Replace config max counterpary dust_limit_satoshis by a constant.

Current Bitcoin Core's policy will reject a p2wsh as a dust if it's
under 330 satoshis. A typical p2wsh output is 43 bytes big to which
Core's `GetDustThreshold()` sums up a minimal spend of 67 bytes (even
if a p2wsh witnessScript might be smaller). `dustRelayFee` is set
to 3000 sat/kb, thus 110 * 3000 / 1000 = 330. As all time-sensitive
outputs are p2wsh, a value of 330 sat is the lower bound desired
to ensure good propagation of transactions. We give a bit margin to
our counterparty and pick up 660 satoshis as an accepted
`dust_limit_satoshis` upper bound.

As this reasoning is tricky and error-prone we hardcode it instead of
letting the user picking up a non-sense value.

Author: Antoine Riard

lightning/src/ln/channel.rs

@@ -444,6 +444,9 @@ pub const COMMITMENT_TX_WEIGHT_PER_HTLC: u64 = 172;
 /// it's 2^24.
 pub const MAX_FUNDING_SATOSHIS: u64 = 1 << 24;
 
+/// Maximum counterparty `dust_limit_satoshis` allowed. 2 * standard dust threshold on p2wsh output
+pub const MAX_DUST_LIMIT_SATOSHIS: u64 = 2 * 330;
+
 /// Used to return a simple Error back to ChannelManager. Will get converted to a
 /// msgs::ErrorAction::SendErrorMessage or msgs::ErrorAction::IgnoreError as appropriate with our
 /// channel_id in ChannelManager.
@@ -690,8 +693,8 @@ impl<Signer: Sign> Channel<Signer> {
 		if msg.dust_limit_satoshis < config.peer_channel_config_limits.min_dust_limit_satoshis {
 			return Err(ChannelError::Close(format!("dust_limit_satoshis ({}) is less than the user specified limit ({})", msg.dust_limit_satoshis, config.peer_channel_config_limits.min_dust_limit_satoshis)));
 		}
-		if msg.dust_limit_satoshis > config.peer_channel_config_limits.max_dust_limit_satoshis {
-			return Err(ChannelError::Close(format!("dust_limit_satoshis ({}) is greater than the user specified limit ({})", msg.dust_limit_satoshis, config.peer_channel_config_limits.max_dust_limit_satoshis)));
+		if msg.dust_limit_satoshis >  MAX_DUST_LIMIT_SATOSHIS {
+			return Err(ChannelError::Close(format!("dust_limit_satoshis ({}) is greater than the implementation limit ({})", msg.dust_limit_satoshis, MAX_DUST_LIMIT_SATOSHIS)));
 		}
 
 		// Convert things into internal flags and prep our state:
@@ -1429,8 +1432,8 @@ impl<Signer: Sign> Channel<Signer> {
 		if msg.dust_limit_satoshis < config.peer_channel_config_limits.min_dust_limit_satoshis {
 			return Err(ChannelError::Close(format!("dust_limit_satoshis ({}) is less than the user specified limit ({})", msg.dust_limit_satoshis, config.peer_channel_config_limits.min_dust_limit_satoshis)));
 		}
-		if msg.dust_limit_satoshis > config.peer_channel_config_limits.max_dust_limit_satoshis {
-			return Err(ChannelError::Close(format!("dust_limit_satoshis ({}) is greater than the user specified limit ({})", msg.dust_limit_satoshis, config.peer_channel_config_limits.max_dust_limit_satoshis)));
+		if msg.dust_limit_satoshis > MAX_DUST_LIMIT_SATOSHIS {
+			return Err(ChannelError::Close(format!("dust_limit_satoshis ({}) is greater than the implementation limit ({})", msg.dust_limit_satoshis, MAX_DUST_LIMIT_SATOSHIS)));
 		}
 		if msg.minimum_depth > config.peer_channel_config_limits.max_minimum_depth {
 			return Err(ChannelError::Close(format!("We consider the minimum depth to be unreasonably large. Expected minimum: ({}). Actual: ({})", config.peer_channel_config_limits.max_minimum_depth, msg.minimum_depth)));

lightning/src/util/config.rs

@@ -105,12 +105,6 @@ pub struct ChannelHandshakeLimits {
 	///
 	/// Default value: 546, the current dust limit on the Bitcoin network.
 	pub min_dust_limit_satoshis: u64,
-	/// Maximum allowed threshold above which outputs will not be generated in their commitment
-	/// transactions.
-	/// HTLCs below this amount plus HTLC transaction fees are not enforceable on-chain.
-	///
-	/// Default value: u64::max_value.
-	pub max_dust_limit_satoshis: u64,
 	/// Before a channel is usable the funding transaction will need to be confirmed by at least a
 	/// certain number of blocks, specified by the node which is not the funder (as the funder can
 	/// assume they aren't going to double-spend themselves).
@@ -143,7 +137,6 @@ impl Default for ChannelHandshakeLimits {
 			max_channel_reserve_satoshis: <u64>::max_value(),
 			min_max_accepted_htlcs: 0,
 			min_dust_limit_satoshis: 546,
-			max_dust_limit_satoshis: <u64>::max_value(),
 			max_minimum_depth: 144,
 			force_announced_channel_preference: true,
 			their_to_self_delay: MAX_LOCAL_BREAKDOWN_TIMEOUT,