Add funding_fee_credit feature

We add an optional feature that lets on-the-fly funding clients accept
payments that are too small to pay the fees for an on-the-fly funding.
When that happens, the payment amount is added as "fee credit" without
performing an on-chain operation. Once enough fee credit has been
obtained, we can initiate an on-chain operation to create a channel or
a splice by paying part of the fees from our fee credit.

This feature makes more efficient use of on-chain transactions by
trusting that our peer will honor our fee credit in the future. The
fee credit takes precedence over other ways of paying the fees (from
our channel balance or future HTLCs), which guarantees that the fee
credit eventually converges to 0.

src/commonMain/kotlin/fr/acinq/lightning/Features.kt

@@ -263,6 +263,13 @@ sealed class Feature {
         override val scopes: Set<FeatureScope> get() = setOf(FeatureScope.Init, FeatureScope.Node)
     }
 
+    @Serializable
+    object FundingFeeCredit : Feature() {
+        override val rfcName get() = "funding_fee_credit"
+        override val mandatory get() = 562
+        override val scopes: Set<FeatureScope> get() = setOf(FeatureScope.Init, FeatureScope.Node)
+    }
+
 }
 
 @Serializable
@@ -345,7 +352,8 @@ data class Features(val activated: Map<Feature, FeatureSupport>, val unknown: Se
             Feature.ChannelBackupClient,
             Feature.ChannelBackupProvider,
             Feature.ExperimentalSplice,
-            Feature.OnTheFlyFunding
+            Feature.OnTheFlyFunding,
+            Feature.FundingFeeCredit
         )
 
         operator fun invoke(bytes: ByteVector): Features = invoke(bytes.toByteArray())
@@ -378,7 +386,8 @@ data class Features(val activated: Map<Feature, FeatureSupport>, val unknown: Se
             Feature.AnchorOutputs to listOf(Feature.StaticRemoteKey),
             Feature.TrampolinePayment to listOf(Feature.PaymentSecret),
             Feature.ExperimentalTrampolinePayment to listOf(Feature.PaymentSecret),
-            Feature.OnTheFlyFunding to listOf(Feature.ExperimentalSplice)
+            Feature.OnTheFlyFunding to listOf(Feature.ExperimentalSplice),
+            Feature.FundingFeeCredit to listOf(Feature.OnTheFlyFunding)
         )
 
         class FeatureException(message: String) : IllegalArgumentException(message)

src/commonMain/kotlin/fr/acinq/lightning/channel/InteractiveTx.kt

@@ -98,6 +98,20 @@ data class InteractiveTxParams(
         val fundingTxIndex = (sharedInput as? SharedFundingInput.Multisig2of2)?.let { it.fundingTxIndex + 1 } ?: 0
         return Helpers.Funding.makeFundingPubKeyScript(channelKeys.fundingPubKey(fundingTxIndex), remoteFundingPubkey)
     }
+
+    fun liquidityFees(purchase: LiquidityAds.Purchase?): MilliSatoshi = purchase?.let { l ->
+        val fees = when (l) {
+            is LiquidityAds.Purchase.Standard -> l.fees.total.toMilliSatoshi()
+            is LiquidityAds.Purchase.WithFeeCredit -> l.fees.total.toMilliSatoshi() - l.feeCreditUsed
+        }
+        when (l.paymentDetails) {
+            is LiquidityAds.PaymentDetails.FromChannelBalance -> if (isInitiator) fees else -fees
+            is LiquidityAds.PaymentDetails.FromChannelBalanceForFutureHtlc -> if (isInitiator) fees else -fees
+            // Fees will be paid later, from relayed HTLCs.
+            is LiquidityAds.PaymentDetails.FromFutureHtlc -> 0.msat
+            is LiquidityAds.PaymentDetails.FromFutureHtlcWithPreimage -> 0.msat
+        }
+    } ?: 0.msat
 }
 
 sealed class InteractiveTxInput {
@@ -209,7 +223,12 @@ sealed class InteractiveTxOutput {
      */
     data class Remote(override val serialId: Long, override val amount: Satoshi, override val pubkeyScript: ByteVector) : InteractiveTxOutput(), Incoming
 
