Remove superfluous channel-value-goes-below-0 checks

Author: optout21

lightning/src/ln/channel.rs

@@ -4617,6 +4617,7 @@ pub(super) fn calculate_our_funding_satoshis(
 /// the fees of the inputs, fees of the inputs weight, and for the initiator,
 /// the fees of the common fields as well as the output and extra input weights.
 /// Returns estimated (partial) fees as additional information
+#[cfg(splicing)]
 pub(super) fn check_v2_funding_inputs_sufficient(
 	contribution_amount: i64, funding_inputs: &[(TxIn, Transaction, Weight)], is_initiator: bool,
 	extra_common_input_weight: Option<Weight>, funding_feerate_sat_per_1000_weight: u32,
@@ -8289,13 +8290,6 @@ impl<SP: Deref> FundedChannel<SP> where
 		// TODO(splicing): check for quiescence
 
 		let pre_channel_value = self.context.get_value_satoshis();
-		// Sanity check: capacity cannot decrease below 0
-		if (pre_channel_value as i64).saturating_add(our_funding_contribution_satoshis) < 0 {
-			return Err(ChannelError::Warn(format!(
-				"Post-splicing channel value cannot be negative. It was {} + {}",
-				pre_channel_value, our_funding_contribution_satoshis
-			)));
-		}
 
 		if our_funding_contribution_satoshis < 0 {
 			return Err(ChannelError::Warn(format!(
@@ -8304,6 +8298,9 @@ impl<SP: Deref> FundedChannel<SP> where
 			)));
 		}
 
+		// TODO(splicing): Once splice-out is supported, check that channel balance does not go below 0
+		// (or below channel reserve)
+
 		// Note: post-splice channel value is not yet known at this point, counterparty contribution is not known
 		// (Cannot test for miminum required post-splice channel value)
 
@@ -8347,16 +8344,6 @@ impl<SP: Deref> FundedChannel<SP> where
 		}
 
 		let pre_channel_value = self.context.get_value_satoshis();
-		// Sanity check: capacity cannot decrease below 0
-		if (pre_channel_value as i64)
-			.saturating_add(their_funding_contribution_satoshis)
-			.saturating_add(our_funding_contribution_satoshis) < 0
-		{
-			return Err(ChannelError::Warn(format!(
-				"Post-splicing channel value cannot be negative. It was {} + {} + {}",
-				pre_channel_value, their_funding_contribution_satoshis, our_funding_contribution_satoshis,
-			)));
-		}
 
 		if their_funding_contribution_satoshis.saturating_add(our_funding_contribution_satoshis) < 0 {
 			return Err(ChannelError::Warn(format!(
@@ -8365,8 +8352,11 @@ impl<SP: Deref> FundedChannel<SP> where
 			)));
 		}
 
+		// TODO(splicing): Once splice acceptor can contribute, check that inputs are sufficient,
+		// similarly to the check in `splice_channel`.
+
 		// Note on channel reserve requirement pre-check: as the splice acceptor does not contribute,
-		// it can go below reserve, therefore no pre-check is done here.
+		// it can't go below reserve, therefore no pre-check is done here.
 		// TODO(splicing): Once splice acceptor can contribute, add reserve pre-check, similar to the one in `splice_ack`.
 
 		// TODO(splicing): Store msg.funding_pubkey