Lower minimum remote dust limit

We are slowly dropping support for non-segwit outputs, as proposed in
lightning/bolts#894

We can thus safely allow dust limits all the way down to 354 satoshis.

eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala

@@ -233,7 +233,7 @@ object NodeParams extends Logging {
 
     val dustLimitSatoshis = Satoshi(config.getLong("dust-limit-satoshis"))
     if (chainHash == Block.LivenetGenesisBlock.hash) {
-      require(dustLimitSatoshis >= Channel.MIN_DUSTLIMIT, s"dust limit must be greater than ${Channel.MIN_DUSTLIMIT}")
+      require(dustLimitSatoshis >= Channel.MIN_DUST_LIMIT, s"dust limit must be greater than ${Channel.MIN_DUST_LIMIT}")
     }
 
     val htlcMinimum = MilliSatoshi(config.getInt("htlc-minimum-msat"))

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

@@ -78,8 +78,12 @@ object Channel {
   val MAX_FUNDING: Satoshi = 16777216 sat // = 2^24
   val MAX_ACCEPTED_HTLCS = 483
 
-  // we don't want the counterparty to use a dust limit lower than that, because they wouldn't only hurt themselves we may need them to publish their commit tx in certain cases (backup/restore)
-  val MIN_DUSTLIMIT: Satoshi = 546 sat
+  // We may need to rely on our peer's commit tx in certain cases (backup/restore) so we must ensure their transactions
+  // can propagate through the bitcoin network (assuming bitcoin core nodes with default policies).
+  // The various dust limits enforced by the bitcoin network are summarized here:
+  // https://github.com/lightningnetwork/lightning-rfc/blob/master/03-transactions.md#dust-limits
+  // A dust limit of 354 sat ensures all segwit outputs will relay with default relay policies.
+  val MIN_DUST_LIMIT: Satoshi = 354 sat
 
   // we won't exchange more than this many signatures when negotiating the closing fee
   val MAX_NEGOTIATION_ITERATIONS = 20

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

@@ -138,7 +138,7 @@ object Helpers {
     if (nodeParams.onChainFeeConf.feerateToleranceFor(remoteNodeId).isFeeDiffTooHigh(channelType, localFeeratePerKw, open.feeratePerKw)) return Left(FeerateTooDifferent(open.temporaryChannelId, localFeeratePerKw, open.feeratePerKw))
     // only enforce dust limit check on mainnet
     if (nodeParams.chainHash == Block.LivenetGenesisBlock.hash) {
-      if (open.dustLimitSatoshis < Channel.MIN_DUSTLIMIT) return Left(DustLimitTooSmall(open.temporaryChannelId, open.dustLimitSatoshis, Channel.MIN_DUSTLIMIT))
+      if (open.dustLimitSatoshis < Channel.MIN_DUST_LIMIT) return Left(DustLimitTooSmall(open.temporaryChannelId, open.dustLimitSatoshis, Channel.MIN_DUST_LIMIT))
     }
 
     // we don't check that the funder's amount for the initial commitment transaction is sufficient for full fee payment
@@ -169,7 +169,7 @@ object Helpers {
     if (accept.maxAcceptedHtlcs > Channel.MAX_ACCEPTED_HTLCS) return Left(InvalidMaxAcceptedHtlcs(accept.temporaryChannelId, accept.maxAcceptedHtlcs, Channel.MAX_ACCEPTED_HTLCS))
     // only enforce dust limit check on mainnet
     if (nodeParams.chainHash == Block.LivenetGenesisBlock.hash) {
-      if (accept.dustLimitSatoshis < Channel.MIN_DUSTLIMIT) return Left(DustLimitTooSmall(accept.temporaryChannelId, accept.dustLimitSatoshis, Channel.MIN_DUSTLIMIT))
+      if (accept.dustLimitSatoshis < Channel.MIN_DUST_LIMIT) return Left(DustLimitTooSmall(accept.temporaryChannelId, accept.dustLimitSatoshis, Channel.MIN_DUST_LIMIT))
     }
 
     if (accept.dustLimitSatoshis > nodeParams.maxRemoteDustLimit) return Left(DustLimitTooLarge(open.temporaryChannelId, accept.dustLimitSatoshis, nodeParams.maxRemoteDustLimit))

eclair-core/src/test/scala/fr/acinq/eclair/channel/states/a/WaitForAcceptChannelStateSpec.scala

@@ -184,11 +184,11 @@ class WaitForAcceptChannelStateSpec extends TestKitBaseClass with FixtureAnyFunS
   test("recv AcceptChannel (dust limit too low)", Tag("mainnet")) { f =>
     import f._
     val accept = bob2alice.expectMsgType[AcceptChannel]
-    // we don't want their dust limit to be below 546
-    val lowDustLimitSatoshis = 545.sat
+    // we don't want their dust limit to be below 354
+    val lowDustLimitSatoshis = 353.sat
     alice ! accept.copy(dustLimitSatoshis = lowDustLimitSatoshis)
     val error = alice2bob.expectMsgType[Error]
-    assert(error === Error(accept.temporaryChannelId, DustLimitTooSmall(accept.temporaryChannelId, lowDustLimitSatoshis, Channel.MIN_DUSTLIMIT).getMessage))
+    assert(error === Error(accept.temporaryChannelId, DustLimitTooSmall(accept.temporaryChannelId, lowDustLimitSatoshis, Channel.MIN_DUST_LIMIT).getMessage))
     awaitCond(alice.stateName == CLOSED)
   }