Allow funder to dip into reserve if increased fee

See lightning/bolts#728

Allow a funder to dip into its channel reserve to pay the increased
commit tx fee for one incoming HTLC.
This prevents being stuck with an unusable channel.

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

@@ -190,6 +190,8 @@ object Commitments {
     } else if (missingForReceiver < 0.msat) {
       if (commitments.localParams.isFunder) {
         // receiver is fundee; it is ok if it can't maintain its channel_reserve for now, as long as its balance is increasing, which is the case if it is receiving a payment
+      } else if (missingForReceiver + commitments1.localParams.channelReserve > 0.msat && outgoingHtlcs.size <= 1) {
+        // receiver is funder; it is allowed to dip into its channel reserve to pay the fee for a single non-dust HTLC
       } else {
         return Left(RemoteCannotAffordFeesForNewHtlc(commitments.channelId, amount = cmd.amount, missing = -missingForReceiver.truncateToSatoshi, reserve = commitments1.remoteParams.channelReserve, fees = fees))
       }
@@ -230,10 +232,12 @@ object Commitments {
     if (missingForSender < 0.sat) {
       throw InsufficientFunds(commitments.channelId, amount = add.amountMsat, missing = -missingForSender.truncateToSatoshi, reserve = commitments1.localParams.channelReserve, fees = if (commitments1.localParams.isFunder) 0.sat else fees)
     } else if (missingForReceiver < 0.sat) {
-      if (commitments.localParams.isFunder) {
-        throw CannotAffordFees(commitments.channelId, missing = -missingForReceiver.truncateToSatoshi, reserve = commitments1.remoteParams.channelReserve, fees = fees)
-      } else {
+      if (!commitments.localParams.isFunder) {
         // receiver is fundee; it is ok if it can't maintain its channel_reserve for now, as long as its balance is increasing, which is the case if it is receiving a payment
+      } else if (missingForReceiver + commitments1.remoteParams.channelReserve > 0.msat && incomingHtlcs.size <= 1) {
+        // receiver is funder; it is allowed to dip into its channel reserve to pay the fee for a single non-dust HTLC
+      } else {
+        throw CannotAffordFees(commitments.channelId, missing = -missingForReceiver.truncateToSatoshi, reserve = commitments1.remoteParams.channelReserve, fees = fees)
       }
     }
 

eclair-core/src/test/scala/fr/acinq/eclair/channel/states/e/NormalStateSpec.scala

@@ -216,10 +216,21 @@ class NormalStateSpec extends TestkitBaseClass with StateTestsHelperMethods {
 
     // actual test begins
     // at this point alice has the minimal amount to sustain a channel (29000 sat ~= alice reserve + commit fee)
-    val add = CMD_ADD_HTLC(120000000 msat, randomBytes32, CltvExpiry(400144), TestConstants.emptyOnionPacket, Upstream.Local(UUID.randomUUID()))
+    // alice should be allowed to dip into her reserve to pay for the commit tx fee increase (see https://github.com/lightningnetwork/lightning-rfc/issues/728)
+    sender.send(bob, CMD_ADD_HTLC(85000000 msat, randomBytes32, CltvExpiry(400144), TestConstants.emptyOnionPacket, Upstream.Local(UUID.randomUUID())))
+    sender.expectMsg("ok")
+    val htlc = bob2alice.expectMsgType[UpdateAddHtlc]
+    bob2alice.forward(alice)
+
+    // but only one pending HTLC is allowed to dip into alice's reserve; bob must wait for that HTLC to settle before sending another one
+    val add = CMD_ADD_HTLC(80000000 msat, randomBytes32, CltvExpiry(400144), TestConstants.emptyOnionPacket, Upstream.Local(UUID.randomUUID()))
     sender.send(bob, add)
-    val error = RemoteCannotAffordFeesForNewHtlc(channelId(bob), add.amount, missing = 1680 sat, 10000 sat, 10680 sat)
+    val error = RemoteCannotAffordFeesForNewHtlc(channelId(bob), add.amount, missing = 3400 sat, 10000 sat, 12400 sat)
     sender.expectMsg(Failure(AddHtlcFailed(channelId(bob), add.paymentHash, error, Origin.Local(add.upstream.asInstanceOf[Upstream.Local].id, Some(sender.ref)), Some(bob.stateData.asInstanceOf[DATA_NORMAL].channelUpdate), Some(add))))
+    bob2alice.expectNoMsg(100 millis)
+
+    // alice should accept the first incoming htlc
+    awaitCond(alice.stateData.asInstanceOf[HasCommitments].commitments.remoteChanges.proposed.contains(htlc))
   }
 
   test("recv CMD_ADD_HTLC (insufficient funds w/ pending htlcs and 0 balance)") { f =>