Add support for OP_RETURN closing scripts

The spec allows the closer to use an OP_RETURN output if their amount is
too low when using `option_simple_close`.

Author: t-bast

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

@@ -113,10 +113,11 @@ case class ChannelParams(channelId: ByteVector32,
     // README: if we set our bitcoin node to generate taproot addresses and our peer does not support option_shutdown_anysegwit, we will not be able to mutual-close
     // channels as the isValidFinalScriptPubkey() check would fail.
     val allowAnySegwit = Features.canUseFeature(localParams.initFeatures, remoteParams.initFeatures, Features.ShutdownAnySegwit)
+    val allowOpReturn = Features.canUseFeature(localParams.initFeatures, remoteParams.initFeatures, Features.SimpleClose)
     val mustUseUpfrontShutdownScript = channelFeatures.hasFeature(Features.UpfrontShutdownScript)
     // we only enforce using the pre-generated shutdown script if option_upfront_shutdown_script is set
     if (mustUseUpfrontShutdownScript && localParams.upfrontShutdownScript_opt.exists(_ != localScriptPubKey)) Left(InvalidFinalScript(channelId))
-    else if (!Closing.MutualClose.isValidFinalScriptPubkey(localScriptPubKey, allowAnySegwit)) Left(InvalidFinalScript(channelId))
+    else if (!Closing.MutualClose.isValidFinalScriptPubkey(localScriptPubKey, allowAnySegwit, allowOpReturn)) Left(InvalidFinalScript(channelId))
     else Right(localScriptPubKey)
   }
 
@@ -127,10 +128,11 @@ case class ChannelParams(channelId: ByteVector32,
   def validateRemoteShutdownScript(remoteScriptPubKey: ByteVector): Either[ChannelException, ByteVector] = {
     // to check whether shutdown_any_segwit is active we check features in local and remote parameters, which are negotiated each time we connect to our peer.
     val allowAnySegwit = Features.canUseFeature(localParams.initFeatures, remoteParams.initFeatures, Features.ShutdownAnySegwit)
+    val allowOpReturn = Features.canUseFeature(localParams.initFeatures, remoteParams.initFeatures, Features.SimpleClose)
     val mustUseUpfrontShutdownScript = channelFeatures.hasFeature(Features.UpfrontShutdownScript)
     // we only enforce using the pre-generated shutdown script if option_upfront_shutdown_script is set
     if (mustUseUpfrontShutdownScript && remoteParams.upfrontShutdownScript_opt.exists(_ != remoteScriptPubKey)) Left(InvalidFinalScript(channelId))
-    else if (!Closing.MutualClose.isValidFinalScriptPubkey(remoteScriptPubKey, allowAnySegwit)) Left(InvalidFinalScript(channelId))
+    else if (!Closing.MutualClose.isValidFinalScriptPubkey(remoteScriptPubKey, allowAnySegwit, allowOpReturn)) Left(InvalidFinalScript(channelId))
     else Right(remoteScriptPubKey)
   }
 

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

@@ -67,14 +67,15 @@ object Helpers {
   private def extractShutdownScript(channelId: ByteVector32, localFeatures: Features[InitFeature], remoteFeatures: Features[InitFeature], upfrontShutdownScript_opt: Option[ByteVector]): Either[ChannelException, Option[ByteVector]] = {
     val canUseUpfrontShutdownScript = Features.canUseFeature(localFeatures, remoteFeatures, Features.UpfrontShutdownScript)
     val canUseAnySegwit = Features.canUseFeature(localFeatures, remoteFeatures, Features.ShutdownAnySegwit)
-    extractShutdownScript(channelId, canUseUpfrontShutdownScript, canUseAnySegwit, upfrontShutdownScript_opt)
+    val canUseOpReturn = Features.canUseFeature(localFeatures, remoteFeatures, Features.SimpleClose)
+    extractShutdownScript(channelId, canUseUpfrontShutdownScript, canUseAnySegwit, canUseOpReturn, upfrontShutdownScript_opt)
   }
 
-  private def extractShutdownScript(channelId: ByteVector32, hasOptionUpfrontShutdownScript: Boolean, allowAnySegwit: Boolean, upfrontShutdownScript_opt: Option[ByteVector]): Either[ChannelException, Option[ByteVector]] = {
+  private def extractShutdownScript(channelId: ByteVector32, hasOptionUpfrontShutdownScript: Boolean, allowAnySegwit: Boolean, allowOpReturn: Boolean, upfrontShutdownScript_opt: Option[ByteVector]): Either[ChannelException, Option[ByteVector]] = {
     (hasOptionUpfrontShutdownScript, upfrontShutdownScript_opt) match {
       case (true, None) => Left(MissingUpfrontShutdownScript(channelId))
       case (true, Some(script)) if script.isEmpty => Right(None) // but the provided script can be empty
-      case (true, Some(script)) if !Closing.MutualClose.isValidFinalScriptPubkey(script, allowAnySegwit) => Left(InvalidFinalScript(channelId))
+      case (true, Some(script)) if !Closing.MutualClose.isValidFinalScriptPubkey(script, allowAnySegwit, allowOpReturn) => Left(InvalidFinalScript(channelId))
       case (true, Some(script)) => Right(Some(script))
       case (false, Some(_)) => Right(None) // they provided a script but the feature is not active, we just ignore it
       case _ => Right(None)
@@ -644,13 +645,14 @@ object Helpers {
 
