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Define a method for determining the max HTLCs rather than a const
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Once we support zero-fee commitment transactions, we will no longer
have a constant number of maximum HTLCs in-flight per channel but
rather it will depend on the channel type.

Here we prepare for this by removing the `MAX_HTLCS` constant and
replacing it with a function.
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@TheBlueMatt
TheBlueMatt committed Mar 10, 2025
1 parent 45127a3 commit 67c5993
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Showing 5 changed files with 45 additions and 31 deletions.
10 changes: 8 additions & 2 deletions lightning/src/ln/chan_utils.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -52,8 +52,14 @@ use super::channel_keys::{DelayedPaymentBasepoint, DelayedPaymentKey, HtlcKey, H
#[allow(unused_imports)]
use crate::prelude::*;

/// Maximum number of one-way in-flight HTLC (protocol-level value).
pub const MAX_HTLCS: u16 = 483;
/// Maximum number of in-flight HTLCs in each direction allowed by the lightning protocol.
///
/// 483 for non-zero-fee-commitment channels and 114 for zero-fee-commitment channels.
///
/// Actual maximums can be set equal to or below this value by each channel participant.
pub fn max_htlcs(_channel_type: &ChannelTypeFeatures) -> u16 {
483
}
/// The weight of a BIP141 witnessScript for a BOLT3's "offered HTLC output" on a commitment transaction, non-anchor variant.
pub const OFFERED_HTLC_SCRIPT_WEIGHT: usize = 133;
/// The weight of a BIP141 witnessScript for a BOLT3's "offered HTLC output" on a commitment transaction, anchor variant.
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47 changes: 25 additions & 22 deletions lightning/src/ln/channel.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -43,7 +43,7 @@ use crate::ln::chan_utils::{
CounterpartyCommitmentSecrets, TxCreationKeys, HTLCOutputInCommitment, htlc_success_tx_weight,
htlc_timeout_tx_weight, ChannelPublicKeys, CommitmentTransaction,
HolderCommitmentTransaction, ChannelTransactionParameters,
CounterpartyChannelTransactionParameters, MAX_HTLCS,
CounterpartyChannelTransactionParameters, max_htlcs,
get_commitment_transaction_number_obscure_factor,
ClosingTransaction, commit_tx_fee_sat,
};
Expand Down Expand Up @@ -2457,8 +2457,8 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
if open_channel_fields.max_accepted_htlcs < 1 {
return Err(ChannelError::close("0 max_accepted_htlcs makes for a useless channel".to_owned()));
}
if open_channel_fields.max_accepted_htlcs > MAX_HTLCS {
return Err(ChannelError::close(format!("max_accepted_htlcs was {}. It must not be larger than {}", open_channel_fields.max_accepted_htlcs, MAX_HTLCS)));
if open_channel_fields.max_accepted_htlcs > max_htlcs(&channel_type) {
return Err(ChannelError::close(format!("max_accepted_htlcs was {}. It must not be larger than {}", open_channel_fields.max_accepted_htlcs, max_htlcs(&channel_type))));
}

// Now check against optional parameters as set by config...
Expand Down Expand Up @@ -2686,7 +2686,7 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
counterparty_htlc_minimum_msat: open_channel_fields.htlc_minimum_msat,
holder_htlc_minimum_msat: if config.channel_handshake_config.our_htlc_minimum_msat == 0 { 1 } else { config.channel_handshake_config.our_htlc_minimum_msat },
counterparty_max_accepted_htlcs: open_channel_fields.max_accepted_htlcs,
holder_max_accepted_htlcs: cmp::min(config.channel_handshake_config.our_max_accepted_htlcs, MAX_HTLCS),
holder_max_accepted_htlcs: cmp::min(config.channel_handshake_config.our_max_accepted_htlcs, max_htlcs(&channel_type)),
minimum_depth,

