Select `channel_type` for automatic channel creation (#3250)

diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/channel/ChannelFeatures.scala b/eclair-core/src/main/scala/fr/acinq/eclair/channel/ChannelFeatures.scala
index e557cee..04fda87 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/channel/ChannelFeatures.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/channel/ChannelFeatures.scala
@@ -116,7 +116,6 @@ object ChannelTypes {
     override def commitmentFormat: CommitmentFormat = PhoenixSimpleTaprootChannelCommitmentFormat
     override def toString: String = "phoenix_simple_taproot_channel"
   }
-
   // @formatter:on
 
   private val features2ChannelType: Map[Features[_ <: InitFeature], SupportedChannelType] = Set(
@@ -149,4 +148,13 @@ object ChannelTypes {
     case Some(proposedChannelType: SupportedChannelType) => Left(InvalidChannelType(channelId, proposedChannelType))
   }
 
+  /** Returns our preferred channel type for public channels, if supported by our peer. */
+  def preferredForPublicChannels(localFeatures: Features[InitFeature], remoteFeatures: Features[InitFeature]): Option[SupportedChannelType] = {
+    if (Features.canUseFeature(localFeatures, remoteFeatures, Features.AnchorOutputsZeroFeeHtlcTx)) {
+      Some(AnchorOutputsZeroFeeHtlcTx(scidAlias = Features.canUseFeature(localFeatures, remoteFeatures, Features.ScidAlias)))
+    } else {
+      None
+    }
+  }
+
 }
diff --git a/eclair-core/src/main/scala/fr/acinq/eclair/io/OpenChannelInterceptor.scala b/eclair-core/src/main/scala/fr/acinq/eclair/io/OpenChannelInterceptor.scala
index 8c50e39..fa0fd9b 100644
--- a/eclair-core/src/main/scala/fr/acinq/eclair/io/OpenChannelInterceptor.scala
+++ b/eclair-core/src/main/scala/fr/acinq/eclair/io/OpenChannelInterceptor.scala
@@ -108,23 +108,23 @@ private class OpenChannelInterceptor(peer: ActorRef[Any],
   }
 
   private def sanityCheckInitiator(request: OpenChannelInitiator): Behavior[Command] = {
+    val channelType_opt = request.open.channelType_opt.orElse(ChannelTypes.preferredForPublicChannels(request.localFeatures, request.remoteFeatures))
     if (request.open.fundingAmount >= Channel.MAX_FUNDING_WITHOUT_WUMBO && !request.localFeatures.hasFeature(Wumbo)) {
       request.replyTo ! OpenChannelResponse.Rejected(s"fundingAmount=${request.open.fundingAmount} is too big, you must enable large channels support in 'eclair.features' to use funding above ${Channel.MAX_FUNDING_WITHOUT_WUMBO} (see eclair.conf)")
       waitForRequest()
     } else if (request.open.fundingAmount >= Channel.MAX_FUNDING_WITHOUT_WUMBO && !request.remoteFeatures.hasFeature(Wumbo)) {
       request.replyTo ! OpenChannelResponse.Rejected(s"fundingAmount=${request.open.fundingAmount} is too big, the remote peer doesn't support wumbo")
       waitForRequest()
-    } else if (request.open.channelType_opt.isEmpty) {
-      request.replyTo ! OpenChannelResponse.Rejected("channel_type must be provided")
+    } else if (channelType_opt.isEmpty) {
+      request.replyTo ! OpenChannelResponse.Rejected("channel_type must be provided and compatible with our peer's features")
       waitForRequest()
     } else {
-      val channelType = request.open.channelType_opt.get
       val dualFunded = Features.canUseFeature(request.localFeatures, request.remoteFeatures, Features.DualFunding)
       val upfrontShutdownScript = Features.canUseFeature(request.localFeatures, request.remoteFeatures, Features.UpfrontShutdownScript)
       // If we're purchasing liquidity, we expect our peer to contribute at least the amount we're purchasing, otherwise we'll cancel the funding attempt.
       val expectedFundingAmount = request.open.fundingAmount + request.open.requestFunding_opt.map(_.requestedAmount).getOrElse(0 sat)
       val localParams = createLocalParams(nodeParams, request.localFeatures, upfrontShutdownScript, isChannelOpener = true, paysCommitTxFees = true, dualFunded = dualFunded, expectedFundingAmount)
-      peer ! Peer.SpawnChannelInitiator(request.replyTo, request.open, ChannelConfig.standard, channelType, localParams)
+      peer ! Peer.SpawnChannelInitiator(request.replyTo, request.open, ChannelConfig.standard, channelType_opt.get, localParams)
       waitForRequest()
     }
   }
diff --git a/eclair-core/src/test/scala/fr/acinq/eclair/io/OpenChannelInterceptorSpec.scala b/eclair-core/src/test/scala/fr/acinq/eclair/io/OpenChannelInterceptorSpec.scala
index 6b582ec..4d1bba9 100644
--- a/eclair-core/src/test/scala/fr/acinq/eclair/io/OpenChannelInterceptorSpec.scala
+++ b/eclair-core/src/test/scala/fr/acinq/eclair/io/OpenChannelInterceptorSpec.scala
@@ -282,4 +282,26 @@ class OpenChannelInterceptorSpec extends ScalaTestWithActorTestKit(ConfigFactory
     eventListener.expectMessageType[ChannelAborted]
   }
 
+  test("don't spawn a channel if we cannot find a satisfying channel type") { f =>
+    import f._
+
+    val probe = TestProbe[Any]()
+
+    // If we both support anchor outputs, it is selected by default.
+    {
+      val features = Features[InitFeature](StaticRemoteKey -> Optional, AnchorOutputsZeroFeeHtlcTx -> Optional, ChannelType -> Optional, ScidAlias -> Optional)
+      val open = Peer.OpenChannel(remoteNodeId, 500_000 sat, None, None, None, None, None, None, None)
+      openChannelInterceptor ! OpenChannelInitiator(probe.ref, remoteNodeId, open, features, features)
+      assert(peer.expectMessageType[Peer.SpawnChannelInitiator].channelType == ChannelTypes.AnchorOutputsZeroFeeHtlcTx(scidAlias = true))
+    }
+    // If our peer doesn't support anchor outputs, we can't find a satisfying channel type.
+    {
+      val localFeatures = Features[InitFeature](StaticRemoteKey -> Optional, AnchorOutputsZeroFeeHtlcTx -> Optional, ChannelType -> Optional, ScidAlias -> Optional)
+      val remoteFeatures = Features[InitFeature](StaticRemoteKey -> Optional, ChannelType -> Optional, ScidAlias -> Optional)
+      val open = Peer.OpenChannel(remoteNodeId, 500_000 sat, None, None, None, None, None, None, None)
+      openChannelInterceptor ! OpenChannelInitiator(probe.ref, remoteNodeId, open, localFeatures, remoteFeatures)
+      assert(probe.expectMessageType[OpenChannelResponse.Rejected].reason == "channel_type must be provided and compatible with our peer's features")
+    }
+  }
+
 }
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