     object MutualClose {
 
-      def isValidFinalScriptPubkey(scriptPubKey: ByteVector, allowAnySegwit: Boolean): Boolean = {
+      def isValidFinalScriptPubkey(scriptPubKey: ByteVector, allowAnySegwit: Boolean, allowOpReturn: Boolean): Boolean = {
         Try(Script.parse(scriptPubKey)) match {
           case Success(OP_DUP :: OP_HASH160 :: OP_PUSHDATA(pubkeyHash, _) :: OP_EQUALVERIFY :: OP_CHECKSIG :: Nil) if pubkeyHash.size == 20 => true
           case Success(OP_HASH160 :: OP_PUSHDATA(scriptHash, _) :: OP_EQUAL :: Nil) if scriptHash.size == 20 => true
           case Success(OP_0 :: OP_PUSHDATA(pubkeyHash, _) :: Nil) if pubkeyHash.size == 20 => true
           case Success(OP_0 :: OP_PUSHDATA(scriptHash, _) :: Nil) if scriptHash.size == 32 => true
           case Success((OP_1 | OP_2 | OP_3 | OP_4 | OP_5 | OP_6 | OP_7 | OP_8 | OP_9 | OP_10 | OP_11 | OP_12 | OP_13 | OP_14 | OP_15 | OP_16) :: OP_PUSHDATA(program, _) :: Nil) if allowAnySegwit && 2 <= program.length && program.length <= 40 => true
+          case Success(OP_RETURN :: _) if allowOpReturn => true
           case _ => false
         }
       }
@@ -717,16 +719,7 @@ object Helpers {
        * The various dust limits are detailed in https://github.com/lightningnetwork/lightning-rfc/blob/master/03-transactions.md#dust-limits
        */
       def checkClosingDustAmounts(closingTx: ClosingTx): Boolean = {
-        closingTx.tx.txOut.forall(txOut => {
-          Try(Script.parse(txOut.publicKeyScript)) match {
-            case Success(OP_DUP :: OP_HASH160 :: OP_PUSHDATA(pubkeyHash, _) :: OP_EQUALVERIFY :: OP_CHECKSIG :: Nil) if pubkeyHash.size == 20 => txOut.amount >= 546.sat
-            case Success(OP_HASH160 :: OP_PUSHDATA(scriptHash, _) :: OP_EQUAL :: Nil) if scriptHash.size == 20 => txOut.amount >= 540.sat
-            case Success(OP_0 :: OP_PUSHDATA(pubkeyHash, _) :: Nil) if pubkeyHash.size == 20 => txOut.amount >= 294.sat
-            case Success(OP_0 :: OP_PUSHDATA(scriptHash, _) :: Nil) if scriptHash.size == 32 => txOut.amount >= 330.sat
-            case Success((OP_1 | OP_2 | OP_3 | OP_4 | OP_5 | OP_6 | OP_7 | OP_8 | OP_9 | OP_10 | OP_11 | OP_12 | OP_13 | OP_14 | OP_15 | OP_16) :: OP_PUSHDATA(program, _) :: Nil) if 2 <= program.length && program.length <= 40 => txOut.amount >= 354.sat
-            case _ => txOut.amount >= 546.sat
-          }
-        })
+        closingTx.tx.txOut.forall(txOut => txOut.amount >= Transactions.dustLimit(txOut.publicKeyScript))
       }
     }
 

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

@@ -3010,7 +3010,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
     if (fundingContribution < 0.sat && parentCommitment.localCommit.spec.toLocal + fundingContribution < parentCommitment.localChannelReserve(d.commitments.params).max(commitTxFees)) {
       log.warning(s"cannot do splice: insufficient funds (commitTxFees=$commitTxFees reserve=${parentCommitment.localChannelReserve(d.commitments.params)})")
       Left(InvalidSpliceRequest(d.channelId))
-    } else if (cmd.spliceOut_opt.map(_.scriptPubKey).exists(!MutualClose.isValidFinalScriptPubkey(_, allowAnySegwit = true))) {
+    } else if (cmd.spliceOut_opt.map(_.scriptPubKey).exists(!MutualClose.isValidFinalScriptPubkey(_, allowAnySegwit = true, allowOpReturn = false))) {
       log.warning("cannot do splice: invalid splice-out script")
       Left(InvalidSpliceRequest(d.channelId))
     } else {