counterparty_forwarding_info: None,
Expand Down Expand Up @@ -2923,7 +2923,7 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
counterparty_htlc_minimum_msat: 0,
holder_htlc_minimum_msat: if config.channel_handshake_config.our_htlc_minimum_msat == 0 { 1 } else { config.channel_handshake_config.our_htlc_minimum_msat },
counterparty_max_accepted_htlcs: 0,
holder_max_accepted_htlcs: cmp::min(config.channel_handshake_config.our_max_accepted_htlcs, MAX_HTLCS),
holder_max_accepted_htlcs: cmp::min(config.channel_handshake_config.our_max_accepted_htlcs, max_htlcs(&channel_type)),
minimum_depth: None, // Filled in in accept_channel

counterparty_forwarding_info: None,
Expand Down Expand Up @@ -3183,6 +3183,22 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
if !matches!(self.channel_state, ChannelState::NegotiatingFunding(flags) if flags == NegotiatingFundingFlags::OUR_INIT_SENT) {
return Err(ChannelError::close("Got an accept_channel message at a strange time".to_owned()));
}

if let Some(ty) = &common_fields.channel_type {
if *ty != self.channel_type {
return Err(ChannelError::close("Channel Type in accept_channel didn't match the one sent in open_channel.".to_owned()));
}
} else if their_features.supports_channel_type() {
// Assume they've accepted the channel type as they said they understand it.
} else {
let channel_type = ChannelTypeFeatures::from_init(&their_features);
if channel_type != ChannelTypeFeatures::only_static_remote_key() {
return Err(ChannelError::close("Only static_remote_key is supported for non-negotiated channel types".to_owned()));
}
self.channel_type = channel_type.clone();
funding.channel_transaction_parameters.channel_type_features = channel_type;
}

if common_fields.dust_limit_satoshis > 21000000 * 100000000 {
return Err(ChannelError::close(format!("Peer never wants payout outputs? dust_limit_satoshis was {}", common_fields.dust_limit_satoshis)));
}
Expand All @@ -3207,8 +3223,10 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
if common_fields.max_accepted_htlcs < 1 {
return Err(ChannelError::close("0 max_accepted_htlcs makes for a useless channel".to_owned()));
}
if common_fields.max_accepted_htlcs > MAX_HTLCS {
return Err(ChannelError::close(format!("max_accepted_htlcs was {}. It must not be larger than {}", common_fields.max_accepted_htlcs, MAX_HTLCS)));

let channel_type = &funding.channel_transaction_parameters.channel_type_features;
if common_fields.max_accepted_htlcs > max_htlcs(channel_type) {
return Err(ChannelError::close(format!("max_accepted_htlcs was {}. It must not be larger than {}", common_fields.max_accepted_htlcs, max_htlcs(channel_type))));
}

// Now check against optional parameters as set by config...
Expand All @@ -3234,21 +3252,6 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
return Err(ChannelError::close(format!("We consider the minimum depth to be unreasonably large. Expected minimum: ({}). Actual: ({})", peer_limits.max_minimum_depth, common_fields.minimum_depth)));
}

if let Some(ty) = &common_fields.channel_type {
if *ty != self.channel_type {
return Err(ChannelError::close("Channel Type in accept_channel didn't match the one sent in open_channel.".to_owned()));
}
} else if their_features.supports_channel_type() {
// Assume they've accepted the channel type as they said they understand it.
} else {
let channel_type = ChannelTypeFeatures::from_init(&their_features);
if channel_type != ChannelTypeFeatures::only_static_remote_key() {
return Err(ChannelError::close("Only static_remote_key is supported for non-negotiated channel types".to_owned()));
}
self.channel_type = channel_type.clone();
funding.channel_transaction_parameters.channel_type_features = channel_type;
}

