Add new config setting max_balance_dust_htlc_msat

Trimmed-to-dust HTLCs are at risk of being burnt as miner fees
at anytime during their lifetime due to the broadcast of either
holder commitment transaction or counterparty's one.

To hedge against this risk, we introduce a new config setting
`max_balance_dust_htlc_msat`, with the initial value of
5_000_000 msat.

Author: Antoine Riard

lightning/src/util/config.rs

@@ -206,7 +206,23 @@ pub struct ChannelConfig {
 	/// This cannot be changed after a channel has been initialized.
 	///
 	/// Default value: true.
-	pub commit_upfront_shutdown_pubkey: bool
+	pub commit_upfront_shutdown_pubkey: bool,
+	/// Limit our total exposure to in-flight HTLCs which are burned to fees as they are too
+	/// small to claim on-chain.
+	///
+	/// When an HTLC present in one of our channels is below a "dust" threshold, the HTLC will
+	/// not be claimable on-chain, instead being turned into additional miner fees if either
+	/// party force-closes the channel. Because the threshold is per-HTLC, our total exposure
+	/// to such payments may be sustantial if there are many dust HTLCs present when the
+	/// channel is force-closed.
+	///
+	/// This limit is applied for sent, forwarded, and received HTLCs and limits the total
+	/// exposure across all three types per-channel. Setting this too low may prevent the
+	/// sending or receipt of low-value HTLCs on high-traffic nodes, and this limit is very
+	/// important to prevent stealing of dust HTLCs by miners.
+	///
+	/// Default value: 5_000_000 msat.
+	pub max_dust_htlc_exposure_msat: u64,
 }
 
 impl Default for ChannelConfig {
@@ -218,12 +234,14 @@ impl Default for ChannelConfig {
 			cltv_expiry_delta: 6 * 12, // 6 blocks/hour * 12 hours
 			announced_channel: false,
 			commit_upfront_shutdown_pubkey: true,
+			max_dust_htlc_exposure_msat: 5_000_000,
 		}
 	}
 }
 
 impl_writeable_tlv_based!(ChannelConfig, {
 	(0, forwarding_fee_proportional_millionths, required),
+	(1, max_dust_htlc_exposure_msat, (default_value, 5_000_000)),
 	(2, cltv_expiry_delta, required),
 	(4, announced_channel, required),
 	(6, commit_upfront_shutdown_pubkey, required),