Remove reserve check if splice contribution is positive (#2771)

If remote has a positive contribution, we do not check their
post-splice reserve level, because they are improving their
situation, even if they stay below the requirement. Note that if
local splices-in some funds in the same operation, remote
post-splice reserve may actually be worse than before, but that's
not their fault.

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

@@ -2690,7 +2690,7 @@ class Channel(val nodeParams: NodeParams, val wallet: OnChainChannelFunder with
         spliceOut = cmd.spliceOutputs,
         targetFeerate = targetFeerate)
       val commitTxFees = Transactions.commitTxTotalCost(d.commitments.params.remoteParams.dustLimit, parentCommitment.remoteCommit.spec, d.commitments.params.commitmentFormat)
-      if (parentCommitment.localCommit.spec.toLocal + fundingContribution < parentCommitment.localChannelReserve(d.commitments.params).max(commitTxFees)) {
+      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))) {

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

@@ -660,11 +660,17 @@ private class InteractiveTxBuilder(replyTo: ActorRef[InteractiveTxBuilder.Respon
     }
 
     val sharedInput_opt = fundingParams.sharedInput_opt.map(_ => {
-      val remoteReserve = channelParams.remoteChannelReserveForCapacity(fundingParams.fundingAmount, isSplice = true)
-      // We ignore the reserve requirement if we are splicing funds into the channel, which increases the size of the reserve.
-      if (sharedOutput.remoteAmount < remoteReserve && remoteOutputs.nonEmpty && localInputs.isEmpty) {
-        log.warn("invalid interactive tx: peer takes too much funds out and falls below the channel reserve ({} < {})", sharedOutput.remoteAmount, remoteReserve)
-        return Left(InvalidCompleteInteractiveTx(fundingParams.channelId))
+      if (fundingParams.remoteContribution >= 0.sat) {
+        // If remote has a positive contribution, we do not check their post-splice reserve level, because they are improving
+        // their situation, even if they stay below the requirement. Note that if local splices-in some funds in the same
+        // operation, remote post-splice reserve may actually be worse than before, but that's not their fault.
+      } else {
+        // If remote removes funds from the channel, it must meet reserve requirements post-splice
+        val remoteReserve = channelParams.remoteChannelReserveForCapacity(fundingParams.fundingAmount, isSplice = true)
+        if (sharedOutput.remoteAmount < remoteReserve) {
+          log.warn("invalid interactive tx: peer takes too much funds out and falls below the channel reserve ({} < {})", sharedOutput.remoteAmount, remoteReserve)
+          return Left(InvalidCompleteInteractiveTx(fundingParams.channelId))
+        }
       }
       if (sharedInputs.length > 1) {
         log.warn("invalid interactive tx: shared input included multiple times")