feat: Support anchor outputs on ln channels

[holzeis]

fixes #1596

Adds anchor outputs to the commitment transaction see below an example of a commitment transaction with anchor outputs.

{
    "addresses": [
        "bc1q2zfkaerpl0xe2n0wnkkcxqnj9xx6dac8qnft7kndyxlu8hjpu62qusa7ga",
        "bc1q0jf0ncmpa70frgdkjnkx9khap24yrwgdtjnwx6m4096w7mh2wyns2azzc7",
        "bc1qwgkl04thfchyqctndvvr7w9wc4tdw80078dzdy2cnsnvjdmmrmusapmcl7",
        "bc1q64hyx7qexkzvwa6spwlwek33hwsxy42sty2yeq9vtr5jhkyg68tsyzm68d"
    ],
    "block_height": -1,
    "block_index": -1,
    "confirmations": 0,
    "double_spend": false,
    "fees": 0,
    "hash": "3b94c7ff64af9bbe0821df7e397565a903ae7824e8817ae4b47ef2c878074724",
    "inputs": [
        {
            "age": 0,
            "output_index": 0,
            "prev_hash": "84151c91fd82eee06a0b0b0bf0fb0f0bfdae9915ec5c87b28f86f504f7b49c79",
            "script_type": "empty",
            "sequence": 2160200047
        }
    ],
    "lock_time": 548510010,
    "opt_in_rbf": true,
    "outputs": [
        {
            "addresses": [
                "bc1q2zfkaerpl0xe2n0wnkkcxqnj9xx6dac8qnft7kndyxlu8hjpu62qusa7ga"
            ],
            "script": "002050936ee461fbcd954dee9dad830272298da6f70704d2bf5a6d21bfc3de41e694",
            "script_type": "pay-to-witness-script-hash",
            "value": 330
        },
        {
            "addresses": [
                "bc1q0jf0ncmpa70frgdkjnkx9khap24yrwgdtjnwx6m4096w7mh2wyns2azzc7"
            ],
            "script": "00207c92f9e361ef9e91a1b694ec62dafd0aaa41b90d5ca6e36b757974ef6eea7127",
            "script_type": "pay-to-witness-script-hash",
            "value": 330
        },
        {
            "addresses": [
                "bc1qwgkl04thfchyqctndvvr7w9wc4tdw80078dzdy2cnsnvjdmmrmusapmcl7"
            ],
            "script": "0020722df7d5774e2e4061736b183f38aec556d71deff1da2691589c26c9377b1ef9",
            "script_type": "pay-to-witness-script-hash",
            "value": 90000
        },
        {
            "addresses": [
                "bc1q64hyx7qexkzvwa6spwlwek33hwsxy42sty2yeq9vtr5jhkyg68tsyzm68d"
            ],
            "script": "0020d56e4378193584c777500bbeecda31bba062555059144c80ac58e92bd888d1d7",
            "script_type": "pay-to-witness-script-hash",
            "value": 109056
        }
    ],
    "preference": "low",
    "received": "2023-11-17T07:54:21.743033463Z",
    "relayed_by": "44.197.112.107",
    "size": 443,
    "total": 199716,
    "ver": 2,
    "vin_sz": 1,
    "vout_sz": 4,
    "vsize": 278
}

---

Judging from the rust-lightning release notes regarding anchor nodes (copied below). It looks like though it would make sense to upgrade as soon as possible at least to v0.0.117.

v0.0.118

• Anchor outputs are now properly considered when calculating the amount
  available to send in HTLCs. This can prevent force-closes in anchor channels
  when sending payments which overflow the available balance (#2674).

v0.0.117

• Anchor channels which were closed by a counterparty broadcasting its
  commitment transaction (i.e. force-closing) would previously not generate a
  SpendableOutputs event for our to_remote (i.e. non-HTLC-encumbered)
  balance. Those with such balances available should fetch the missing
  SpendableOutputDescriptors using the new
  ChannelMonitor::get_spendable_outputs method (#2605).

• Anchor channels may result in spurious or missing Balance entries for HTLC
  balances (fix: Show the number of seed words correctly (don't split across lines) #2610).

Security

0.0.117 fixes several loss-of-funds vulnerabilities in anchor output channels,
support for which was added in 0.0.116, in reorg handling, and when accepting
channel(s) from counterparties which are miners.

• When a counterparty broadcasts their latest commitment transaction for a
  channel with anchor outputs, we'd previously fail to build claiming
  transactions against any HTLC outputs in that transaction. This could lead
  to loss of funds if the counterparty is able to eventually claim the HTLC
  after a timeout (feat: use mutinynet faucet for channel opening fundings #2606).

• Anchor channels HTLC claims on-chain previously spent the entire value of any
  HTLCs as fee, which has now been fixed (Failed to run DLC manager periodic check: Storage error Packet extended beyond the provided bytes  #2587).

[luckysori]

Did you look at how ldk-node, ldk-sample and mutiny handle these events when anchor outputs are enabled? Looking at those projects is helpful for me when I upgrade or change the config.

[holzeis]

Did you look at how ldk-node, ldk-sample and mutiny handle these events when anchor outputs are enabled? Looking at those projects is helpful for me when I upgrade or change the config.

I just had a look

• ldk-node and mutiny do not use anchor channels
• ldk-sample uses it, but does not check anything on Event::OpenChannelRequest

[holzeis]

Given the bugs in 116 in regards to anchor channels, I don't think we should go with that change before we've upgraded to 117.

[holzeis]

Closing this PR for now as it is depending on the 117 upgrade.