Consider anchor outputs value on channel open

We should make sure the funding amount of a channel can cover all its
associated costs, including the value of anchor outputs, to make sure
that it is actually usable once "opened".

Author: wpaulino

lightning/src/ln/channel.rs

@@ -5739,16 +5739,16 @@ impl<SP: Deref> OutboundV1Channel<SP> where SP::Target: SignerProvider {
 		let channel_type = Self::get_initial_channel_type(&config, their_features);
 		debug_assert!(channel_type.is_subset(&channelmanager::provided_channel_type_features(&config)));
 
-		let commitment_conf_target = if channel_type.supports_anchors_zero_fee_htlc_tx() {
-			ConfirmationTarget::AnchorChannelFee
+		let (commitment_conf_target, anchor_outputs_value_msat)  = if channel_type.supports_anchors_zero_fee_htlc_tx() {
+			(ConfirmationTarget::AnchorChannelFee, ANCHOR_OUTPUT_VALUE_SATOSHI * 2 * 1000)
 		} else {
-			ConfirmationTarget::NonAnchorChannelFee
+			(ConfirmationTarget::NonAnchorChannelFee, 0)
 		};
 		let commitment_feerate = fee_estimator.bounded_sat_per_1000_weight(commitment_conf_target);
 
 		let value_to_self_msat = channel_value_satoshis * 1000 - push_msat;
 		let commitment_tx_fee = commit_tx_fee_msat(commitment_feerate, MIN_AFFORDABLE_HTLC_COUNT, &channel_type);
-		if value_to_self_msat < commitment_tx_fee {
+		if value_to_self_msat.saturating_sub(anchor_outputs_value_msat) < commitment_tx_fee {
 			return Err(APIError::APIMisuseError{ err: format!("Funding amount ({}) can't even pay fee for initial commitment transaction fee of {}.", value_to_self_msat / 1000, commitment_tx_fee / 1000) });
 		}
 
@@ -6365,13 +6365,18 @@ impl<SP: Deref> InboundV1Channel<SP> where SP::Target: SignerProvider {
 
 		// check if the funder's amount for the initial commitment tx is sufficient
 		// for full fee payment plus a few HTLCs to ensure the channel will be useful.
+		let anchor_outputs_value = if channel_type.supports_anchors_zero_fee_htlc_tx() {
+			ANCHOR_OUTPUT_VALUE_SATOSHI * 2
+		} else {
+			0
+		};
 		let funders_amount_msat = msg.funding_satoshis * 1000 - msg.push_msat;
 		let commitment_tx_fee = commit_tx_fee_msat(msg.feerate_per_kw, MIN_AFFORDABLE_HTLC_COUNT, &channel_type) / 1000;
-		if funders_amount_msat / 1000 < commitment_tx_fee {
-			return Err(ChannelError::Close(format!("Funding amount ({} sats) can't even pay fee for initial commitment transaction fee of {} sats.", funders_amount_msat / 1000, commitment_tx_fee)));
+		if (funders_amount_msat / 1000).saturating_sub(anchor_outputs_value) < commitment_tx_fee {
+			return Err(ChannelError::Close(format!("Funding amount ({} sats) can't even pay fee for initial commitment transaction fee of {} sats.", (funders_amount_msat / 1000).saturating_sub(anchor_outputs_value), commitment_tx_fee)));
 		}
 
-		let to_remote_satoshis = funders_amount_msat / 1000 - commitment_tx_fee;
+		let to_remote_satoshis = funders_amount_msat / 1000 - commitment_tx_fee - anchor_outputs_value;
 		// While it's reasonable for us to not meet the channel reserve initially (if they don't
 		// want to push much to us), our counterparty should always have more than our reserve.
 		if to_remote_satoshis < holder_selected_channel_reserve_satoshis {