Add non_initiator_pays_commit_fees channel flag

We add a non-standard channel flag to `open_channel2` to allow wallets
to ask their peer to pay the commit tx fees, even when they're not the
channel opener. This is necessary for on-the-fly funding, until we can
move to 0-fee commit txs which will make it obsolete.

Author: t-bast

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

@@ -666,8 +666,16 @@ case class RemoteParams(nodeId: PublicKey,
                         initFeatures: Features[InitFeature],
                         upfrontShutdownScript_opt: Option[ByteVector])
 
-case class ChannelFlags(announceChannel: Boolean) {
-  override def toString: String = s"ChannelFlags(announceChannel=$announceChannel)"
+/**
+ * The [[nonInitiatorPaysCommitFees]] parameter is set to true when the sender wants the receiver to pay the commitment transaction fees.
+ * This is not part of the BOLTs and won't be needed anymore once commitment transactions don't pay any on-chain fees.
+ */
+case class ChannelFlags(nonInitiatorPaysCommitFees: Boolean, announceChannel: Boolean) {
+  override def toString: String = s"ChannelFlags(announceChannel=$announceChannel, nonInitiatorPaysCommitFees=$nonInitiatorPaysCommitFees)"
+}
+
+object ChannelFlags {
+  def apply(announceChannel: Boolean): ChannelFlags = ChannelFlags(nonInitiatorPaysCommitFees = false, announceChannel = announceChannel)
 }
 
 /** Information about what triggered the opening of the channel */

eclair-core/src/main/scala/fr/acinq/eclair/wire/protocol/CommonCodecs.scala

@@ -119,7 +119,7 @@ object CommonCodecs {
 
   val listofsignatures: Codec[List[ByteVector64]] = listOfN(uint16, bytes64)
 
-  val channelflags: Codec[ChannelFlags] = (ignore(7) :: bool).as[ChannelFlags]
+  val channelflags: Codec[ChannelFlags] = (ignore(6) :: bool :: bool).as[ChannelFlags]
 
   val ipv4address: Codec[Inet4Address] = bytes(4).xmap(b => InetAddress.getByAddress(b.toArray).asInstanceOf[Inet4Address], a => ByteVector(a.getAddress))
 

eclair-core/src/test/resources/nonreg/codecs/000003-DATA_NORMAL/fundee-announced/data.json

@@ -47,6 +47,7 @@
         }
       },
       "channelFlags" : {
+        "nonInitiatorPaysCommitFees" : false,
         "announceChannel" : true
       }
     },

eclair-core/src/test/resources/nonreg/codecs/000003-DATA_NORMAL/funder/data.json

@@ -47,6 +47,7 @@
         }
       },
       "channelFlags" : {
+        "nonInitiatorPaysCommitFees" : false,
         "announceChannel" : true
       }
     },

eclair-core/src/test/resources/nonreg/codecs/020002-DATA_NORMAL/funder-announced/data.json

@@ -61,6 +61,7 @@
         }
       },
       "channelFlags" : {
+        "nonInitiatorPaysCommitFees" : false,
         "announceChannel" : true
       }
     },

eclair-core/src/test/resources/nonreg/codecs/030000-DATA_WAIT_FOR_FUNDING_CONFIRMED/funder/data.json

@@ -54,6 +54,7 @@
         }
       },
       "channelFlags" : {
+        "nonInitiatorPaysCommitFees" : false,
         "announceChannel" : false
       }
     },

eclair-core/src/test/resources/nonreg/codecs/03000a-DATA_WAIT_FOR_CHANNEL_READY/funder/data.json

@@ -56,6 +56,7 @@
         }
       },
       "channelFlags" : {
+        "nonInitiatorPaysCommitFees" : false,
         "announceChannel" : false
       }
     },

eclair-core/src/test/resources/nonreg/codecs/03000c-DATA_WAIT_FOR_DUAL_FUNDING_READY/funder/data.json

@@ -62,6 +62,7 @@
         }
       },
       "channelFlags" : {
+        "nonInitiatorPaysCommitFees" : false,
         "announceChannel" : false
       }
     },

eclair-core/src/test/scala/fr/acinq/eclair/json/JsonSerializersSpec.scala

@@ -185,6 +185,7 @@ class JsonSerializersSpec extends TestKitBaseClass with AnyFunSuiteLike with Mat
         |         "initFeatures": { "activated": {}, "unknown": [] }
         |       },
         |       "channelFlags": {
+        |         "nonInitiatorPaysCommitFees": false,
         |         "announceChannel": true
         |       }
         |     },

eclair-core/src/test/scala/fr/acinq/eclair/wire/protocol/CommonCodecsSpec.scala

@@ -140,17 +140,20 @@ class CommonCodecsSpec extends AnyFunSuite {
 
   test("encode/decode channel flags") {
     val testCases = Map(
-      bin"00000000" -> ChannelFlags(announceChannel = false),
-      bin"00000001" -> ChannelFlags(announceChannel = true),
+      bin"00000000" -> ChannelFlags(nonInitiatorPaysCommitFees = false, announceChannel = false),
+      bin"00000001" -> ChannelFlags(nonInitiatorPaysCommitFees = false, announceChannel = true),
+      bin"00000010" -> ChannelFlags(nonInitiatorPaysCommitFees = true, announceChannel = false),
+      bin"00000011" -> ChannelFlags(nonInitiatorPaysCommitFees = true, announceChannel = true),
     )
     testCases.foreach { case (bin, obj) =>
       assert(channelflags.decode(bin).require == DecodeResult(obj, BitVector.empty))
       assert(channelflags.encode(obj).require == bin)
     }
 
     // BOLT 2: The receiving node MUST [...] ignore undefined bits in channel_flags.
-    assert(channelflags.decode(bin"11111111").require == DecodeResult(ChannelFlags(announceChannel = true), BitVector.empty))
-    assert(channelflags.decode(bin"11111110").require == DecodeResult(ChannelFlags(announceChannel = false), BitVector.empty))
+    assert(channelflags.decode(bin"11111111").require == DecodeResult(ChannelFlags(nonInitiatorPaysCommitFees = true, announceChannel = true), BitVector.empty))
+    assert(channelflags.decode(bin"11111110").require == DecodeResult(ChannelFlags(nonInitiatorPaysCommitFees = true, announceChannel = false), BitVector.empty))
+    assert(channelflags.decode(bin"11111100").require == DecodeResult(ChannelFlags(nonInitiatorPaysCommitFees = false, announceChannel = false), BitVector.empty))
   }
 
   test("encode/decode with rgb codec") {