Avoid negative on-the-fly funding fee (#3189)

Since we don't have access to channel params in the `Peer` actor, we
don't know the remote `htlc_minimum` when receiving a splice. This may
lead to cases where we later fail because we end up with a negative
funding fee, which doesn't make any sense.

To avoid those failures, we hard-code the `htlc_minimum` value used by
Phoenix (which is the only consumer of this protocol so far) and use
the max with our local `htlc_minimum`.

Author: t-bast

eclair-core/src/main/scala/fr/acinq/eclair/io/Peer.scala

@@ -467,10 +467,11 @@ class Peer(val nodeParams: NodeParams,
       case Event(msg: SpliceInit, d: ConnectedData) =>
         d.channels.get(FinalChannelId(msg.channelId)) match {
           case Some(_) if msg.usesOnTheFlyFunding && !d.fundingFeerateOk(msg.feerate) =>
-            log.info("rejecting open_channel2: feerate too low ({} < {})", msg.feerate, d.currentFeerates.fundingFeerate)
+            log.info("rejecting splice_init: feerate too low ({} < {})", msg.feerate, d.currentFeerates.fundingFeerate)
             self ! Peer.OutgoingMessage(TxAbort(msg.channelId, FundingFeerateTooLow(msg.channelId, msg.feerate, d.currentFeerates.fundingFeerate).getMessage), d.peerConnection)
           case Some(channel) =>
-            OnTheFlyFunding.validateSplice(nodeParams.onTheFlyFundingConfig, msg, nodeParams.channelConf.htlcMinimum, pendingOnTheFlyFunding, feeCredit.getOrElse(0 msat)) match {
+            // We don't have access to the remote htlc_minimum here, so we hard-code the value Phoenix uses (1000 msat).
+            OnTheFlyFunding.validateSplice(nodeParams.onTheFlyFundingConfig, msg, nodeParams.channelConf.htlcMinimum.max(1000 msat), pendingOnTheFlyFunding, feeCredit.getOrElse(0 msat)) match {
               case reject: OnTheFlyFunding.ValidationResult.Reject =>
                 log.warning("rejecting on-the-fly splice: {}", reject.cancel.toAscii)
                 self ! Peer.OutgoingMessage(reject.cancel, d.peerConnection)

eclair-core/src/main/scala/fr/acinq/eclair/payment/relay/OnTheFlyFunding.scala

@@ -91,7 +91,7 @@ object OnTheFlyFunding {
   /** An on-the-fly funding proposal sent to our peer. */
   case class Proposal(htlc: WillAddHtlc, upstream: Upstream.Hot, onionSharedSecrets: Seq[Sphinx.SharedSecret]) {
     /** Maximum fees that can be collected from this HTLC. */
-    def maxFees(htlcMinimum: MilliSatoshi): MilliSatoshi = htlc.amount - htlcMinimum
+    def maxFees(htlcMinimum: MilliSatoshi): MilliSatoshi = (htlc.amount - htlcMinimum).max(0 msat)
 
     /** Create commands to fail all upstream HTLCs. */
     def createFailureCommands(failure_opt: Option[FailureReason])(implicit log: LoggingAdapter): Seq[(ByteVector32, CMD_FAIL_HTLC)] = upstream match {