Add support for RBF-ing splice transactions

If the latest splice transaction doesn't confirm, we allow exchanging
`tx_init_rbf` and `tx_ack_rbf` to create another splice transaction to
replace it. We use the same funding contribution as the previous splice.

We disallow creating another splice transaction using `splice_init` if
we have several RBF attempts for the latest splice: we cannot know which
one of them will confirm and should be spent by the new splice.

Disallow chains of unconfirmed splice transactions

When 0-conf isn't used, we reject `splice_init` while the previous
splice transaction hasn't confirmed. Our peer should either use RBF
instead of creating a new splice, or they should wait for our node
to receive the block that confirmed the previous transaction. This
protects against chains of unconfirmed transactions.

Author: t-bast

docs/release-notes/eclair-vnext.md

@@ -13,6 +13,39 @@ Every node advertizes the rates at which they sell their liquidity, and buyers c
 The liquidity ads specification is still under review and will likely change.
 This feature isn't meant to be used on mainnet yet and is thus disabled by default.
 
+### Channel Splicing
+
+With this release, we add support for the final version of [splicing](https://github.com/lightning/bolts/pull/1160) that was recently added to the BOLTs.
+Splicing allows node operators to change the size of their existing channels, which makes it easier and more efficient to allocate liquidity where it is most needed.
+Most node operators can now have a single channel with each of their peer, which costs less on-chain fees and resources, and makes path-finding easier.
+
+The size of an existing channel can be increased with the `splicein` API:
+
+```sh
+eclair-cli splicein --channelId=<channel_id> --amountIn=<amount_satoshis>
+```
+
+Once that transaction confirms, the additional liquidity can be used to send outgoing payments.
+If the transaction doesn't confirm, the node operator can speed up confirmation with the `rbfsplice` API:
+
+```sh
+eclair-cli rbfsplice --channelId=<channel_id> --targetFeerateSatByte=<feerate_satoshis_per_byte> --fundingFeeBudgetSatoshis=<maximum_on_chain_fee_satoshis>
+```
+
+If the node operator wants to reduce the size of a channel, or send some of the channel funds to an on-chain address, they can use the `spliceout` API:
+
+```sh
+eclair-cli spliceout --channelId=<channel_id> --amountOut=<amount_satoshis> --scriptPubKey=<on_chain_address>
+```
+
+That operation can also be RBF-ed with the `rbfsplice` API to speed up confirmation if necessary.
+
+Note that when 0-conf is used for the channel, it is not possible to RBF splice transactions.
+Node operators should instead create a new splice transaction (with `splicein` or `spliceout`) to CPFP the previous transaction.
+
+Note that eclair had already introduced support for a splicing prototype in v0.9.0, which helped improve the BOLT proposal.
+We're removing support for the previous splicing prototype feature: users that depended on this protocol must upgrade to create official splice transactions.
+
 ### Update minimal version of Bitcoin Core
 
 With this release, eclair requires using Bitcoin Core 27.1.
@@ -38,6 +71,7 @@ Eclair will not allow remote peers to open new obsolete channels that do not sup
 - `channelstats` now takes optional parameters `--count` and `--skip` to control pagination. By default, it will return first 10 entries. (#2890)
 - `createinvoice` now takes an optional `--privateChannelIds` parameter that can be used to add routing hints through private channels. (#2909)
 - `nodes` allows filtering nodes that offer liquidity ads (#2848)
+- `rbfsplice` lets any channel participant RBF the current unconfirmed splice transaction (#2887)
 
 ### Miscellaneous improvements and bug fixes
 

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

@@ -94,6 +94,8 @@ trait Eclair {
 
   def spliceOut(channelId: ByteVector32, amountOut: Satoshi, scriptOrAddress: Either[ByteVector, String])(implicit timeout: Timeout): Future[CommandResponse[CMD_SPLICE]]
 
+  def rbfSplice(channelId: ByteVector32, targetFeerate: FeeratePerKw, fundingFeeBudget: Satoshi, lockTime_opt: Option[Long])(implicit timeout: Timeout): Future[CommandResponse[CMD_BUMP_FUNDING_FEE]]
+
   def close(channels: List[ApiTypes.ChannelIdentifier], scriptPubKey_opt: Option[ByteVector], closingFeerates_opt: Option[ClosingFeerates])(implicit timeout: Timeout): Future[Map[ApiTypes.ChannelIdentifier, Either[Throwable, CommandResponse[CMD_CLOSE]]]]
 
   def forceClose(channels: List[ApiTypes.ChannelIdentifier])(implicit timeout: Timeout): Future[Map[ApiTypes.ChannelIdentifier, Either[Throwable, CommandResponse[CMD_FORCECLOSE]]]]
@@ -228,17 +230,18 @@ class EclairImpl(appKit: Kit) extends Eclair with Logging {
   }
 
   override def rbfOpen(channelId: ByteVector32, targetFeerate: FeeratePerKw, fundingFeeBudget: Satoshi, lockTime_opt: Option[Long])(implicit timeout: Timeout): Future[CommandResponse[CMD_BUMP_FUNDING_FEE]] = {
-    sendToChannelTyped(channel = Left(channelId),
-      cmdBuilder = CMD_BUMP_FUNDING_FEE(_, targetFeerate, fundingFeeBudget, lockTime_opt.getOrElse(appKit.nodeParams.currentBlockHeight.toLong), requestFunding_opt = None))
+    sendToChannelTyped(
+      channel = Left(channelId),
+      cmdBuilder = CMD_BUMP_FUNDING_FEE(_, targetFeerate, fundingFeeBudget, lockTime_opt.getOrElse(appKit.nodeParams.currentBlockHeight.toLong), requestFunding_opt = None)
+    )
   }
 
