Automate lnd on-chain allowence to be sufficient for anchor closing

[DanGould]

By

Note: if CHAIN_WALLET_AMOUNT_SATS is present another output will be added to send the amount to the internal wallet. This may be required in case the wallet is empty as in such case LND can not reserve sats for anchor commitments. However, to truly work, you need LND 0.14.2.

I thought LND 14.2 would be able to open a channel without setting CHAIN_WALLET_AMOUNT_SATS. What am I missing? Is there some setting to allow 0 internal wallet balance opens? Is that behavior even desirable or is the on-chain utxo better?

Running LND 14.2 (and 14.0, 14.3, 15.0) from lightning polar. Loptos works like a charm with CHAIN_WALLET_AMOUNT_SATS set sufficiently high.

[Kixunil]

This is LND restriction because for anchors to safely work you need to have some onchain sats ready for fee bumping.
So if you already have min((number_of_currently_open_channels + number_of_channels_opened_by_loptos) * 10000, 100000) sats in the wallet you don't need CHAIN_WALLET_AMOUNT_SATS.
If you have less sats you need to add more sats to satisfy the condition. This can be done in the same transaction you open the channels by specifying the amount in CHAIN_WALLET_AMOUNT_SATS. In that case the transaction has these outputs (should be in random order):

• channel_0
• channel_1
• ...
• channel_n
• change_to_payer
• lnd_output

Where lnd_output is segwit-native p2pk belonging to LND. LND 14.2 and higher detects that this is happening and allows opening of the channel. In case the reserve was low older LND versions will incorrectly reject such transaction thinking the reserve is not high enough.

Hope it's clear. I guess this should be documented somewhere more clearly.

[Kixunil]

Also forgot to mention there's a new LND RPC to reliably calculate the amount in sufficiently recent LND. We should use that to check the amount before attempting to open.

[DanGould]

That RequiredReserve RPC is in the WalletKit service. We need this PR in tonic_lnd to support walletrpc without hacks