Remove minInboundLiquidityTarget

We previously forced wallets to purchase additional inbound liquidity
every time an on-chain transaction was created. We now allow wallets
to disable automatic liquidity purchases: the LSP will need to add
enough funds on their side to cover the commitment fees, which the
wallet won't be paying for. But we still make a dummy purchase of 1 sat
to ensure that the liquidity ads flow is used and the wallet refunds
the mining fees paid by the LSP.

src/commonMain/kotlin/fr/acinq/lightning/io/Peer.kt

@@ -1272,9 +1272,11 @@ class Peer(
                         val currentFeerates = peerFeeratesFlow.filterNotNull().first()
                         val requestRemoteFunding = run {
                             // We need our peer to contribute, because they must have enough funds to pay the commitment fees.
+                            // That means they will add at least one input to the funding transaction, and pay the corresponding mining fees.
+                            // We always request a liquidity purchase, even for a dummy amount, which ensures that we refund their mining fees.
                             val inboundLiquidityTarget = when (val policy = nodeParams.liquidityPolicy.first()) {
-                                is LiquidityPolicy.Disable -> LiquidityPolicy.minInboundLiquidityTarget
-                                is LiquidityPolicy.Auto -> policy.inboundLiquidityTarget ?: LiquidityPolicy.minInboundLiquidityTarget
+                                is LiquidityPolicy.Disable -> 1.sat
+                                is LiquidityPolicy.Auto -> policy.inboundLiquidityTarget ?: 1.sat
                             }
                             // We assume that the liquidity policy is correctly configured to match a funding lease offered by our peer.
                             val fundingRate = walletParams.remoteFundingRates.findRate(inboundLiquidityTarget)!!

src/commonMain/kotlin/fr/acinq/lightning/payment/IncomingPaymentHandler.kt

@@ -298,9 +298,9 @@ class IncomingPaymentHandler(val nodeParams: NodeParams, val db: PaymentsDb, pri
         return when (val liquidityPolicy = nodeParams.liquidityPolicy.value) {
             is LiquidityPolicy.Disable -> Either.Left(LiquidityEvents.Rejected(willAddHtlcAmount, 0.msat, LiquidityEvents.Source.OffChainPayment, LiquidityEvents.Rejected.Reason.PolicySetToDisabled))
             is LiquidityPolicy.Auto -> {
-                // Whenever we receive on-the-fly funding, we take this opportunity to purchase inbound liquidity.
+                // Whenever we receive on-the-fly funding, we take this opportunity to purchase inbound liquidity, if configured.
                 // This reduces the frequency of on-chain funding and thus the overall on-chain fees paid.
-                val additionalInboundLiquidity = liquidityPolicy.inboundLiquidityTarget ?: LiquidityPolicy.minInboundLiquidityTarget
+                val additionalInboundLiquidity = liquidityPolicy.inboundLiquidityTarget ?: 0.sat
                 // We must round up to the nearest satoshi value instead of rounding down.
                 val requestedAmount = (willAddHtlcAmount + 999.msat).truncateToSatoshi() + additionalInboundLiquidity
                 when (val fundingRate = remoteFundingRates.findRate(requestedAmount)) {

src/commonMain/kotlin/fr/acinq/lightning/payment/LiquidityPolicy.kt

@@ -38,12 +38,4 @@ sealed class LiquidityPolicy {
             }
         }?.let { reason -> LiquidityEvents.Rejected(amount, fee, source, reason) }
     }
-
-    companion object {
-        /**
-         * We usually need our peer to contribute to channel funding, because they must have enough funds to pay the commitment fees.
-         * When we don't have an inbound liquidity target set, we use the following default amount.
-         */
-        val minInboundLiquidityTarget: Satoshi = 100_000.sat
-    }
 }

src/commonTest/kotlin/fr/acinq/lightning/io/peer/PeerTest.kt

@@ -222,7 +222,7 @@ class PeerTest : LightningTestSuite() {
     @Test
     fun `swap funds into a channel`() = runSuspendTest {
         val nodeParams = Pair(TestConstants.Alice.nodeParams, TestConstants.Bob.nodeParams)
-        nodeParams.second.liquidityPolicy.emit(LiquidityPolicy.Auto(inboundLiquidityTarget = null, maxAbsoluteFee = 20_000.sat, maxRelativeFeeBasisPoints = 1000, skipAbsoluteFeeCheck = false))
+        nodeParams.second.liquidityPolicy.emit(LiquidityPolicy.Auto(inboundLiquidityTarget = 100_000.sat, maxAbsoluteFee = 20_000.sat, maxRelativeFeeBasisPoints = 1000, skipAbsoluteFeeCheck = false))
         val walletParams = Pair(TestConstants.Alice.walletParams, TestConstants.Bob.walletParams)
         val (_, bob, _, bob2alice) = newPeers(this, nodeParams, walletParams, automateMessaging = false)
 

src/commonTest/kotlin/fr/acinq/lightning/payment/IncomingPaymentHandlerTestsCommon.kt

