Fix anchor reserves when splicing in all funds

In an upcoming commit, we will fix `check_sufficient_funds_for_channel`
to check that we have on-chain funds to cover the anchor reserve for an
additional anchor channel in the validation of outbound channel opens.

Before we do this, we stop using this function to check that any
splice-ins leave enough on-chain anchor reserves. This function keeps
an anchor reserve for an additional anchor channel on top of the
existing set of anchor channels, but after splice-ins, our anchor
reserve only needs to cover the existing set of anchor channels.

Author: tankyleo

src/lib.rs

@@ -1251,7 +1251,7 @@ impl Node {
 			FundingAmount::Exact { amount_sats } => {
 				// Check funds availability after connection (includes anchor reserve
 				// calculation).
-				self.check_sufficient_funds_for_channel(amount_sats, &peer_info.node_id)?;
+				self.check_sufficient_onchain_funds(amount_sats, &peer_info.node_id, true)?;
 				amount_sats
 			},
 			FundingAmount::Max => {
@@ -1357,33 +1357,37 @@ impl Node {
 		Ok(new_channel_anchor_reserve_sats(&self.config, peer_node_id, anchor_channel))
 	}
 
-	fn check_sufficient_funds_for_channel(
-		&self, amount_sats: u64, peer_node_id: &PublicKey,
+	fn check_sufficient_onchain_funds(
+		&self, amount_sats: u64, peer_node_id: &PublicKey, for_new_channel: bool,
 	) -> Result<(), Error> {
+		let action_str = if for_new_channel { "create channel" } else { "splice-in" };
 		let cur_anchor_reserve_sats =
 			total_anchor_channels_reserve_sats(&self.channel_manager, &self.config);
 		let spendable_amount_sats =
 			self.wallet.get_spendable_amount_sats(cur_anchor_reserve_sats).unwrap_or(0);
 
-		// Fail early if we have less than the channel value available.
 		if spendable_amount_sats < amount_sats {
-			log_error!(self.logger,
-				"Unable to create channel due to insufficient funds. Available: {}sats, Required: {}sats",
-				spendable_amount_sats, amount_sats
+			log_error!(
+				self.logger,
+				"Unable to {} due to insufficient funds. Available: {}sats, Required: {}sats",
+				action_str,
+				spendable_amount_sats,
+				amount_sats
 			);
 			return Err(Error::InsufficientFunds);
 		}
 
-		// Fail if we have less than the channel value + anchor reserve available (if applicable).
-		let required_funds_sats =
-			amount_sats + self.new_channel_anchor_reserve_sats(peer_node_id)?;
+		if for_new_channel {
+			let required_funds_sats =
+				amount_sats + self.new_channel_anchor_reserve_sats(peer_node_id)?;
 
-		if spendable_amount_sats < required_funds_sats {
-			log_error!(self.logger,
-				"Unable to create channel due to insufficient funds. Available: {}sats, Required: {}sats",
-				spendable_amount_sats, required_funds_sats
-			);
-			return Err(Error::InsufficientFunds);
+			if spendable_amount_sats < required_funds_sats {
+				log_error!(self.logger,
+					"Unable to create channel due to insufficient funds. Available: {}sats, Required: {}sats",
+					spendable_amount_sats, required_funds_sats
+				);
+				return Err(Error::InsufficientFunds);
+			}
 		}
 
 		Ok(())
@@ -1659,7 +1663,7 @@ impl Node {
 				},
 			};
 
-			self.check_sufficient_funds_for_channel(splice_amount_sats, &counterparty_node_id)?;
+			self.check_sufficient_onchain_funds(splice_amount_sats, &counterparty_node_id, false)?;
 
 			let funding_template = self
 				.channel_manager