Always subtract both anchors from funder amount

For simplicity's sake, we always subtract both anchors from the funder's
main output, even if only one anchor materializes.

eclair-core/src/main/scala/fr/acinq/eclair/transactions/Transactions.scala

@@ -185,8 +185,13 @@ object Transactions {
   def commitTxFeeMsat(dustLimit: Satoshi, spec: CommitmentSpec, commitmentFormat: CommitmentFormat): MilliSatoshi = {
     val trimmedOfferedHtlcs = trimOfferedHtlcs(dustLimit, spec, commitmentFormat)
     val trimmedReceivedHtlcs = trimReceivedHtlcs(dustLimit, spec, commitmentFormat)
+    val anchorsCost = commitmentFormat match {
+      case DefaultCommitmentFormat => Satoshi(0)
+      // the funder pays for both anchors all the time, even if only one anchor is present
+      case AnchorOutputsCommitmentFormat => AnchorOutputsCommitmentFormat.anchorAmount * 2
+    }
     val weight = commitmentFormat.commitWeight + commitmentFormat.htlcOutputWeight * (trimmedOfferedHtlcs.size + trimmedReceivedHtlcs.size)
-    weight2feeMsat(spec.feeratePerKw, weight)
+    weight2feeMsat(spec.feeratePerKw, weight) + anchorsCost
   }
 
   def commitTxFee(dustLimit: Satoshi, spec: CommitmentSpec, commitmentFormat: CommitmentFormat): Satoshi = commitTxFeeMsat(dustLimit, spec, commitmentFormat).truncateToSatoshi
@@ -291,24 +296,11 @@ object Transactions {
 
     val hasHtlcs = outputs.nonEmpty
 
-    def computeMainOutputs(funderSpecAmount: MilliSatoshi, fundeeSpecAmount: MilliSatoshi): (Satoshi, Satoshi) = commitmentFormat match {
-      // NB: we don't care if values are < 0, they will be trimmed if they are < dust limit anyway
-      case DefaultCommitmentFormat =>
-        val commitFee = commitTxFee(localDustLimit, spec, commitmentFormat)
-        (funderSpecAmount.truncateToSatoshi - commitFee, fundeeSpecAmount.truncateToSatoshi)
-      case AnchorOutputsCommitmentFormat =>
-        val commitFee = commitTxFee(localDustLimit, spec, commitmentFormat)
-        // We won't add an anchor for the fundee if he has nothing at stake (which typically happens when a new channel is opened without any push_msat)
-        val fundeeAnchorSkipped = fundeeSpecAmount < localDustLimit && !hasHtlcs
-        val anchorsCost = if (fundeeAnchorSkipped) AnchorOutputsCommitmentFormat.anchorAmount else AnchorOutputsCommitmentFormat.anchorAmount * 2
-        (funderSpecAmount.truncateToSatoshi - commitFee - anchorsCost, fundeeSpecAmount.truncateToSatoshi)
-    }
-
     val (toLocalAmount: Satoshi, toRemoteAmount: Satoshi) = if (localIsFunder) {
-      computeMainOutputs(spec.toLocal, spec.toRemote)
+      (spec.toLocal.truncateToSatoshi - commitTxFee(localDustLimit, spec, commitmentFormat), spec.toRemote.truncateToSatoshi)
     } else {
-      computeMainOutputs(spec.toRemote, spec.toLocal).swap
-    }
+      (spec.toLocal.truncateToSatoshi, spec.toRemote.truncateToSatoshi - commitTxFee(localDustLimit, spec, commitmentFormat))
+    } // NB: we don't care if values are < 0, they will be trimmed if they are < dust limit anyway
 
     if (toLocalAmount >= localDustLimit) {
       outputs.append(CommitmentOutputLink(

eclair-core/src/test/resources/bolt3-tx-test-vectors-anchor-outputs-format.txt

@@ -158,7 +158,7 @@
     to_local_msat: 6988000000
     to_remote_msat: 3000000000
     local_feerate_per_kw: 6216009
-    remote_signature = 3045022100ee67ddd6b3739f3189151063365585ff153ac17e34751da0c90e8034e388a6e2022077aa76cd469604e6af1c00b600d0f4ed0876b11f06336edde3388d517a1ef5e8
+    remote_signature = 30450221008fd5dbff02e4b59020d4cd23a3c30d3e287065fda75a0a09b402980adf68ccda022001e0b8b620cd915ddff11f1de32addf23d81d51b90e6841b2cb8dcaf3faa5ecf
     output commit_tx: 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
     num_htlcs: 0
 