@@ -247,7 +247,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
         assertIs<AddLiquidityForIncomingPayment>(addLiquidity)
         assertEquals(incomingPayment.preimage, addLiquidity.preimage)
         assertEquals(defaultAmount, addLiquidity.paymentAmount)
-        assertTrue(defaultAmount < addLiquidity.requestedAmount)
+        assertEquals(defaultAmount, addLiquidity.requestedAmount.toMilliSatoshi())
         assertEquals(TestConstants.fundingRates.fundingRates.first(), addLiquidity.fundingRate)
         assertEquals(listOf(willAddHtlc), addLiquidity.willAddHtlcs)
         // We don't update the payments DB: we're waiting to receive HTLCs after the open/splice.
@@ -359,7 +359,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
         assertIs<AddLiquidityForIncomingPayment>(addLiquidity)
         assertEquals(incomingPayment.preimage, addLiquidity.preimage)
         assertEquals(defaultAmount, addLiquidity.paymentAmount)
-        assertTrue(defaultAmount < addLiquidity.requestedAmount)
+        assertEquals(defaultAmount, addLiquidity.requestedAmount.toMilliSatoshi())
         assertEquals(TestConstants.fundingRates.fundingRates.first(), addLiquidity.fundingRate)
         assertNull(paymentHandler.db.getIncomingPayment(incomingPayment.paymentHash)?.received)
     }
@@ -464,7 +464,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
     @Test
     fun `receive multipart payment with a mix of HTLC and will_add_htlc`() = runSuspendTest {
         val channelId = randomBytes32()
-        val (amount1, amount2) = listOf(50_000_000.msat, 60_000_000.msat)
+        val (amount1, amount2) = listOf(50_000_000.msat, 100_000_000.msat)
         val totalAmount = amount1 + amount2
         val (paymentHandler, incomingPayment, paymentSecret) = createFixture(totalAmount)
         checkDbPayment(incomingPayment, paymentHandler.db)
@@ -489,7 +489,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
             assertEquals(1, result.actions.size)
             val addLiquidity = result.actions.first() as AddLiquidityForIncomingPayment
             assertEquals(incomingPayment.preimage, addLiquidity.preimage)
-            assertEquals(amount2.truncateToSatoshi() + LiquidityPolicy.minInboundLiquidityTarget, addLiquidity.requestedAmount)
+            assertEquals(amount2.truncateToSatoshi(), addLiquidity.requestedAmount)
             assertEquals(totalAmount, addLiquidity.paymentAmount)
             assertNull(paymentHandler.db.getIncomingPayment(incomingPayment.paymentHash)?.received)
         }
@@ -617,7 +617,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
     @Test
     fun `receive multipart payment with funding fee`() = runSuspendTest {
         val channelId = randomBytes32()
-        val (amount1, amount2) = listOf(50_000_000.msat, 60_000_000.msat)
+        val (amount1, amount2) = listOf(50_000_000.msat, 100_000_000.msat)
         val totalAmount = amount1 + amount2
         val (paymentHandler, incomingPayment, paymentSecret) = createFixture(totalAmount)
         checkDbPayment(incomingPayment, paymentHandler.db)
@@ -676,7 +676,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
     @Test
     fun `receive payment with funding fee -- from channel balance`() = runSuspendTest {
         val channelId = randomBytes32()
-        val amount = 50_000_000.msat
+        val amount = 100_000_000.msat
         val (paymentHandler, incomingPayment, paymentSecret) = createFixture(amount)
         checkDbPayment(incomingPayment, paymentHandler.db)
 
@@ -743,7 +743,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
 
         // We have a matching transaction in our DB.
         val purchase = LiquidityAds.Purchase.Standard(
-            defaultAmount.truncateToSatoshi() + LiquidityPolicy.minInboundLiquidityTarget,
+            defaultAmount.truncateToSatoshi(),
             LiquidityAds.Fees(2000.sat, 3000.sat),
             LiquidityAds.PaymentDetails.FromFutureHtlc(listOf(incomingPayment.paymentHash)),
         )
@@ -779,7 +779,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
 
         // We have a matching transaction in our DB, but we paid the fees from our channel balance already.
         val purchase = LiquidityAds.Purchase.Standard(
-            defaultAmount.truncateToSatoshi() + LiquidityPolicy.minInboundLiquidityTarget,
+            defaultAmount.truncateToSatoshi(),
             LiquidityAds.Fees(2000.sat, 3000.sat),
             LiquidityAds.PaymentDetails.FromChannelBalanceForFutureHtlc(listOf(incomingPayment.paymentHash)),
         )
@@ -801,7 +801,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
 
         // We have a matching transaction in our DB, but the fees must be paid with a different payment_hash.
         val purchase = LiquidityAds.Purchase.Standard(
-            defaultAmount.truncateToSatoshi() + LiquidityPolicy.minInboundLiquidityTarget,
+            defaultAmount.truncateToSatoshi(),
             LiquidityAds.Fees(2000.sat, 3000.sat),
             LiquidityAds.PaymentDetails.FromFutureHtlc(listOf(randomBytes32())),
         )
@@ -1436,7 +1436,7 @@ class IncomingPaymentHandlerTestsCommon : LightningTestSuite() {
 
         // We have a matching transaction in our DB, but the fees must be paid with a different payment_hash.
         val purchase = LiquidityAds.Purchase.Standard(
-            defaultAmount.truncateToSatoshi() + LiquidityPolicy.minInboundLiquidityTarget,
+            defaultAmount.truncateToSatoshi(),
             LiquidityAds.Fees(2000.sat, 3000.sat),
             LiquidityAds.PaymentDetails.FromFutureHtlcWithPreimage(listOf(preimage)),
         )