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

@@ -578,7 +578,7 @@ private class InteractiveTxBuilder(replyTo: ActorRef[InteractiveTxBuilder.Respon
       Left(OutputBelowDust(fundingParams.channelId, addOutput.serialId, addOutput.amount, fundingParams.dustLimit))
     } else if (addOutput.pubkeyScript == fundingPubkeyScript && addOutput.amount != fundingParams.fundingAmount) {
       Left(InvalidSharedOutputAmount(fundingParams.channelId, addOutput.serialId, addOutput.amount, fundingParams.fundingAmount))
-    } else if (!MutualClose.isValidFinalScriptPubkey(addOutput.pubkeyScript, allowAnySegwit = true)) {
+    } else if (!MutualClose.isValidFinalScriptPubkey(addOutput.pubkeyScript, allowAnySegwit = true, allowOpReturn = false)) {
       Left(InvalidSpliceOutputScript(fundingParams.channelId, addOutput.serialId, addOutput.pubkeyScript))
     } else if (addOutput.pubkeyScript == fundingPubkeyScript) {
       Right(Output.Shared(addOutput.serialId, addOutput.pubkeyScript, purpose.previousLocalBalance + fundingParams.localContribution, purpose.previousRemoteBalance + fundingParams.remoteContribution, purpose.htlcBalance))

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

@@ -30,7 +30,7 @@ import fr.acinq.eclair.wire.protocol.UpdateAddHtlc
 import scodec.bits.ByteVector
 
 import java.nio.ByteOrder
-import scala.util.Try
+import scala.util.{Success, Try}
 
 /**
  * Created by PM on 15/12/2016.
@@ -271,6 +271,27 @@ object Transactions {
    */
   def fee2rate(fee: Satoshi, weight: Int): FeeratePerKw = FeeratePerKw((fee * 1000L) / weight)
 
+  /** As defined in https://github.com/lightning/bolts/blob/master/03-transactions.md#dust-limits */
+  def dustLimit(scriptPubKey: ByteVector): Satoshi = {
+    Try(Script.parse(scriptPubKey)) match {
+      case Success(OP_DUP :: OP_HASH160 :: OP_PUSHDATA(pubkeyHash, _) :: OP_EQUALVERIFY :: OP_CHECKSIG :: Nil) if pubkeyHash.size == 20 => 546.sat
+      case Success(OP_HASH160 :: OP_PUSHDATA(scriptHash, _) :: OP_EQUAL :: Nil) if scriptHash.size == 20 => 540.sat
+      case Success(OP_0 :: OP_PUSHDATA(pubkeyHash, _) :: Nil) if pubkeyHash.size == 20 => 294.sat
+      case Success(OP_0 :: OP_PUSHDATA(scriptHash, _) :: Nil) if scriptHash.size == 32 => 330.sat
+      case Success((OP_1 | OP_2 | OP_3 | OP_4 | OP_5 | OP_6 | OP_7 | OP_8 | OP_9 | OP_10 | OP_11 | OP_12 | OP_13 | OP_14 | OP_15 | OP_16) :: OP_PUSHDATA(program, _) :: Nil) if 2 <= program.length && program.length <= 40 => 354.sat
+      case Success(OP_RETURN :: _) => 0.sat // OP_RETURN is never dust
+      case _ => 546.sat
+    }
+  }
+
+  /** When an output is using OP_RETURN, we usually want to make sure its amount is 0, otherwise bitcoind won't accept it. */
+  def isOpReturn(scriptPubKey: ByteVector): Boolean = {
+    Try(Script.parse(scriptPubKey)) match {
+      case Success(OP_RETURN :: _) => true
+      case _ => false
+    }
+  }
+
   /** Offered HTLCs below this amount will be trimmed. */
   def offeredHtlcTrimThreshold(dustLimit: Satoshi, spec: CommitmentSpec, commitmentFormat: CommitmentFormat): Satoshi =
     dustLimit + weight2fee(spec.htlcTxFeerate(commitmentFormat), commitmentFormat.htlcTimeoutWeight)

eclair-core/src/test/scala/fr/acinq/eclair/channel/HelpersSpec.scala

@@ -225,6 +225,7 @@ class HelpersSpec extends TestKitBaseClass with AnyFunSuiteLike with ChannelStat
       TxOut(294 sat, OP_0 :: OP_PUSHDATA(hex"0000000000000000000000000000000000000000") :: Nil),
       TxOut(330 sat, OP_0 :: OP_PUSHDATA(hex"0000000000000000000000000000000000000000000000000000000000000000") :: Nil),
       TxOut(354 sat, OP_3 :: OP_PUSHDATA(hex"0000000000") :: Nil),
+      TxOut(0 sat, OP_RETURN :: OP_PUSHDATA(hex"deadbeef") :: Nil),
     )
 
     def toClosingTx(txOut: Seq[TxOut]): ClosingTx = {