   override def spliceIn(channelId: ByteVector32, amountIn: Satoshi, pushAmount_opt: Option[MilliSatoshi])(implicit timeout: Timeout): Future[CommandResponse[CMD_SPLICE]] = {
-    sendToChannelTyped(channel = Left(channelId),
-      cmdBuilder = CMD_SPLICE(_,
-        spliceIn_opt = Some(SpliceIn(additionalLocalFunding = amountIn, pushAmount = pushAmount_opt.getOrElse(0.msat))),
-        spliceOut_opt = None,
-        requestFunding_opt = None,
-      ))
+    val spliceIn = SpliceIn(additionalLocalFunding = amountIn, pushAmount = pushAmount_opt.getOrElse(0.msat))
+    sendToChannelTyped(
+      channel = Left(channelId),
+      cmdBuilder = CMD_SPLICE(_, spliceIn_opt = Some(spliceIn), spliceOut_opt = None, requestFunding_opt = None)
+    )
   }
 
   override def spliceOut(channelId: ByteVector32, amountOut: Satoshi, scriptOrAddress: Either[ByteVector, String])(implicit timeout: Timeout): Future[CommandResponse[CMD_SPLICE]] = {
@@ -249,12 +252,18 @@ class EclairImpl(appKit: Kit) extends Eclair with Logging {
         case Right(script) => Script.write(script)
       }
     }
-    sendToChannelTyped(channel = Left(channelId),
-      cmdBuilder = CMD_SPLICE(_,
-        spliceIn_opt = None,
-        spliceOut_opt = Some(SpliceOut(amount = amountOut, scriptPubKey = script)),
-        requestFunding_opt = None,
-      ))
+    val spliceOut = SpliceOut(amount = amountOut, scriptPubKey = script)
+    sendToChannelTyped(
+      channel = Left(channelId),
+      cmdBuilder = CMD_SPLICE(_, spliceIn_opt = None, spliceOut_opt = Some(spliceOut), requestFunding_opt = None)
+    )
+  }
+
+  override def rbfSplice(channelId: ByteVector32, targetFeerate: FeeratePerKw, fundingFeeBudget: Satoshi, lockTime_opt: Option[Long])(implicit timeout: Timeout): Future[CommandResponse[CMD_BUMP_FUNDING_FEE]] = {
+    sendToChannelTyped(
+      channel = Left(channelId),
+      cmdBuilder = CMD_BUMP_FUNDING_FEE(_, targetFeerate, fundingFeeBudget, lockTime_opt.getOrElse(appKit.nodeParams.currentBlockHeight.toLong), requestFunding_opt = None)
+    )
   }
 
   override def close(channels: List[ApiTypes.ChannelIdentifier], scriptPubKey_opt: Option[ByteVector], closingFeerates_opt: Option[ClosingFeerates])(implicit timeout: Timeout): Future[Map[ApiTypes.ChannelIdentifier, Either[Throwable, CommandResponse[CMD_CLOSE]]]] = {
@@ -575,9 +584,9 @@ class EclairImpl(appKit: Kit) extends Eclair with Logging {
     case Left(channelId) => appKit.register ? Register.Forward(null, channelId, request)
     case Right(shortChannelId) => appKit.register ? Register.ForwardShortId(null, shortChannelId, request)
   }).map {
-    case t: R@unchecked => t
-    case t: Register.ForwardFailure[C]@unchecked => throw ChannelNotFound(Left(t.fwd.channelId))
-    case t: Register.ForwardShortIdFailure[C]@unchecked => throw ChannelNotFound(Right(t.fwd.shortChannelId))
+    case t: R @unchecked => t
+    case t: Register.ForwardFailure[C] @unchecked => throw ChannelNotFound(Left(t.fwd.channelId))
+    case t: Register.ForwardShortIdFailure[C] @unchecked => throw ChannelNotFound(Right(t.fwd.shortChannelId))
   }
 
   private def sendToChannelTyped[C <: Command, R <: CommandResponse[C]](channel: ApiTypes.ChannelIdentifier, cmdBuilder: akka.actor.typed.ActorRef[Any] => C)(implicit timeout: Timeout): Future[R] =
@@ -588,9 +597,9 @@ class EclairImpl(appKit: Kit) extends Eclair with Logging {
         case Right(shortChannelId) => Register.ForwardShortId(replyTo, shortChannelId, cmd)
       }
     }.map {
-      case t: R@unchecked => t
-      case t: Register.ForwardFailure[C]@unchecked => throw ChannelNotFound(Left(t.fwd.channelId))
-      case t: Register.ForwardShortIdFailure[C]@unchecked => throw ChannelNotFound(Right(t.fwd.shortChannelId))
+      case t: R @unchecked => t
+      case t: Register.ForwardFailure[C] @unchecked => throw ChannelNotFound(Left(t.fwd.channelId))
+      case t: Register.ForwardShortIdFailure[C] @unchecked => throw ChannelNotFound(Right(t.fwd.shortChannelId))
     }
 
   /**