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 the fee credit.

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

eclair-core/src/main/scala/fr/acinq/eclair/Features.scala

@@ -332,6 +332,14 @@ object Features {
     val mandatory = 560
   }
 
+  // TODO:
+  //  - add NodeFeature once stable
+  //  - add link to bLIP
+  case object FundingFeeCredit extends Feature with InitFeature {
+    val rfcName = "funding_fee_credit"
+    val mandatory = 562
+  }
+
   val knownFeatures: Set[Feature] = Set(
     DataLossProtect,
     InitialRoutingSync,
@@ -358,7 +366,8 @@ object Features {
     TrampolinePaymentPrototype,
     AsyncPaymentPrototype,
     SplicePrototype,
-    OnTheFlyFunding
+    OnTheFlyFunding,
+    FundingFeeCredit
   )
 
   // Features may depend on other features, as specified in Bolt 9.
@@ -372,7 +381,8 @@ object Features {
     TrampolinePaymentPrototype -> (PaymentSecret :: Nil),
     KeySend -> (VariableLengthOnion :: Nil),
     AsyncPaymentPrototype -> (TrampolinePaymentPrototype :: Nil),
-    OnTheFlyFunding -> (SplicePrototype :: Nil)
+    OnTheFlyFunding -> (SplicePrototype :: Nil),
+    FundingFeeCredit -> (OnTheFlyFunding :: Nil)
   )
 
   case class FeatureException(message: String) extends IllegalArgumentException(message)

eclair-core/src/main/scala/fr/acinq/eclair/channel/Helpers.scala

@@ -171,7 +171,7 @@ object Helpers {
 
     for {
       script_opt <- extractShutdownScript(open.temporaryChannelId, localFeatures, remoteFeatures, open.upfrontShutdownScript_opt)
-      willFund_opt <- LiquidityAds.validateRequest(nodeParams.privateKey, open.temporaryChannelId, fundingScript, open.fundingFeerate, isChannelCreation = true, open.requestFunding_opt, addFunding_opt.flatMap(_.rates_opt))
+      willFund_opt <- LiquidityAds.validateRequest(nodeParams.privateKey, open.temporaryChannelId, fundingScript, open.fundingFeerate, isChannelCreation = true, open.requestFunding_opt, addFunding_opt.flatMap(_.rates_opt), open.useFeeCredit_opt)
     } yield (channelFeatures, script_opt, willFund_opt)
   }
 

eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/Channel.scala

@@ -952,7 +952,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
             val parentCommitment = d.commitments.latest.commitment
             val localFundingPubKey = nodeParams.channelKeyManager.fundingPublicKey(d.commitments.params.localParams.fundingKeyPath, parentCommitment.fundingTxIndex + 1).publicKey
             val fundingScript = Funding.makeFundingPubKeyScript(localFundingPubKey, msg.fundingPubKey)
-            LiquidityAds.validateRequest(nodeParams.privateKey, d.channelId, fundingScript, msg.feerate, isChannelCreation = false, msg.requestFunding_opt, nodeParams.willFundRates_opt) match {
+            LiquidityAds.validateRequest(nodeParams.privateKey, d.channelId, fundingScript, msg.feerate, isChannelCreation = false, msg.requestFunding_opt, nodeParams.willFundRates_opt, msg.useFeeCredit_opt) match {
               case Left(t) =>
                 log.warning("rejecting splice request with invalid liquidity ads: {}", t.getMessage)
                 stay() using d.copy(spliceStatus = SpliceStatus.SpliceAborted) sending TxAbort(d.channelId, t.getMessage)
@@ -963,7 +963,8 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
                   fundingPubKey = localFundingPubKey,
                   pushAmount = 0.msat,
                   requireConfirmedInputs = nodeParams.channelConf.requireConfirmedInputsForDualFunding,
-                  willFund_opt = willFund_opt.map(_.willFund)
+                  willFund_opt = willFund_opt.map(_.willFund),
+                  feeCreditUsed_opt = msg.useFeeCredit_opt
                 )
                 val fundingParams = InteractiveTxParams(
                   channelId = d.channelId,

eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/ChannelOpenDualFunded.scala

