel with an overlay channel in terms of the witness size by 32 bytes => 8 vbytes!

https://github.com/lightningnetwork/lnd/blob/0c2f045f5defac7215ab7b6f6eea9ac4297c1b01/input/size.go#L625

See this example LocalAssetSweep:

 "txid": "f10c2568e0761ae1237ad5f9cf97937634496e4951bcb7ef498c0dea854991b8",
  "hash": "b493125886ac03a76bacc94162b0d8131970bfb2631e470b3125452cc9125795",
  "version": 2,
  "size": 234,
  "vsize": 129,
  "weight": 516,
  "locktime": 513,
  "vin": [
    {
      "txid": "6208f51938bc204cf158599a0764800a795b00029dca556a0e0be950779be0b8",
      "vout": 2,
      "scriptSig": {
        "asm": "",
        "hex": ""
      },
      "txinwitness": [
        "e62913970013a8336106f210395c14faa3724f22898936b1ca3f74a9cf287291bd59ac2f0c138b6e3bc7f724b4bafcf02dfcb7447ed7166ea484ef8d120b7c6f",
        "20d803c541003b94cfee17af01d9b9f757ec2e566f1d1213275a3f7ec549b7abebac51b275",
        "c1dca094751109d0bd055d03565874e8276dd53e926b44e3bd1bb6bf4bc130a279"
      ],
      "sequence": 1
    }
  ],
  "vout": [
    {
      "value": 0.00010870,
      "n": 0,
      "scriptPubKey": {
        "asm": "1 4758048401a0ae19bea5a530602c8e56a62127c95694622beee0e2a3e734ca00",
        "desc": "rawtr(4758048401a0ae19bea5a530602c8e56a62127c95694622beee0e2a3e734ca00)#6200sr9e",
        "hex": "51204758048401a0ae19bea5a530602c8e56a62127c95694622beee0e2a3e734ca00",
        "address": "bcrt1pgavqfpqp5zhpn04955cxqtyw26nzzf7f262xy2lwur328ee5egqqw8t3yx",
        "type": "witness_v1_taproot"
      }
    }
  ],
  "hex": "02000000000101b8e09b7750e90b0e6a55ca9d02005b790a8064079a5958f14c20bc3819f5086202000000000100000001762a0000000000002251204758048401a0ae19bea5a530602c8e56a62127c95694622beee0e2a3e734ca000340e62913970013a8336106f210395c14faa3724f22898936b1ca3f74a9cf287291bd59ac2f0c138b6e3bc7f724b4bafcf02dfcb7447ed7166ea484ef8d120b7c6f2520d803c541003b94cfee17af01d9b9f757ec2e566f1d1213275a3f7ec549b7abebac51b27521c1dca094751109d0bd055d03565874e8276dd53e926b44e3bd1bb6bf4bc130a27901020000"
}```

[ViktorTigerstrom]

Looking at the litd logs in https://github.com/lightninglabs/taproot-assets/issues/1770#issuecomment-3234150914,`litd` gets blocked here:

lightning-terminal/terminal.go

Line 924 in 911f8ca

for {

as it it'll never successfully setup the full lnd-client which is evident by the missing log lines:

lnd is now fully synced to its chain backend

&

Full lnd client connected

We need the full lnd-client before we can start the integrated sub-servers in litd, which include tapd. Here's the line that start those:

lightning-terminal/terminal.go

Line 779 in 911f8ca

g.subServerMgr.StartIntegratedServers(

So the main issue of why tapd is not started is that lnd never gets marked as gets marked as synced to its chain backend. There's no easy workaround for this currently unfortunately.

[Roasbeef]

I think there was a regression here: https://github.com/lightningnetwork/lnd/blob/f51684dd0f97574ab11e984f10e5c53314ed9922/sweep/fee_bumper.go#L1652

That should be adding the additional pk script as change output. The getWeightEstimate function was modified to enable this.