[bug]: InboundFees are overwritten if they were not explicitly specified in lncli

[feelancer21]

Background

I run the current master on testnet and tested the setting of inbound fees with lncli updatechanpolicy. But the inbound fees are not preserved if I run the command without the command line flags

Your environment

• testnet
• version of lnd
  fn/v1.0.5-129-gb1175514f

Steps to reproduce

1. set inbound fees with
   lncli updatechanpolicy --base_fee_msat 0 --fee_rate_ppm 200 --time_lock_delta 144 --inbound_fee_rate_ppm -100 --inbound_base_fee_msat -1 --chan_point 9952586a20fecd4d99c3f400c44f10e17c674f1050f773656a42acbf7b80901c:0

2. check the inbound fees

lncli getchaninfo 2579374014416486400
{
    "channel_id":  "2579374014416486400",
    "chan_point":  "9952586a20fecd4d99c3f400c44f10e17c674f1050f773656a42acbf7b80901c:0",
    "last_update":  1712092954,
    "node1_pub":  "024e97ccd1382cb736f86694e0d1312cb53fdcbffb93f13f2626702c3d74cabf99",
    "node2_pub":  "038214ce28b285a6233300fc114184c1e1da7bab146d705679339c304e118330a1",
    "capacity":  "20000",
    "node1_policy":  {
        "time_lock_delta":  144,
        "min_htlc":  "2100000",
        "fee_base_msat":  "0",
        "fee_rate_milli_msat":  "200",
        "disabled":  false,
        "max_htlc_msat":  "19800000",
        "last_update":  1712092954,
        "custom_records":  {
            "55555":  "ffffffffffffff9c"
        },
        "inbound_fee_base_msat":  -1,
        "inbound_fee_rate_milli_msat":  -100
    },
    "node2_policy":  {
        "time_lock_delta":  144,
        "min_htlc":  "2100000",
        "fee_base_msat":  "0",
        "fee_rate_milli_msat":  "250",
        "disabled":  false,
        "max_htlc_msat":  "7858400",
        "last_update":  1712092021,
        "custom_records":  {},
        "inbound_fee_base_msat":  0,
        "inbound_fee_rate_milli_msat":  0
    },
    "custom_records":  {}
}

3. Set the policy again without inbound fees
   lncli updatechanpolicy --base_fee_msat 0 --fee_rate_ppm 300 --time_lock_delta 144 --chan_point 9952586a20fecd4d99c3f400c44f10e17c674f1050f773656a42acbf7b80901c:0

4. Check the inbound fees again

lncli getchaninfo 2579374014416486400
{
    "channel_id":  "2579374014416486400",
    "chan_point":  "9952586a20fecd4d99c3f400c44f10e17c674f1050f773656a42acbf7b80901c:0",
    "last_update":  1712093293,
    "node1_pub":  "024e97ccd1382cb736f86694e0d1312cb53fdcbffb93f13f2626702c3d74cabf99",
    "node2_pub":  "038214ce28b285a6233300fc114184c1e1da7bab146d705679339c304e118330a1",
    "capacity":  "20000",
    "node1_policy":  {
        "time_lock_delta":  144,
        "min_htlc":  "2100000",
        "fee_base_msat":  "0",
        "fee_rate_milli_msat":  "300",
        "disabled":  false,
        "max_htlc_msat":  "19800000",
        "last_update":  1712093293,
        "custom_records":  {
            "55555":  "0000000000000000"
        },
        "inbound_fee_base_msat":  0,
        "inbound_fee_rate_milli_msat":  0
    },
    "node2_policy":  {
        "time_lock_delta":  144,
        "min_htlc":  "2100000",
        "fee_base_msat":  "0",
        "fee_rate_milli_msat":  "250",
        "disabled":  false,
        "max_htlc_msat":  "7858400",
        "last_update":  1712092021,
        "custom_records":  {},
        "inbound_fee_base_msat":  0,
        "inbound_fee_rate_milli_msat":  0
    },
    "custom_records":  {}
}

Expected behaviour

Inbound Fees have to be preserved.

[Roasbeef]

I think it's an existing bug with the way updatechanpolicy works in that you need to set everything otherwise it doesn't know what was a default vs just setting that value to nothing (eg: zero fees).

[feelancer21]

Looked in the code because I wondered why max_htlc_msat and min_htlc_msat are preseverd. max_htlc_msat is replaced by the current value if a value of zero is submitted. And min_htlc_msat is set only if min_htlc_msat_specified is set.

I think we need also a boolean like inbound_fees_specified to make UpdateChanPolicy backwards compatible. If it is set then both inbound_base_fee_msat and inbound_fee_rate_ppm are updated.

[SulaimanAminuBarkindo]

I'll jump on this. If there are any more relevant details that i might need, please share them. Thanks

[feelancer21]

I'll jump on this. If there are any more relevant details that i might need, please share them. Thanks

Pretty cool. Had a short look on this some days ago. But haven't really started. So let's go.

[feelancer21]

Started working on this. PR soon (tm)

https://github.com/feelancer21/lnd/tree/preserve-inbound-fees

[SulaimanAminuBarkindo]

Started working on this. PR soon (tm)

https://github.com/feelancer21/lnd/tree/preserve-inbound-fees

Awesome I am caught up with something. Looking forward to the PR