@@ -180,6 +180,7 @@ trait ChannelOpenDualFunded extends DualFundingHandlers with ErrorHandlers {
             Some(ChannelTlv.ChannelTypeTlv(d.init.channelType)),
             if (d.init.requireConfirmedInputs) Some(ChannelTlv.RequireConfirmedInputsTlv()) else None,
             willFund_opt.map(l => ChannelTlv.ProvideFundingTlv(l.willFund)),
+            open.useFeeCredit_opt.map(c => ChannelTlv.FeeCreditUsedTlv(c)),
             d.init.pushAmount_opt.map(amount => ChannelTlv.PushAmountTlv(amount)),
           ).flatten
           val accept = AcceptDualFundedChannel(
@@ -547,7 +548,7 @@ trait ChannelOpenDualFunded extends DualFundingHandlers with ErrorHandlers {
               stay() using d.copy(rbfStatus = RbfStatus.RbfAborted) sending TxAbort(d.channelId, InvalidRbfAttemptTooSoon(d.channelId, d.latestFundingTx.createdAt, d.latestFundingTx.createdAt + nodeParams.channelConf.remoteRbfLimits.attemptDeltaBlocks).getMessage)
             } else {
               val fundingScript = d.commitments.latest.commitInput.txOut.publicKeyScript
-              LiquidityAds.validateRequest(nodeParams.privateKey, d.channelId, fundingScript, msg.feerate, isChannelCreation = true, msg.requestFunding_opt, nodeParams.willFundRates_opt) match {
+              LiquidityAds.validateRequest(nodeParams.privateKey, d.channelId, fundingScript, msg.feerate, isChannelCreation = true, msg.requestFunding_opt, nodeParams.willFundRates_opt, None) match {
                 case Left(t) =>
                   log.warning("rejecting rbf attempt: invalid liquidity ads request ({})", t.getMessage)
                   stay() using d.copy(rbfStatus = RbfStatus.RbfAborted) sending TxAbort(d.channelId, t.getMessage)

eclair-core/src/main/scala/fr/acinq/eclair/channel/fund/InteractiveTxBuilder.scala

@@ -37,7 +37,7 @@ import fr.acinq.eclair.crypto.keymanager.ChannelKeyManager
 import fr.acinq.eclair.transactions.Transactions.{CommitTx, HtlcTx, InputInfo, TxOwner}
 import fr.acinq.eclair.transactions.{CommitmentSpec, DirectedHtlc, Scripts, Transactions}
 import fr.acinq.eclair.wire.protocol._
-import fr.acinq.eclair.{Logs, MilliSatoshi, MilliSatoshiLong, NodeParams, UInt64}
+import fr.acinq.eclair.{Logs, MilliSatoshi, MilliSatoshiLong, NodeParams, ToMilliSatoshiConversion, UInt64}
 import scodec.bits.ByteVector
 
 import scala.concurrent.{ExecutionContext, Future}
@@ -157,13 +157,18 @@ object InteractiveTxBuilder {
     // BOLT 2: the initiator's serial IDs MUST use even values and the non-initiator odd values.
     val serialIdParity: Int = if (isInitiator) 0 else 1
 