-    /** The shared output can be added by us or by our peer, depending on who initiated the protocol. */
+    /**
+     * The shared output can be added by us or by our peer, depending on who initiated the protocol.
+     *
+     * @param localAmount amount contributed by us, before applying push_amount and (optional) liquidity fees: this is different from the channel balance.
+     * @param remoteAmount amount contributed by our peer, before applying push_amount and (optional) liquidity fees: this is different from the channel balance.
+     */
     data class Shared(override val serialId: Long, override val pubkeyScript: ByteVector, val localAmount: MilliSatoshi, val remoteAmount: MilliSatoshi, val htlcAmount: MilliSatoshi) : InteractiveTxOutput(), Incoming, Outgoing {
         // Note that the truncation is a no-op: the sum of balances in a channel must be a satoshi amount.
         override val amount: Satoshi = (localAmount + remoteAmount + htlcAmount).truncateToSatoshi()
@@ -246,8 +265,17 @@ data class FundingContributions(val inputs: List<InteractiveTxInput.Outgoing>, v
         /**
          * @param walletInputs 2-of-2 swap-in wallet inputs.
          */
-        fun create(channelKeys: KeyManager.ChannelKeys, swapInKeys: KeyManager.SwapInOnChainKeys, params: InteractiveTxParams, walletInputs: List<WalletState.Utxo>): Either<FundingContributionFailure, FundingContributions> =
-            create(channelKeys, swapInKeys, params, null, walletInputs, listOf())
+        fun create(
+            channelKeys: KeyManager.ChannelKeys,
+            swapInKeys: KeyManager.SwapInOnChainKeys,
+            params: InteractiveTxParams,
+            walletInputs: List<WalletState.Utxo>,
+            localPushAmount: MilliSatoshi,
+            remotePushAmount: MilliSatoshi,
+            liquidityPurchase: LiquidityAds.Purchase?
+        ): Either<FundingContributionFailure, FundingContributions> {
+            return create(channelKeys, swapInKeys, params, null, walletInputs, listOf(), localPushAmount, remotePushAmount, liquidityPurchase)
+        }
 
         /**
          * @param sharedUtxo previous input shared between the two participants (e.g. previous funding output when splicing) and our corresponding balance.
@@ -262,6 +290,9 @@ data class FundingContributions(val inputs: List<InteractiveTxInput.Outgoing>, v
             sharedUtxo: Pair<SharedFundingInput, SharedFundingInputBalances>?,
             walletInputs: List<WalletState.Utxo>,
             localOutputs: List<TxOut>,
+            localPushAmount: MilliSatoshi,
+            remotePushAmount: MilliSatoshi,
+            liquidityPurchase: LiquidityAds.Purchase?,
             changePubKey: PublicKey? = null
         ): Either<FundingContributionFailure, FundingContributions> {
             walletInputs.forEach { utxo ->
@@ -277,14 +308,18 @@ data class FundingContributions(val inputs: List<InteractiveTxInput.Outgoing>, v
                 return Either.Left(FundingContributionFailure.NotEnoughFunding(params.localContribution, localOutputs.map { it.amount }.sum(), totalAmountIn))
             }
 
