Drop ChannelHandshakeLimits::max_funding_satoshis

Previously, LDK would by default limit channels pre-Wumbo sizes and
leave it to the user to bump
`ChannelHandshakeLimits::max_funding_satoshis`. This has mostly
historical reasons that aimed to allow limiting risk when Lightning and
LDK were not as matured as today. By now, we do however expect ~all
users to eventually want to bump this limit, and having them pick an
arbitrary value (or pick a default ourselves) is kinda odd. Users that
still want to limit risks have ample other means to do so, for example
manually rejecting inbound channels via the manual-acceptence flow (via
`Event::OpenChannelRequest`) or soon even limiting risk on a per-HTLC
basis via general purpose HTLC interception.

Furthermore, it turns out that our current implementation is wrong, as
we do always announce `Wumbo`/`option_supports_large_channels` support
via the `IN` feature in `ChannelManager` defaults, irrespective of what
limit is configured. This has us announcing support for Wumbo channels
to only then reject inbound requests in case a counterparty dares to
actually try to open one.

To address this, we here simply propose to drop the
`max_funding_satoshis` field and corresponding checks entirely, and do
what we've announced to the network for a long time: enable Wumbo by
default.

Author: tnull

lightning/src/ln/channel.rs

@@ -3413,13 +3413,6 @@ where
 			return Err(ChannelError::close(format!("Configured with an unreasonable our_to_self_delay ({}) putting user funds at risks. It must be greater than {}", config.channel_handshake_config.our_to_self_delay, BREAKDOWN_TIMEOUT)));
 		}
 
-		// Check sanity of message fields:
-		if channel_value_satoshis > config.channel_handshake_limits.max_funding_satoshis {
-			return Err(ChannelError::close(format!(
-				"Per our config, funding must be at most {}. It was {}. Peer contribution: {}. Our contribution: {}",
-				config.channel_handshake_limits.max_funding_satoshis, channel_value_satoshis,
-				open_channel_fields.funding_satoshis, our_funding_satoshis)));
-		}
 		if channel_value_satoshis >= TOTAL_BITCOIN_SUPPLY_SATOSHIS {
 			return Err(ChannelError::close(format!("Funding must be smaller than the total bitcoin supply. It was {}", channel_value_satoshis)));
 		}
@@ -16068,11 +16061,7 @@ mod tests {
 	use crate::ln::channel::{
 		AwaitingChannelReadyFlags, ChannelState, FundedChannel, HTLCCandidate, HTLCInitiator,
 		HTLCUpdateAwaitingACK, InboundHTLCOutput, InboundHTLCState, InboundV1Channel,
-		OutboundHTLCOutput, OutboundHTLCState, OutboundV1Channel,
-	};
-	use crate::ln::channel::{
-		MAX_FUNDING_SATOSHIS_NO_WUMBO, MIN_THEIR_CHAN_RESERVE_SATOSHIS,
-		TOTAL_BITCOIN_SUPPLY_SATOSHIS,
+		OutboundHTLCOutput, OutboundHTLCState, OutboundV1Channel, MIN_THEIR_CHAN_RESERVE_SATOSHIS,
 	};
 	use crate::ln::channel_keys::{RevocationBasepoint, RevocationKey};
 	use crate::ln::channelmanager::{self, HTLCSource, PaymentId};
@@ -16125,15 +16114,6 @@ mod tests {
 		assert!(ChannelState::ChannelReady(ChannelReadyFlags::new()) < ChannelState::ShutdownComplete);
 	}
 
-	#[test]
-	fn test_max_funding_satoshis_no_wumbo() {
-		assert_eq!(TOTAL_BITCOIN_SUPPLY_SATOSHIS, 21_000_000 * 100_000_000);
-		assert!(
-			MAX_FUNDING_SATOSHIS_NO_WUMBO <= TOTAL_BITCOIN_SUPPLY_SATOSHIS,
-			"MAX_FUNDING_SATOSHIS_NO_WUMBO is greater than all satoshis in existence"
-		);
-	}
-
 	#[cfg(ldk_test_vectors)]
 	struct Keys {
 		signer: crate::sign::InMemorySigner,

lightning/src/ln/channel_open_tests.rs

@@ -499,8 +499,7 @@ fn test_channel_resumption_fail_post_funding() {
 pub fn test_insane_channel_opens() {
 	// Stand up a network of 2 nodes
 	use crate::ln::channel::TOTAL_BITCOIN_SUPPLY_SATOSHIS;
-	let mut cfg = UserConfig::default();
-	cfg.channel_handshake_limits.max_funding_satoshis = TOTAL_BITCOIN_SUPPLY_SATOSHIS + 1;
+	let cfg = UserConfig::default();
 	let chanmon_cfgs = create_chanmon_cfgs(2);
 	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, Some(cfg.clone())]);

lightning/src/util/config.rs

@@ -10,7 +10,6 @@
 //! Various user-configurable channel limits and settings which ChannelManager
 //! applies for you.
 
-use crate::ln::channel::MAX_FUNDING_SATOSHIS_NO_WUMBO;
 use crate::ln::channelmanager::{BREAKDOWN_TIMEOUT, MAX_LOCAL_BREAKDOWN_TIMEOUT};
 
 #[cfg(fuzzing)]
@@ -302,11 +301,6 @@ pub struct ChannelHandshakeLimits {
 	/// Default value: `1000`
 	/// (Minimum of [`ChannelHandshakeConfig::their_channel_reserve_proportional_millionths`])
 	pub min_funding_satoshis: u64,
-	/// Maximum allowed satoshis when a channel is funded. This is supplied by the sender and so
-	/// only applies to inbound channels.
-	///
-	/// Default value: `2^24 - 1`
-	pub max_funding_satoshis: u64,
 	/// The remote node sets a limit on the minimum size of HTLCs we can send to them. This allows
 	/// you to limit the maximum minimum-size they can require.
 	///
@@ -376,7 +370,6 @@ impl Default for ChannelHandshakeLimits {
 	fn default() -> Self {
 		ChannelHandshakeLimits {
 			min_funding_satoshis: 1000,
-			max_funding_satoshis: MAX_FUNDING_SATOSHIS_NO_WUMBO,
 			max_htlc_minimum_msat: u64::MAX,
 			min_max_htlc_value_in_flight_msat: 0,
 			max_channel_reserve_satoshis: u64::MAX,
@@ -397,7 +390,6 @@ impl Readable for ChannelHandshakeLimits {
 	fn read<R: crate::io::Read>(reader: &mut R) -> Result<Self, crate::ln::msgs::DecodeError> {
 		Ok(Self {
 			min_funding_satoshis: Readable::read(reader)?,
-			max_funding_satoshis: Readable::read(reader)?,
 			max_htlc_minimum_msat: Readable::read(reader)?,
 			min_max_htlc_value_in_flight_msat: Readable::read(reader)?,
 			max_channel_reserve_satoshis: Readable::read(reader)?,