-    def liquidityFees(liquidityPurchase_opt: Option[LiquidityAds.Purchase]): Satoshi = {
+    def liquidityFees(liquidityPurchase_opt: Option[LiquidityAds.Purchase]): MilliSatoshi = {
       liquidityPurchase_opt.map(l => l.paymentDetails match {
         // The initiator of the interactive-tx is the liquidity buyer (if liquidity ads is used).
-        case LiquidityAds.PaymentDetails.FromChannelBalance | _: LiquidityAds.PaymentDetails.FromChannelBalanceForFutureHtlc => if (isInitiator) l.fees.total else -l.fees.total
+        case LiquidityAds.PaymentDetails.FromChannelBalance | _: LiquidityAds.PaymentDetails.FromChannelBalanceForFutureHtlc =>
+          val feesOwed = l match {
+            case l: LiquidityAds.Purchase.Standard => l.fees.total.toMilliSatoshi
+            case l: LiquidityAds.Purchase.WithFeeCredit => l.fees.total.toMilliSatoshi - l.feeCreditUsed
+          }
+          if (isInitiator) feesOwed else -feesOwed
         // Fees will be paid later, when relaying HTLCs.
-        case _: LiquidityAds.PaymentDetails.FromFutureHtlc | _: LiquidityAds.PaymentDetails.FromFutureHtlcWithPreimage => 0.sat
-      }).getOrElse(0 sat)
+        case _: LiquidityAds.PaymentDetails.FromFutureHtlc | _: LiquidityAds.PaymentDetails.FromFutureHtlcWithPreimage => 0 msat
+      }).getOrElse(0 msat)
     }
   }
 
@@ -744,6 +749,16 @@ private class InteractiveTxBuilder(replyTo: ActorRef[InteractiveTxBuilder.Respon
       return Left(InvalidCompleteInteractiveTx(fundingParams.channelId))
     }
 
+    liquidityPurchase_opt match {
+      case Some(p: LiquidityAds.Purchase.WithFeeCredit) if !fundingParams.isInitiator =>
+        val currentFeeCredit = nodeParams.db.liquidity.getFeeCredit(remoteNodeId)
+        if (currentFeeCredit < p.feeCreditUsed) {
+          log.warn("not enough fee credit: our peer may be malicious ({} < {})", currentFeeCredit, p.feeCreditUsed)
+          return Left(InvalidCompleteInteractiveTx(fundingParams.channelId))
+        }
+      case _ => ()
+    }
+
     previousTransactions.headOption match {
       case Some(previousTx) =>
         // This is an RBF attempt: even if our peer does not contribute to the feerate increase, we'd like to broadcast

eclair-core/src/main/scala/fr/acinq/eclair/db/DualDatabases.scala

@@ -463,4 +463,19 @@ case class DualLiquidityDb(primary: LiquidityDb, secondary: LiquidityDb) extends
     primary.getOnTheFlyFundingPreimage(paymentHash)
   }
 
+  override def addFeeCredit(nodeId: PublicKey, amount: MilliSatoshi, receivedAt: TimestampMilli): MilliSatoshi = {
+    runAsync(secondary.addFeeCredit(nodeId, amount, receivedAt))
+    primary.addFeeCredit(nodeId, amount, receivedAt)
+  }
+
+  override def getFeeCredit(nodeId: PublicKey): MilliSatoshi = {
+    runAsync(secondary.getFeeCredit(nodeId))
+    primary.getFeeCredit(nodeId)
+  }
+
+  override def removeFeeCredit(nodeId: PublicKey, amountUsed: MilliSatoshi): MilliSatoshi = {
+    runAsync(secondary.removeFeeCredit(nodeId, amountUsed))
+    primary.removeFeeCredit(nodeId, amountUsed)
+  }
+
 }

eclair-core/src/main/scala/fr/acinq/eclair/db/LiquidityDb.scala

@@ -20,6 +20,7 @@ import fr.acinq.bitcoin.scalacompat.Crypto.PublicKey
 import fr.acinq.bitcoin.scalacompat.{ByteVector32, TxId}
 import fr.acinq.eclair.channel.{ChannelLiquidityPurchased, LiquidityPurchase}
 import fr.acinq.eclair.payment.relay.OnTheFlyFunding
+import fr.acinq.eclair.{MilliSatoshi, TimestampMilli}
 
 /**
  * Created by t-bast on 13/09/2024. 
@@ -57,4 +58,13 @@ trait LiquidityDb {
   /** Check if we received the preimage for the given payment hash of an on-the-fly payment. */
   def getOnTheFlyFundingPreimage(paymentHash: ByteVector32): Option[ByteVector32]
 