-            val nextLocalBalance = (sharedUtxo?.second?.toLocal ?: 0.msat) + params.localContribution.toMilliSatoshi()
-            val nextRemoteBalance = (sharedUtxo?.second?.toRemote ?: 0.msat) + params.remoteContribution.toMilliSatoshi()
-            if (nextLocalBalance < 0.msat || nextRemoteBalance < 0.msat) {
-                return Either.Left(FundingContributionFailure.InvalidFundingBalances(params.fundingAmount, nextLocalBalance, nextRemoteBalance))
+            val liquidityFees = params.liquidityFees(liquidityPurchase)
+            val nextLocalBalanceBeforePush = (sharedUtxo?.second?.toLocal ?: 0.msat) + params.localContribution.toMilliSatoshi()
+            val nextLocalBalanceAfterPush = (sharedUtxo?.second?.toLocal ?: 0.msat) + params.localContribution.toMilliSatoshi() - localPushAmount + remotePushAmount - liquidityFees
+            val nextRemoteBalanceBeforePush = (sharedUtxo?.second?.toRemote ?: 0.msat) + params.remoteContribution.toMilliSatoshi()
+            val nextRemoteBalanceAfterPush = (sharedUtxo?.second?.toRemote ?: 0.msat) + params.remoteContribution.toMilliSatoshi() + localPushAmount - remotePushAmount + liquidityFees
+            if (nextLocalBalanceAfterPush < 0.msat || nextRemoteBalanceAfterPush < 0.msat) {
+                return Either.Left(FundingContributionFailure.InvalidFundingBalances(params.fundingAmount, nextLocalBalanceAfterPush, nextRemoteBalanceAfterPush))
             }
 
             val fundingPubkeyScript = params.fundingPubkeyScript(channelKeys)
-            val sharedOutput = listOf(InteractiveTxOutput.Shared(0, fundingPubkeyScript, nextLocalBalance, nextRemoteBalance, sharedUtxo?.second?.toHtlcs ?: 0.msat))
+            // We use local and remote balances before amounts are pushed to allow computing the local and remote mining fees.
+            val sharedOutput = listOf(InteractiveTxOutput.Shared(0, fundingPubkeyScript, nextLocalBalanceBeforePush, nextRemoteBalanceBeforePush, sharedUtxo?.second?.toHtlcs ?: 0.msat))
             val nonChangeOutputs = localOutputs.map { o -> InteractiveTxOutput.Local.NonChange(0, o.amount, o.publicKeyScript) }
             val changeOutput = when (changePubKey) {
                 null -> listOf()
@@ -1068,16 +1103,7 @@ data class InteractiveTxSigningSession(
             val channelKeys = channelParams.localParams.channelKeys(keyManager)
             val unsignedTx = sharedTx.buildUnsignedTx()
             val sharedOutputIndex = unsignedTx.txOut.indexOfFirst { it.publicKeyScript == fundingParams.fundingPubkeyScript(channelKeys) }
-            val liquidityFees = liquidityPurchase?.let { l ->
-                val fees = l.fees.total.toMilliSatoshi()
-                when (l.paymentDetails) {
-                    is LiquidityAds.PaymentDetails.FromChannelBalance -> if (fundingParams.isInitiator) fees else -fees
-                    is LiquidityAds.PaymentDetails.FromChannelBalanceForFutureHtlc -> if (fundingParams.isInitiator) fees else -fees
-                    // Fees will be paid later, from relayed HTLCs.
-                    is LiquidityAds.PaymentDetails.FromFutureHtlc -> 0.msat
-                    is LiquidityAds.PaymentDetails.FromFutureHtlcWithPreimage -> 0.msat
-                }
-            } ?: 0.msat
+            val liquidityFees = fundingParams.liquidityFees(liquidityPurchase)
             return Helpers.Funding.makeCommitTxs(
                 channelKeys,
                 channelParams.channelId,

