From 073efa0e531bcc7375dddd9132d20d83fe9e011c Mon Sep 17 00:00:00 2001
From: t-bast <bastien@acinq.fr>
Date: Thu, 22 Aug 2024 14:01:56 +0200
Subject: [PATCH] Correctly set `htlc_minimum` in blinded paths

We must use the most restrictive value for `htlc_minimum`, not the
least restrictive one. When we don't have the remote channel update
we should use the value from our node params.

The `htlc_maximum` is allowed to be twice the payment amount according
to the BOLTs: we reflect that in the blinded path data.
---
 .../fr/acinq/lightning/payment/OfferManager.kt       | 12 ++++++++----
 .../lightning/payment/OfferManagerTestsCommon.kt     |  4 ++++
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/commonMain/kotlin/fr/acinq/lightning/payment/OfferManager.kt b/src/commonMain/kotlin/fr/acinq/lightning/payment/OfferManager.kt
index 030f113f3..2b04369ff 100644
--- a/src/commonMain/kotlin/fr/acinq/lightning/payment/OfferManager.kt
+++ b/src/commonMain/kotlin/fr/acinq/lightning/payment/OfferManager.kt
@@ -4,9 +4,12 @@ import fr.acinq.bitcoin.ByteVector32
 import fr.acinq.bitcoin.PublicKey
 import fr.acinq.bitcoin.utils.Either.Left
 import fr.acinq.bitcoin.utils.Either.Right
-import fr.acinq.lightning.*
+import fr.acinq.lightning.EncodedNodeId
+import fr.acinq.lightning.Features
 import fr.acinq.lightning.Lightning.randomBytes32
 import fr.acinq.lightning.Lightning.randomKey
+import fr.acinq.lightning.NodeParams
+import fr.acinq.lightning.WalletParams
 import fr.acinq.lightning.crypto.RouteBlinding
 import fr.acinq.lightning.io.OfferInvoiceReceived
 import fr.acinq.lightning.io.OfferNotPaid
@@ -18,7 +21,6 @@ import fr.acinq.lightning.message.OnionMessages.Destination
 import fr.acinq.lightning.message.OnionMessages.IntermediateNode
 import fr.acinq.lightning.message.OnionMessages.buildMessage
 import fr.acinq.lightning.utils.currentTimestampMillis
-import fr.acinq.lightning.utils.msat
 import fr.acinq.lightning.utils.toByteVector
 import fr.acinq.lightning.wire.*
 import kotlinx.coroutines.flow.MutableSharedFlow
@@ -168,8 +170,10 @@ class OfferManager(val nodeParams: NodeParams, val walletParams: WalletParams, v
                     // This ensures that even when payers haven't received the latest block(s) or don't include a safety margin in the
                     // expiry they use, we can still safely receive their payment.
                     cltvExpiryDelta = cltvExpiryDelta + nodeParams.minFinalCltvExpiryDelta,
-                    minHtlc = remoteChannelUpdates.minOfOrNull { it.htlcMinimumMsat } ?: 1.msat,
-                    maxHtlc = amount,
+                    // We must use the most restrictive minimum HTLC value between local and remote.
+                    minHtlc = (listOf(nodeParams.htlcMinimum) + remoteChannelUpdates.map { it.htlcMinimumMsat }).max(),
+                    // Payments are allowed to overpay at most two times the invoice amount.
+                    maxHtlc = amount * 2,
                     allowedFeatures = Features.empty
                 )
                 val remoteNodePayload = RouteBlindingEncryptedData(
diff --git a/src/commonTest/kotlin/fr/acinq/lightning/payment/OfferManagerTestsCommon.kt b/src/commonTest/kotlin/fr/acinq/lightning/payment/OfferManagerTestsCommon.kt
index 974664394..851eedddf 100644
--- a/src/commonTest/kotlin/fr/acinq/lightning/payment/OfferManagerTestsCommon.kt
+++ b/src/commonTest/kotlin/fr/acinq/lightning/payment/OfferManagerTestsCommon.kt
@@ -89,6 +89,8 @@ class OfferManagerTestsCommon : LightningTestSuite() {
         val path = payInvoice.invoice.blindedPaths.first()
         assertEquals(EncodedNodeId(aliceTrampolineKey.publicKey()), path.route.route.introductionNodeId)
         assertEquals(aliceOfferManager.nodeParams.expiryDeltaBlocks + aliceOfferManager.nodeParams.minFinalCltvExpiryDelta, path.paymentInfo.cltvExpiryDelta)
+        assertEquals(TestConstants.Alice.nodeParams.htlcMinimum, path.paymentInfo.minHtlc)
+        assertEquals(payOffer.amount * 2, path.paymentInfo.maxHtlc)
     }
 
     @Test
@@ -121,6 +123,8 @@ class OfferManagerTestsCommon : LightningTestSuite() {
         val path = payInvoice.invoice.blindedPaths.first()
         assertEquals(EncodedNodeId(aliceTrampolineKey.publicKey()), path.route.route.introductionNodeId)
         assertEquals(aliceOfferManager.nodeParams.expiryDeltaBlocks + aliceOfferManager.nodeParams.minFinalCltvExpiryDelta, path.paymentInfo.cltvExpiryDelta)
+        assertEquals(TestConstants.Alice.nodeParams.htlcMinimum, path.paymentInfo.minHtlc)
+        assertEquals(payOffer.amount * 2, path.paymentInfo.maxHtlc)
     }
 
     @Test