+  /** Add fee credit for the given remote node and return the updated fee credit. */
+  def addFeeCredit(nodeId: PublicKey, amount: MilliSatoshi, receivedAt: TimestampMilli = TimestampMilli.now()): MilliSatoshi
+
+  /** Return the amount owed to the given remote node as fee credit. */
+  def getFeeCredit(nodeId: PublicKey): MilliSatoshi
+
+  /** Remove fee credit for the given remote node and return the remaining fee credit. */
+  def removeFeeCredit(nodeId: PublicKey, amountUsed: MilliSatoshi): MilliSatoshi
+
 }

eclair-core/src/main/scala/fr/acinq/eclair/db/pg/PgLiquidityDb.scala

@@ -25,7 +25,7 @@ import fr.acinq.eclair.db.Monitoring.Tags.DbBackends
 import fr.acinq.eclair.db.pg.PgUtils.PgLock.NoLock.withLock
 import fr.acinq.eclair.payment.relay.OnTheFlyFunding
 import fr.acinq.eclair.wire.protocol.LiquidityAds
-import fr.acinq.eclair.{MilliSatoshi, MilliSatoshiLong}
+import fr.acinq.eclair.{MilliSatoshi, MilliSatoshiLong, TimestampMilli}
 import grizzled.slf4j.Logging
 import scodec.bits.BitVector
 
@@ -58,6 +58,7 @@ class PgLiquidityDb(implicit ds: DataSource) extends LiquidityDb with Logging {
           // On-the-fly funding.
           statement.executeUpdate("CREATE TABLE liquidity.on_the_fly_funding_preimages (payment_hash TEXT NOT NULL PRIMARY KEY, preimage TEXT NOT NULL, received_at TIMESTAMP WITH TIME ZONE NOT NULL)")
           statement.executeUpdate("CREATE TABLE liquidity.pending_on_the_fly_funding (node_id TEXT NOT NULL, payment_hash TEXT NOT NULL, channel_id TEXT NOT NULL, tx_id TEXT NOT NULL, funding_tx_index BIGINT NOT NULL, remaining_fees_msat BIGINT NOT NULL, proposed BYTEA NOT NULL, funded_at TIMESTAMP WITH TIME ZONE NOT NULL, PRIMARY KEY (node_id, payment_hash))")
+          statement.executeUpdate("CREATE TABLE liquidity.fee_credit (node_id TEXT NOT NULL PRIMARY KEY, amount_msat BIGINT NOT NULL, updated_at TIMESTAMP WITH TIME ZONE NOT NULL)")
           // Indexes.
           statement.executeUpdate("CREATE INDEX liquidity_purchases_node_id_idx ON liquidity.purchases(node_id)")
         case Some(CURRENT_VERSION) => () // table is up-to-date, nothing to do
@@ -129,6 +130,7 @@ class PgLiquidityDb(implicit ds: DataSource) extends LiquidityDb with Logging {
   override def addPendingOnTheFlyFunding(remoteNodeId: Crypto.PublicKey, pending: OnTheFlyFunding.Pending): Unit = withMetrics("liquidity/add-pending-on-the-fly-funding", DbBackends.Postgres) {
     pending.status match {
       case _: OnTheFlyFunding.Status.Proposed => ()
+      case _: OnTheFlyFunding.Status.AddedToFeeCredit => ()
       case status: OnTheFlyFunding.Status.Funded => withLock { pg =>
         using(pg.prepareStatement("INSERT INTO liquidity.pending_on_the_fly_funding (node_id, payment_hash, channel_id, tx_id, funding_tx_index, remaining_fees_msat, proposed, funded_at) VALUES (?, ?, ?, ?, ?, ?, ?, ?) ON CONFLICT DO NOTHING")) { statement =>
           statement.setString(1, remoteNodeId.toHex)
@@ -237,4 +239,43 @@ class PgLiquidityDb(implicit ds: DataSource) extends LiquidityDb with Logging {
     }
   }
 