src/commonMain/kotlin/fr/acinq/lightning/channel/states/Normal.kt

@@ -405,15 +405,6 @@ data class Normal(
                                                 add(ChannelAction.Disconnect)
                                             }
                                             Pair(this@Normal.copy(spliceStatus = SpliceStatus.None), actions)
-                                        } else if (!canAffordSpliceLiquidityFees(spliceStatus.command, parentCommitment)) {
-                                            val missing = spliceStatus.command.requestRemoteFunding?.let { r -> r.fees(spliceStatus.command.feerate).total - parentCommitment.localCommit.spec.toLocal.truncateToSatoshi() }
-                                            logger.warning { "cannot do splice: balance is too low to pay for inbound liquidity (missing=$missing)" }
-                                            spliceStatus.command.replyTo.complete(ChannelCommand.Commitment.Splice.Response.Failure.InsufficientFunds)
-                                            val actions = buildList {
-                                                add(ChannelAction.Message.Send(Warning(channelId, InvalidSpliceRequest(channelId).message)))
-                                                add(ChannelAction.Disconnect)
-                                            }
-                                            Pair(this@Normal.copy(spliceStatus = SpliceStatus.None), actions)
                                         } else {
                                             val spliceInit = SpliceInit(
                                                 channelId,
@@ -521,6 +512,7 @@ data class Normal(
                                 Helpers.Funding.makeFundingPubKeyScript(spliceStatus.spliceInit.fundingPubkey, cmd.message.fundingPubkey),
                                 cmd.message.fundingContribution,
                                 spliceStatus.spliceInit.feerate,
+                                cmd.message.feeCreditUsed,
                                 cmd.message.willFund,
                             )) {
                                 is Either.Left<ChannelException> -> {
@@ -550,6 +542,9 @@ data class Normal(
                                         sharedUtxo = Pair(sharedInput, SharedFundingInputBalances(toLocal = parentCommitment.localCommit.spec.toLocal, toRemote = parentCommitment.localCommit.spec.toRemote, toHtlcs = parentCommitment.localCommit.spec.htlcs.map { it.add.amountMsat }.sum())),
                                         walletInputs = spliceStatus.command.spliceIn?.walletInputs ?: emptyList(),
                                         localOutputs = spliceStatus.command.spliceOutputs,
+                                        localPushAmount = spliceStatus.spliceInit.pushAmount,
+                                        remotePushAmount = cmd.message.pushAmount,
+                                        liquidityPurchase = liquidityPurchase.value,
                                         changePubKey = null // we don't want a change output: we're spending every funds available
                                     )) {
                                         is Either.Left -> {
@@ -854,19 +849,6 @@ data class Normal(
         }
     }
 
-    private fun canAffordSpliceLiquidityFees(splice: ChannelCommand.Commitment.Splice.Request, parentCommitment: Commitment): Boolean {
-        return when (val request = splice.requestRemoteFunding) {
-            null -> true
-            else -> when (request.paymentDetails) {
-                is LiquidityAds.PaymentDetails.FromChannelBalance -> request.fees(splice.feerate).total <= parentCommitment.localCommit.spec.toLocal.truncateToSatoshi()
-                is LiquidityAds.PaymentDetails.FromChannelBalanceForFutureHtlc -> request.fees(splice.feerate).total <= parentCommitment.localCommit.spec.toLocal.truncateToSatoshi()
-                // Fees don't need to be paid during the splice, they will be deducted from relayed HTLCs.
-                is LiquidityAds.PaymentDetails.FromFutureHtlc -> true
-                is LiquidityAds.PaymentDetails.FromFutureHtlcWithPreimage -> true
-            }
-        }
-    }
-
     private fun ChannelContext.sendSpliceTxSigs(
         origins: List<Origin>,
         action: InteractiveTxSigningSessionAction.SendTxSigs,

src/commonMain/kotlin/fr/acinq/lightning/channel/states/WaitForAcceptChannel.kt

@@ -58,13 +58,22 @@ data class WaitForAcceptChannel(
                                 fundingParams.fundingPubkeyScript(channelKeys),
                                 accept.fundingAmount,
                                 lastSent.fundingFeerate,
+                                accept.feeCreditUsed,
                                 accept.willFund
                             )) {
                                 is Either.Left -> {
                                     logger.error { "rejecting liquidity proposal: ${liquidityPurchase.value.message}" }
                                     Pair(Aborted, listOf(ChannelAction.Message.Send(Error(cmd.message.temporaryChannelId, liquidityPurchase.value.message))))
                                 }
-                                is Either.Right -> when (val fundingContributions = FundingContributions.create(channelKeys, keyManager.swapInOnChainWallet, fundingParams, init.walletInputs)) {
+                                is Either.Right -> when (val fundingContributions = FundingContributions.create(
+                                    channelKeys,
+                                    keyManager.swapInOnChainWallet,
+                                    fundingParams,
+                                    init.walletInputs,
+                                    lastSent.pushAmount,
+                                    accept.pushAmount,
+                                    liquidityPurchase.value
+                                )) {
                                     is Either.Left -> {
                                         logger.error { "could not fund channel: ${fundingContributions.value}" }
                                         Pair(Aborted, listOf(ChannelAction.Message.Send(Error(channelId, ChannelFundingError(channelId).message))))

src/commonMain/kotlin/fr/acinq/lightning/channel/states/WaitForFundingConfirmed.kt

