Bug: balance reported by /channels disagrees with what LND checks in payinvoice

[Kixunil]

Background

Paying an invoice can fail with "insufficient balance" even if the local balance (with channel reserve subtracted) reported by /channel is higher than amount in the invoice.

Python code calculating the balance

Your environment

• version of lnd: 0.11.0
• which operating system (uname -a on *Nix) :Linux disp7952 4.19.132-1.pvops.qubes.x86_64 #1 SMP Tue Jul 14 03:42:21 UTC 2020 x86_64 GNU/Linux
• version of btcd, bitcoind, or other backend: Bitcoin Core version v0.20.0
• any other relevant environment details: regtest

Steps to reproduce

1. Setup two regtest LN nodes and a bitcoind node
2. Create (and confirm) a 40000 sat channel from A to B
3. Create a 20000 sat invoice at node B and pay it from node A
4. Get balance of the channel
5. Create a 10000 sat invoice and attempt to pay it from A

Expected behaviour

Either balance is reported to be lower than the invoice amount or the payment succeeds.

Actual behaviour

The reported balance is 10337, yet paying a 10000 sat invoice fails.

[Roasbeef]

Are you taking into account the reserve and also the fees we "deduct" when we need to pay for the HTLC on the commitment transaction in the intermediate state?

[Kixunil]

Are you taking into account the reserve

Yes, that's what I meant by "with channel reserve subtracted"

the fees we "deduct"

Oh, this is not a part of reserve? Is there any way to get it from LND API? commit_fee doesn't sound like it, but maybe it is?

[Roasbeef]

The only way to compute that is to do it manually. Here's an example:

• We have 10k satoshis available balance
• We go to add an HTLC that is 9k satoshis
• As a result of this new HTLC, since we're the initiator, we need to pay 2k satoshis of fees
• We now have a 1k satoshi shortage so we reject this HTLC addition

[Kixunil]

Is that 2k sats a constant or do I need to retrieve it from the node somehow?

[Roasbeef]

Is that 2k sats a constant or do I need to retrieve it from the node somehow?

The size/weight of an HTLC is known, but what's variable is the actual fee rate used on the commitment. This value can be checked using listchannels.

So given that, I don't think there's an actual issue here, and would file it under #2708 -- which has some hidden dragons on the way to implementation.

[Kixunil]

Ah, OK, thanks!