+  override def addFeeCredit(nodeId: PublicKey, amount: MilliSatoshi, receivedAt: TimestampMilli): MilliSatoshi = withMetrics("liquidity/add-fee-credit", DbBackends.Postgres) {
+    withLock { pg =>
+      using(pg.prepareStatement("INSERT INTO liquidity.fee_credit(node_id, amount_msat, updated_at) VALUES (?, ?, ?) ON CONFLICT (node_id) DO UPDATE SET (amount_msat, updated_at) = (liquidity.fee_credit.amount_msat + EXCLUDED.amount_msat, EXCLUDED.updated_at) RETURNING amount_msat")) { statement =>
+        statement.setString(1, nodeId.toHex)
+        statement.setLong(2, amount.toLong)
+        statement.setTimestamp(3, receivedAt.toSqlTimestamp)
+        statement.executeQuery().map(_.getLong("amount_msat").msat).headOption.getOrElse(0 msat)
+      }
+    }
+  }
+
+  override def getFeeCredit(nodeId: PublicKey): MilliSatoshi = withMetrics("liquidity/get-fee-credit", DbBackends.Postgres) {
+    withLock { pg =>
+      using(pg.prepareStatement("SELECT amount_msat FROM liquidity.fee_credit WHERE node_id = ?")) { statement =>
+        statement.setString(1, nodeId.toHex)
+        statement.executeQuery().map(_.getLong("amount_msat").msat).headOption.getOrElse(0 msat)
+      }
+    }
+  }
+
+  override def removeFeeCredit(nodeId: PublicKey, amountUsed: MilliSatoshi): MilliSatoshi = withMetrics("liquidity/remove-fee-credit", DbBackends.Postgres) {
+    withLock { pg =>
+      using(pg.prepareStatement("SELECT amount_msat FROM liquidity.fee_credit WHERE node_id = ?")) { statement =>
+        statement.setString(1, nodeId.toHex)
+        statement.executeQuery().map(_.getLong("amount_msat").msat).headOption match {
+          case Some(current) => using(pg.prepareStatement("UPDATE liquidity.fee_credit SET (amount_msat, updated_at) = (?, ?) WHERE node_id = ?")) { statement =>
+            val updated = (current - amountUsed).max(0 msat)
+            statement.setLong(1, updated.toLong)
+            statement.setTimestamp(2, Timestamp.from(Instant.now()))
+            statement.setString(3, nodeId.toHex)
+            statement.executeUpdate()
+            updated
+          }
+          case None => 0 msat
+        }
+      }
+    }
+  }
+
 }

eclair-core/src/main/scala/fr/acinq/eclair/db/sqlite/SqliteLiquidityDb.scala

@@ -53,6 +53,7 @@ class SqliteLiquidityDb(val sqlite: Connection) extends LiquidityDb with Logging
         // On-the-fly funding.
         statement.executeUpdate("CREATE TABLE on_the_fly_funding_preimages (payment_hash BLOB NOT NULL PRIMARY KEY, preimage BLOB NOT NULL, received_at INTEGER NOT NULL)")
         statement.executeUpdate("CREATE TABLE on_the_fly_funding_pending (node_id BLOB NOT NULL, payment_hash BLOB NOT NULL, channel_id BLOB NOT NULL, tx_id BLOB NOT NULL, funding_tx_index INTEGER NOT NULL, remaining_fees_msat INTEGER NOT NULL, proposed BLOB NOT NULL, funded_at INTEGER NOT NULL, PRIMARY KEY (node_id, payment_hash))")
+        statement.executeUpdate("CREATE TABLE fee_credit (node_id BLOB NOT NULL PRIMARY KEY, amount_msat INTEGER NOT NULL, updated_at INTEGER NOT NULL)")
         // Indexes.
         statement.executeUpdate("CREATE INDEX liquidity_purchases_node_id_idx ON liquidity_purchases(node_id)")
       case Some(CURRENT_VERSION) => () // table is up-to-date, nothing to do
@@ -117,6 +118,7 @@ class SqliteLiquidityDb(val sqlite: Connection) extends LiquidityDb with Logging
   override def addPendingOnTheFlyFunding(remoteNodeId: Crypto.PublicKey, pending: OnTheFlyFunding.Pending): Unit = withMetrics("liquidity/add-pending-on-the-fly-funding", DbBackends.Sqlite) {
     pending.status match {
       case _: OnTheFlyFunding.Status.Proposed => ()
+      case _: OnTheFlyFunding.Status.AddedToFeeCredit => ()
       case status: OnTheFlyFunding.Status.Funded =>
         using(sqlite.prepareStatement("INSERT OR IGNORE INTO on_the_fly_funding_pending (node_id, payment_hash, channel_id, tx_id, funding_tx_index, remaining_fees_msat, proposed, funded_at) VALUES (?, ?, ?, ?, ?, ?, ?, ?)")) { statement =>
           statement.setBytes(1, remoteNodeId.value.toArray)
@@ -212,4 +214,50 @@ class SqliteLiquidityDb(val sqlite: Connection) extends LiquidityDb with Logging
     }
   }
 
