Fix anchor outputs closing fee requirements

When using anchor outputs, the mutual close fee is allowed to be greater
than the commit tx fee, because we're targeting a specific confirmation
window.

Author: t-bast

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

@@ -427,8 +427,12 @@ object Helpers {
 
     def firstClosingFee(commitments: Commitments, localScriptPubkey: ByteVector, remoteScriptPubkey: ByteVector, feeEstimator: FeeEstimator, feeTargets: FeeTargets)(implicit log: LoggingAdapter): Satoshi = {
       val requestedFeerate = feeEstimator.getFeeratePerKw(feeTargets.mutualCloseBlockTarget)
-      // we "MUST set fee_satoshis less than or equal to the base fee of the final commitment transaction"
-      val feeratePerKw = requestedFeerate.min(commitments.localCommit.spec.feeratePerKw)
+      val feeratePerKw = if (commitments.channelVersion.hasAnchorOutputs) {
+        requestedFeerate
+      } else {
+        // we "MUST set fee_satoshis less than or equal to the base fee of the final commitment transaction"
+        requestedFeerate.min(commitments.localCommit.spec.feeratePerKw)
+      }
       firstClosingFee(commitments, localScriptPubkey, remoteScriptPubkey, feeratePerKw)
     }
 
@@ -456,7 +460,7 @@ object Helpers {
     def checkClosingSignature(keyManager: ChannelKeyManager, commitments: Commitments, localScriptPubkey: ByteVector, remoteScriptPubkey: ByteVector, remoteClosingFee: Satoshi, remoteClosingSig: ByteVector64)(implicit log: LoggingAdapter): Either[ChannelException, Transaction] = {
       import commitments._
       val lastCommitFeeSatoshi = commitments.commitInput.txOut.amount - commitments.localCommit.publishableTxs.commitTx.tx.txOut.map(_.amount).sum
-      if (remoteClosingFee > lastCommitFeeSatoshi) {
+      if (remoteClosingFee > lastCommitFeeSatoshi && !commitments.channelVersion.hasAnchorOutputs) {
         log.error(s"remote proposed a commit fee higher than the last commitment fee: remoteClosingFeeSatoshi=${remoteClosingFee.toLong} lastCommitFeeSatoshi=$lastCommitFeeSatoshi")
         Left(InvalidCloseFee(commitments.channelId, remoteClosingFee))
       } else {

eclair-core/src/test/scala/fr/acinq/eclair/channel/states/g/NegotiatingStateSpec.scala

@@ -21,10 +21,11 @@ import akka.testkit.TestProbe
 import fr.acinq.bitcoin.{ByteVector32, ByteVector64, SatoshiLong}
 import fr.acinq.eclair.TestConstants.Bob
 import fr.acinq.eclair.blockchain._
-import fr.acinq.eclair.blockchain.fee.{FeeratePerKw, FeeratesPerKw}
+import fr.acinq.eclair.blockchain.fee.{FeeEstimator, FeeratePerKw, FeeratesPerKw}
 import fr.acinq.eclair.channel.Helpers.Closing
 import fr.acinq.eclair.channel._
 import fr.acinq.eclair.channel.states.StateTestsHelperMethods
+import fr.acinq.eclair.transactions.Transactions
 import fr.acinq.eclair.wire.{ClosingSigned, Error, Shutdown}
 import fr.acinq.eclair.{CltvExpiry, MilliSatoshiLong, TestConstants, TestKitBaseClass}
 import org.scalatest.funsuite.FixtureAnyFunSuiteLike
@@ -67,9 +68,6 @@ class NegotiatingStateSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike
       if (test.tags.contains("fee2")) {
         alice.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(4316 sat)))
         bob.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(4316 sat)))
-      } else if (test.tags.contains("anchor_outputs")) {
-        alice.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(1250 sat)))
-        bob.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(1250 sat)))
       } else {
         alice.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(5000 sat)))
         bob.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(5000 sat)))
@@ -108,6 +106,7 @@ class NegotiatingStateSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike
     awaitCond(alice.stateData.asInstanceOf[DATA_NEGOTIATING].closingTxProposed.last.map(_.localClosingSigned) == initialState.closingTxProposed.last.map(_.localClosingSigned) :+ aliceCloseSig2)
     val Some(closingTx) = alice.stateData.asInstanceOf[DATA_NEGOTIATING].bestUnpublishedClosingTx_opt
     assert(closingTx.txOut.length === 2) // NB: in the anchor outputs case, anchors are removed from the closing tx
+    assert(aliceCloseSig2.feeSatoshis > Transactions.weight2fee(FeeEstimator.AnchorOutputMaxCommitFeerate, closingTx.weight())) // NB: closing fee is allowed to be higher than commit tx fee when using anchor outputs
   }
 
   test("recv ClosingSigned (theirCloseFee != ourCloseFee)") {

eclair-core/src/test/scala/fr/acinq/eclair/channel/states/h/ClosingStateSpec.scala

@@ -612,9 +612,6 @@ class ClosingStateSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike with
 
   test("recv BITCOIN_TX_CONFIRMED (remote commit, anchor outputs)", Tag("anchor_outputs")) { f =>
     import f._
-    // Set feerates below the 10 sat/byte anchor outputs threshold to ensure a fee negotiation round-trip takes place.
-    alice.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(1200 sat)))
-    bob.feeEstimator.setFeerate(FeeratesPerKw.single(FeeratePerKw(1200 sat)))
     mutualClose(alice, bob, alice2bob, bob2alice, alice2blockchain, bob2blockchain)
     val initialState = alice.stateData.asInstanceOf[DATA_CLOSING]
     assert(initialState.commitments.channelVersion === ChannelVersion.ANCHOR_OUTPUTS)