eclair-core/src/test/scala/fr/acinq/eclair/transactions/TransactionsSpec.scala

@@ -400,22 +400,22 @@ class TransactionsSpec extends AnyFunSuite with Logging {
     val spec = CommitmentSpec(htlcs = Set.empty, feeratePerKw = feeratePerKw, toLocal = 400.mbtc.toMilliSatoshi, toRemote = 300.mbtc.toMilliSatoshi)
     val commitFee = commitTxFee(localDustLimit, spec, AnchorOutputsCommitmentFormat)
     val belowDust = (localDustLimit * 0.9).toMilliSatoshi
-    val belowDustWithFeeAndAnchors = (localDustLimit + commitFee + anchorAmount * 1.75).toMilliSatoshi
+    val belowDustWithFeeAndAnchors = (localDustLimit + commitFee * 0.9).toMilliSatoshi
 
     {
       val outputs = makeCommitTxOutputs(localIsFunder = true, localDustLimit, localRevocationPriv.publicKey, toLocalDelay, localDelayedPaymentPriv.publicKey, remotePaymentPriv.publicKey, localHtlcPriv.publicKey, remoteHtlcPriv.publicKey, localFundingPriv.publicKey, remoteFundingPriv.publicKey, spec, AnchorOutputsCommitmentFormat)
       assert(outputs.map(_.commitmentOutput).toSet === Set(CommitmentOutput.ToLocal, CommitmentOutput.ToRemote, CommitmentOutput.ToLocalAnchor, CommitmentOutput.ToRemoteAnchor))
       assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToLocalAnchor).get.output.amount === anchorAmount)
       assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToRemoteAnchor).get.output.amount === anchorAmount)
-      assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToLocal).get.output.amount.toMilliSatoshi === spec.toLocal - commitFee - anchorAmount * 2)
+      assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToLocal).get.output.amount.toMilliSatoshi === spec.toLocal - commitFee)
       assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToRemote).get.output.amount.toMilliSatoshi === spec.toRemote)
     }
     {
       val toRemoteFundeeBelowDust = spec.copy(toRemote = belowDust)
       val outputs = makeCommitTxOutputs(localIsFunder = true, localDustLimit, localRevocationPriv.publicKey, toLocalDelay, localDelayedPaymentPriv.publicKey, remotePaymentPriv.publicKey, localHtlcPriv.publicKey, remoteHtlcPriv.publicKey, localFundingPriv.publicKey, remoteFundingPriv.publicKey, toRemoteFundeeBelowDust, AnchorOutputsCommitmentFormat)
       assert(outputs.map(_.commitmentOutput).toSet === Set(CommitmentOutput.ToLocal, CommitmentOutput.ToLocalAnchor))
       assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToLocalAnchor).get.output.amount === anchorAmount)
-      assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToLocal).get.output.amount.toMilliSatoshi === spec.toLocal - commitFee - anchorAmount)
+      assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToLocal).get.output.amount.toMilliSatoshi === spec.toLocal - commitFee)
     }
     {
       val toLocalFunderBelowDust = spec.copy(toLocal = belowDustWithFeeAndAnchors)
@@ -436,7 +436,7 @@ class TransactionsSpec extends AnyFunSuite with Logging {
       val outputs = makeCommitTxOutputs(localIsFunder = false, localDustLimit, localRevocationPriv.publicKey, toLocalDelay, localDelayedPaymentPriv.publicKey, remotePaymentPriv.publicKey, localHtlcPriv.publicKey, remoteHtlcPriv.publicKey, localFundingPriv.publicKey, remoteFundingPriv.publicKey, toLocalFundeeBelowDust, AnchorOutputsCommitmentFormat)
       assert(outputs.map(_.commitmentOutput).toSet === Set(CommitmentOutput.ToRemote, CommitmentOutput.ToRemoteAnchor))
       assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToRemoteAnchor).get.output.amount === anchorAmount)
-      assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToRemote).get.output.amount.toMilliSatoshi === spec.toRemote - commitFee - anchorAmount)
+      assert(outputs.find(_.commitmentOutput == CommitmentOutput.ToRemote).get.output.amount.toMilliSatoshi === spec.toRemote - commitFee)
     }
     {
       val allBelowDust = spec.copy(toLocal = belowDust, toRemote = belowDust)