Take min feerate into account for recommended fees (#2918)

Independently of target and tolerance ratios, transactions must be
publishable in our local mempool. We must be careful that fee ranges
stay above this min fee level.

In theory we shouldn't need to care about the upper range, but I'm not
sure that, depending on the fee estimator, the min fee is always lower
than arbitrary block targets.

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

@@ -115,21 +115,23 @@ case class NodeParams(nodeKeyManager: NodeKeyManager,
 
   /** Returns the feerates we'd like our peer to use when funding channels. */
   def recommendedFeerates(remoteNodeId: PublicKey, localFeatures: Features[InitFeature], remoteFeatures: Features[InitFeature]): RecommendedFeerates = {
+    // Independently of target and tolerance ratios, our transactions must be publishable in our local mempool
+    val minimumFeerate = currentBitcoinCoreFeerates.minimum
     val feerateTolerance = onChainFeeConf.feerateToleranceFor(remoteNodeId)
     val fundingFeerate = onChainFeeConf.getFundingFeerate(currentBitcoinCoreFeerates)
     val fundingRange = RecommendedFeeratesTlv.FundingFeerateRange(
-      min = fundingFeerate * feerateTolerance.ratioLow,
-      max = fundingFeerate * feerateTolerance.ratioHigh,
+      min = (fundingFeerate * feerateTolerance.ratioLow).max(minimumFeerate),
+      max = (fundingFeerate * feerateTolerance.ratioHigh).max(minimumFeerate),
     )
     // We use the most likely commitment format, even though there is no guarantee that this is the one that will be used.
     val commitmentFormat = ChannelTypes.defaultFromFeatures(localFeatures, remoteFeatures, announceChannel = false).commitmentFormat
     val commitmentFeerate = onChainFeeConf.getCommitmentFeerate(currentBitcoinCoreFeerates, remoteNodeId, commitmentFormat, channelConf.minFundingPrivateSatoshis)
     val commitmentRange = RecommendedFeeratesTlv.CommitmentFeerateRange(
-      min = commitmentFeerate * feerateTolerance.ratioLow,
-      max = commitmentFormat match {
+      min = (commitmentFeerate * feerateTolerance.ratioLow).max(minimumFeerate),
+      max = (commitmentFormat match {
         case Transactions.DefaultCommitmentFormat => commitmentFeerate * feerateTolerance.ratioHigh
         case _: Transactions.AnchorOutputsCommitmentFormat => (commitmentFeerate * feerateTolerance.ratioHigh).max(feerateTolerance.anchorOutputMaxCommitFeerate)
-      },
+      }).max(minimumFeerate),
     )
     RecommendedFeerates(chainHash, fundingFeerate, commitmentFeerate, TlvStream(fundingRange, commitmentRange))
   }