@@ -136,7 +136,7 @@ data class WaitForFundingConfirmed(
                             latestFundingTx.fundingParams.dustLimit,
                             rbfStatus.command.targetFeerate
                         )
-                        when (val contributions = FundingContributions.create(channelKeys(), keyManager.swapInOnChainWallet, fundingParams, rbfStatus.command.walletInputs)) {
+                        when (val contributions = FundingContributions.create(channelKeys(), keyManager.swapInOnChainWallet, fundingParams, rbfStatus.command.walletInputs, 0.msat, 0.msat, null)) {
                             is Either.Left -> {
                                 logger.warning { "error creating funding contributions: ${contributions.value}" }
                                 Pair(this@WaitForFundingConfirmed.copy(rbfStatus = RbfStatus.RbfAborted), listOf(ChannelAction.Message.Send(TxAbort(channelId, ChannelFundingError(channelId).message))))

src/commonMain/kotlin/fr/acinq/lightning/channel/states/WaitForOpenChannel.kt

@@ -48,7 +48,7 @@ data class WaitForOpenChannel(
                                 fundingRates == null -> null
                                 requestFunding == null -> null
                                 requestFunding.requestedAmount > fundingAmount -> null
-                                else -> fundingRates.validateRequest(staticParams.nodeParams.nodePrivateKey, fundingScript, open.fundingFeerate, requestFunding)
+                                else -> fundingRates.validateRequest(staticParams.nodeParams.nodePrivateKey, fundingScript, open.fundingFeerate, requestFunding, 0.msat)
                             }
                             val accept = AcceptDualFundedChannel(
                                 temporaryChannelId = open.temporaryChannelId,
@@ -91,7 +91,7 @@ data class WaitForOpenChannel(
                             val remoteFundingPubkey = open.fundingPubkey
                             val dustLimit = open.dustLimit.max(localParams.dustLimit)
                             val fundingParams = InteractiveTxParams(channelId, false, fundingAmount, open.fundingAmount, remoteFundingPubkey, open.lockTime, dustLimit, open.fundingFeerate)
-                            when (val fundingContributions = FundingContributions.create(channelKeys, keyManager.swapInOnChainWallet, fundingParams, walletInputs)) {
+                            when (val fundingContributions = FundingContributions.create(channelKeys, keyManager.swapInOnChainWallet, fundingParams, walletInputs, accept.pushAmount, open.pushAmount, null)) {
                                 is Either.Left -> {
                                     logger.error { "could not fund channel: ${fundingContributions.value}" }
                                     Pair(Aborted, listOf(ChannelAction.Message.Send(Error(temporaryChannelId, ChannelFundingError(temporaryChannelId).message))))

src/commonMain/kotlin/fr/acinq/lightning/db/PaymentsDb.kt

@@ -175,6 +175,15 @@ data class IncomingPayment(val preimage: ByteVector32, val origin: Origin, val r
             override val fees: MilliSatoshi = fundingFee?.amount ?: 0.msat
         }
 
+        /**
+         * Payment was added to our fee credit for future on-chain operations (see [Feature.FundingFeeCredit]).
+         * We didn't really receive this amount yet, but we trust our peer to use it for future on-chain operations.
+         */
+        data class AddedToFeeCredit(override val amount: MilliSatoshi) : ReceivedWith() {
+            // Adding to the fee credit doesn't cost any fees.
+            override val fees: MilliSatoshi = 0.msat
+        }
+
         sealed class OnChainIncomingPayment : ReceivedWith() {
             abstract val serviceFee: MilliSatoshi
             abstract val miningFee: Satoshi