Apply 0-reserve to both peers

We previously only applied 0-reserve to the non-initiator when the feature
bit was active. We should apply it to both sides of the channel.

This reduces the liquidity requirements on the initiator side, and allows
receiving more funds over lightning before requiring a splice to get more
inbound liquidity.

src/commonMain/kotlin/fr/acinq/lightning/channel/Commitments.kt

@@ -146,12 +146,12 @@ data class Commitment(
     val fundingAmount: Satoshi = commitInput.txOut.amount
 
     fun localChannelReserve(params: ChannelParams): Satoshi = when {
-        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) && !params.localParams.isInitiator -> 0.sat
+        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) -> 0.sat
         else -> (fundingAmount / 100).max(params.remoteParams.dustLimit)
     }
 
     fun remoteChannelReserve(params: ChannelParams): Satoshi = when {
-        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) && params.localParams.isInitiator -> 0.sat
+        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) -> 0.sat
         else -> (fundingAmount / 100).max(params.localParams.dustLimit)
     }
 
@@ -390,14 +390,7 @@ data class Commitment(
         // and it would be tricky to check if the conditions are met at signing
         // (it also means that we need to check the fee of the initial commitment tx somewhere)
         val fees = commitTxFee(params.localParams.dustLimit, reduced)
-        // TODO:
-        //  When migrating to the dual-funded model, we removed the explicit channel reserve from LocalParams.
-        //  For channels that were created before the splicing update, this can result in a mismatch where we think
-        //  the channel reserve is bigger than what is actually is, and incorrectly reject the remote update_fee.
-        //  We temporarily ignore the channel reserve to avoid unnecessary force-close.
-        //  We should restore the correct calculation that takes the reserve into account once all users have migrated.
-        // val missing = reduced.toRemote.truncateToSatoshi() - remoteChannelReserve(params) - fees
-        val missing = reduced.toRemote.truncateToSatoshi() - fees
+        val missing = reduced.toRemote.truncateToSatoshi() - remoteChannelReserve(params) - fees
         return if (missing < 0.sat) {
             Either.Left(CannotAffordFees(params.channelId, -missing, remoteChannelReserve(params), fees))
         } else {
@@ -515,11 +508,11 @@ data class FullCommitment(
     val fundingTxId: ByteVector32 = commitInput.outPoint.txid
     val fundingAmount = commitInput.txOut.amount
     val localChannelReserve = when {
-        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) && !params.localParams.isInitiator -> 0.sat
+        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) -> 0.sat
         else -> (fundingAmount / 100).max(params.remoteParams.dustLimit)
     }
     val remoteChannelReserve = when {
-        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) && params.localParams.isInitiator -> 0.sat
+        params.channelFeatures.hasFeature(Feature.ZeroReserveChannels) -> 0.sat
         else -> (fundingAmount / 100).max(params.localParams.dustLimit)
     }
 }

src/commonTest/kotlin/fr/acinq/lightning/channel/states/NormalTestsCommon.kt

@@ -429,6 +429,21 @@ class NormalTestsCommon : LightningTestSuite() {
         assertEquals(alice1.commitments.changes.remoteChanges.proposed, listOf(add))
     }
 
+    @Test
+    fun `recv UpdateAddHtlc -- zero-conf -- zero-reserve -- initiator`() {
+        val (alice0, bob0) = reachNormal(ChannelType.SupportedChannelType.AnchorOutputsZeroReserve, aliceFundingAmount = 10_000_000.sat, bobFundingAmount = 5_000_000.sat, alicePushAmount = 9_800_000_000.msat, zeroConf = true)
+        assertEquals(alice0.commitments.latest.fundingAmount, 15_000_000.sat)
+        assertEquals(alice0.commitments.latest.localCommit.spec.toLocal, 200_000_000.msat)
+        // If we applied the default 1% reserve, Alice's balance couldn't go below 150 000 sats.
+        // Alice still needs to pay the commit tx fees, so she cannot spend her entire balance.
+        val (nodes1, r, htlc) = addHtlc(160_000_000.msat, alice0, bob0)
+        val (alice2, bob2) = crossSign(nodes1.first, nodes1.second)
+        val (alice3, bob3) = fulfillHtlc(htlc.id, r, alice2, bob2)
+        val (bob4, alice4) = crossSign(bob3, alice3)
+        assertEquals(alice4.commitments.latest.localCommit.spec.toLocal, 40_000_000.msat)
+        assertEquals(bob4.commitments.latest.localCommit.spec.toRemote, 40_000_000.msat)
+    }
+
     @Test
     fun `recv UpdateAddHtlc -- unexpected id`() {
         val (_, bob0) = reachNormal()
@@ -1419,8 +1434,7 @@ class NormalTestsCommon : LightningTestSuite() {
         assertEquals(bob.commitments.copy(changes = bob.commitments.changes.copy(remoteChanges = bob.commitments.changes.remoteChanges.copy(proposed = bob.commitments.changes.remoteChanges.proposed + fee2))), bob2.commitments)
     }
 
-    // TODO: restore this test once we take the reserve into account (see canReceiveFee)
-    @Ignore
+    @Test
     fun `recv UpdateFee -- sender cannot afford it`() {
         val (alice, bob) = reachNormal()
         // We put all the balance on Bob's side, so that Alice cannot afford a feerate increase.