let counterparty_shutdown_scriptpubkey = if their_features.supports_upfront_shutdown_script() {
match &common_fields.shutdown_scriptpubkey {
&Some(ref script) => {
Expand Down
7 changes: 4 additions & 3 deletions lightning/src/ln/functional_tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -10609,8 +10609,9 @@ pub fn test_nondust_htlc_excess_fees_are_dust() {
config.channel_config.max_dust_htlc_exposure =
MaxDustHTLCExposure::FeeRateMultiplier(10_000);
// Make sure the HTLC limits don't get in the way
config.channel_handshake_limits.min_max_accepted_htlcs = chan_utils::MAX_HTLCS;
config.channel_handshake_config.our_max_accepted_htlcs = chan_utils::MAX_HTLCS;
let chan_ty = ChannelTypeFeatures::only_static_remote_key();
config.channel_handshake_limits.min_max_accepted_htlcs = chan_utils::max_htlcs(&chan_ty);
config.channel_handshake_config.our_max_accepted_htlcs = chan_utils::max_htlcs(&chan_ty);
config.channel_handshake_config.our_htlc_minimum_msat = 1;
config.channel_handshake_config.max_inbound_htlc_value_in_flight_percent_of_channel = 100;

Expand Down Expand Up @@ -10651,7 +10652,7 @@ pub fn test_nondust_htlc_excess_fees_are_dust() {
let commitment_tx_per_htlc_cost =
htlc_success_tx_weight(&ChannelTypeFeatures::empty()) * EXCESS_FEERATE as u64;
let max_htlcs_remaining = dust_limit * 2 / commitment_tx_per_htlc_cost;
assert!(max_htlcs_remaining < chan_utils::MAX_HTLCS.into(),
assert!(max_htlcs_remaining < chan_utils::max_htlcs(&chan_ty).into(),
"We should be able to fill our dust limit without too many HTLCs");
for i in 0..max_htlcs_remaining + 1 {
assert_ne!(i, max_htlcs_remaining);
Expand Down
6 changes: 4 additions & 2 deletions lightning/src/util/anchor_channel_reserves.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -25,10 +25,11 @@ use crate::chain::chainmonitor::ChainMonitor;
use crate::chain::chainmonitor::Persist;
use crate::chain::Filter;
use crate::events::bump_transaction::Utxo;
use crate::ln::chan_utils::MAX_HTLCS;
use crate::ln::chan_utils::max_htlcs;
use crate::ln::channelmanager::AChannelManager;
use crate::prelude::new_hash_set;
use crate::sign::ecdsa::EcdsaChannelSigner;
use crate::types::features::ChannelTypeFeatures;
use crate::util::logger::Logger;
use bitcoin::constants::WITNESS_SCALE_FACTOR;
use bitcoin::Amount;
Expand Down Expand Up @@ -178,7 +179,8 @@ impl Default for AnchorChannelReserveContext {
fn get_reserve_per_channel_with_input(
context: &AnchorChannelReserveContext, initial_input_weight: Weight,
) -> Amount {
let expected_accepted_htlcs = min(context.expected_accepted_htlcs, MAX_HTLCS) as u64;
let max_max_htlcs = max_htlcs(&ChannelTypeFeatures::only_static_remote_key());
let expected_accepted_htlcs = min(context.expected_accepted_htlcs, max_max_htlcs) as u64;
let weight = Weight::from_wu(
COMMITMENT_TRANSACTION_BASE_WEIGHT +
// Reserves are calculated in terms of accepted HTLCs, as their timeout defines the urgency of
Expand Down
6 changes: 4 additions & 2 deletions lightning/src/util/config.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -193,8 +193,10 @@ pub struct ChannelHandshakeConfig {
///
/// Default value: `50`
///
/// Maximum value: `483` (Any values larger will be treated as `483`. This is the BOLT #2 spec
/// limit on `max_accepted_htlcs`.)
/// Maximum value: depends on channel type, see docs on [`max_htlcs`] (any values over the
/// maximum will be silently reduced to the maximum).
///
/// [`max_htlcs`]: crate::ln::chan_utils::max_htlcs
pub our_max_accepted_htlcs: u16,
}

Expand Down

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