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Round LSPS2 JIT channel amounts up to whole sats #1134

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Round LSPS2 JIT channel amounts up to whole sats #1134

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3 changes: 2 additions & 1 deletion src/liquidity/service/lsps2.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -448,7 +448,8 @@ where
let over_provisioning_msat = (amt_to_forward_msat
* service_config.channel_over_provisioning_ppm as u64)
/ 1_000_000;
let channel_amount_sats = (amt_to_forward_msat + over_provisioning_msat) / 1000;
let channel_amount_sats =
(amt_to_forward_msat + over_provisioning_msat).div_ceil(1000);
Comment on lines +451 to +452

@ajaysehwal ajaysehwal Oct 10, 2026

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The div_ceil fix looks good, and I like that the funds check (L462) and create_channel use the same variable. One caveat: the channel isn't guaranteed to have enough usable capacity for the forward.

With channel_over_provisioning_ppm = 0, the channel is sized to exactly ceil(forward) sats and funded entirely by the LSP (push_msat = 0). The LSP still needs to account for the client's reserve (~1% by default), commitment fees, and anchor outputs, so the HTLC may not fit.

Could we either scope the PR description to fixing sub-sat truncation, or account for the reserve and fees explicitly and document the minimum sensible channel_over_provisioning_ppm?

let cur_anchor_reserve_sats =
total_anchor_channels_reserve_sats(&self.channel_manager, &self.config);
let spendable_amount_sats =
Expand Down
16 changes: 11 additions & 5 deletions tests/integration_tests_rust.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3837,11 +3837,18 @@ async fn unified_send_receive_bip21_uri() {

#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
async fn lsps2_client_service_integration() {
do_lsps2_client_service_integration(true).await;
do_lsps2_client_service_integration(false).await;
do_lsps2_client_service_integration(true, 100_000_000).await;
do_lsps2_client_service_integration(false, 100_000_000).await;
}

async fn do_lsps2_client_service_integration(client_trusts_lsp: bool) {
#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
async fn lsps2_jit_channel_rounds_fractional_msat_up() {
// A forward amount that is not a whole number of satoshis must result in a channel large
// enough to carry it, i.e., the channel amount needs to be rounded up, not down.
do_lsps2_client_service_integration(false, 100_000_100).await;
}

async fn do_lsps2_client_service_integration(client_trusts_lsp: bool, jit_amount_msat: u64) {
let (bitcoind, electrsd) = setup_bitcoind_and_electrsd();
let esplora_url = format!("http://{}", electrsd.esplora_url.as_ref().unwrap());

Expand Down Expand Up @@ -3918,7 +3925,6 @@ async fn do_lsps2_client_service_integration(client_trusts_lsp: bool) {

let invoice_description =
Bolt11InvoiceDescription::Direct(Description::new(String::from("asdf")).unwrap());
let jit_amount_msat = 100_000_000;

println!("Generating JIT invoice!");
let jit_invoice = client_node
Expand Down Expand Up @@ -3951,7 +3957,7 @@ async fn do_lsps2_client_service_integration(client_trusts_lsp: bool) {
let expected_channel_overprovisioning_msat =
(expected_received_amount_msat * channel_over_provisioning_ppm as u64) / 1_000_000;
let expected_channel_size_sat =
(expected_received_amount_msat + expected_channel_overprovisioning_msat) / 1000;
(expected_received_amount_msat + expected_channel_overprovisioning_msat).div_ceil(1000);
let channel_value_sats = client_node.list_channels().first().unwrap().channel_value_sats;
assert_eq!(channel_value_sats, expected_channel_size_sat);

Expand Down
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