Smaller review changes

Remove redundant check on total input amount.
Re-word checking against fee and dust limit.
Add a debug assert on precondition in begin_intera...()

Author: optout21

lightning/src/ln/channel.rs

@@ -2230,6 +2230,8 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 	) -> Result<Option<InteractiveTxMessageSend>, ChannelError>
 	where ES::Target: EntropySource
 	{
+		debug_assert!(self.interactive_tx_constructor.is_none());
+
 		let mut funding_inputs = Vec::new();
 		mem::swap(&mut self.dual_funding_context.our_funding_inputs, &mut funding_inputs);
 
@@ -2239,14 +2241,9 @@ impl<SP: Deref> PendingV2Channel<SP> where SP::Target: SignerProvider {
 
 		let funding_inputs_prev_outputs = DualFundingChannelContext::txouts_from_input_prev_txs(&funding_inputs)?;
 
-		let total_input_satoshis: u64 = funding_inputs_prev_outputs.iter().map(|txout| txout.value.to_sat()).sum();
-		if total_input_satoshis < self.dual_funding_context.our_funding_satoshis {
-			return Err(ChannelError::Warn(format!(
-				"Total value of funding inputs must be at least funding amount. It was {} sats", total_input_satoshis,
-			)));
-		}
-
 		// Add output for funding tx
+		// Note: For the error case when the inputs are insufficient, it will be handled after
+		// the `calculate_change_output_value` call below
 		let mut funding_outputs = Vec::new();
 		let funding_output_value_satoshis = self.funding.get_value_satoshis();
 		let funding_output_script_pubkey = self.funding.get_funding_redeemscript().to_p2wsh();

lightning/src/ln/interactivetxs.rs

@@ -1699,19 +1699,17 @@ pub(super) fn calculate_change_output_value(
 
 	// Note: in case of additional outputs, they will have to be subtracted here
 
-	let min_contribution_and_fees = our_contribution.saturating_add(fees_sats);
-	if total_input_satoshis < min_contribution_and_fees {
+	let total_inputs_less_fees = total_input_satoshis.saturating_sub(fees_sats);
+	if total_inputs_less_fees < our_contribution {
 		// Not enough to cover contribution plus fees
 		return Err(AbortReason::InsufficientFees);
 	}
-	let min_contribution_and_fees_and_dust =
-		min_contribution_and_fees.saturating_add(holder_dust_limit_satoshis);
-	if total_input_satoshis < min_contribution_and_fees_and_dust {
+	let remaining_value = total_inputs_less_fees.saturating_sub(our_contribution);
+	if remaining_value < holder_dust_limit_satoshis {
 		// Enough to cover contribution plus fees, but leftover is below dust limit; no change
 		Ok(None)
 	} else {
 		// Enough to have over-dust change
-		let remaining_value = total_input_satoshis.saturating_sub(min_contribution_and_fees);
 		Ok(Some(remaining_value))
 	}
 }