+  override def addFeeCredit(nodeId: PublicKey, amount: MilliSatoshi, receivedAt: TimestampMilli): MilliSatoshi = withMetrics("liquidity/add-fee-credit", DbBackends.Sqlite) {
+    using(sqlite.prepareStatement("SELECT amount_msat FROM fee_credit WHERE node_id = ?")) { statement =>
+      statement.setBytes(1, nodeId.value.toArray)
+      statement.executeQuery().map(_.getLong("amount_msat").msat).headOption match {
+        case Some(current) => using(sqlite.prepareStatement("UPDATE fee_credit SET (amount_msat, updated_at) = (?, ?) WHERE node_id = ?")) { statement =>
+          statement.setLong(1, (current + amount).toLong)
+          statement.setLong(2, receivedAt.toLong)
+          statement.setBytes(3, nodeId.value.toArray)
+          statement.executeUpdate()
+          amount + current
+        }
+        case None => using(sqlite.prepareStatement("INSERT OR IGNORE INTO fee_credit(node_id, amount_msat, updated_at) VALUES (?, ?, ?)")) { statement =>
+          statement.setBytes(1, nodeId.value.toArray)
+          statement.setLong(2, amount.toLong)
+          statement.setLong(3, receivedAt.toLong)
+          statement.executeUpdate()
+          amount
+        }
+      }
+    }
+  }
+
+  override def getFeeCredit(nodeId: PublicKey): MilliSatoshi = withMetrics("liquidity/get-fee-credit", DbBackends.Sqlite) {
+    using(sqlite.prepareStatement("SELECT amount_msat FROM fee_credit WHERE node_id = ?")) { statement =>
+      statement.setBytes(1, nodeId.value.toArray)
+      statement.executeQuery().map(_.getLong("amount_msat").msat).headOption.getOrElse(0 msat)
+    }
+  }
+
+  override def removeFeeCredit(nodeId: PublicKey, amountUsed: MilliSatoshi): MilliSatoshi = withMetrics("liquidity/remove-fee-credit", DbBackends.Sqlite) {
+    using(sqlite.prepareStatement("SELECT amount_msat FROM fee_credit WHERE node_id = ?")) { statement =>
+      statement.setBytes(1, nodeId.value.toArray)
+      statement.executeQuery().map(_.getLong("amount_msat").msat).headOption match {
+        case Some(current) => using(sqlite.prepareStatement("UPDATE fee_credit SET (amount_msat, updated_at) = (?, ?) WHERE node_id = ?")) { statement =>
+          val updated = (current - amountUsed).max(0 msat)
+          statement.setLong(1, updated.toLong)
+          statement.setLong(2, TimestampMilli.now().toLong)
+          statement.setBytes(3, nodeId.value.toArray)
+          statement.executeUpdate()
+          updated
+        }
+        case None => 0 msat
+      }
+    }
+  }
+
 }

eclair-core/src/main/scala/fr/acinq/eclair/io/Monitoring.scala

@@ -75,6 +75,7 @@ object Monitoring {
       val Rejected = "rejected"
       val Expired = "expired"
       val Timeout = "timeout"
+      val AddedToFeeCredit = "added-to-fee-credit"
       val Funded = "funded"
       val RelaySucceeded = "relay-succeeded"