Ignore lnd's internal errors (#2659)

It seems like lnd sends this error whenever something wrong happens on
their side, regardless of whether the channel actually needs to be closed.
We ignore it to avoid paying the cost of a channel force-close, it's up
to them to broadcast their commitment if they wish.

See lightningnetwork/lnd#7657 for example.

eclair-core/src/main/scala/fr/acinq/eclair/channel/fsm/ErrorHandlers.scala

@@ -28,7 +28,7 @@ import fr.acinq.eclair.channel.fsm.Channel.UnhandledExceptionStrategy
 import fr.acinq.eclair.channel.publish.TxPublisher.{PublishFinalTx, PublishReplaceableTx, PublishTx}
 import fr.acinq.eclair.transactions.Transactions
 import fr.acinq.eclair.transactions.Transactions.ClosingTx
-import fr.acinq.eclair.wire.protocol.{AcceptChannel, ChannelReestablish, Error, OpenChannel}
+import fr.acinq.eclair.wire.protocol.{AcceptChannel, ChannelReestablish, Error, OpenChannel, Warning}
 
 import java.sql.SQLException
 
@@ -134,7 +134,16 @@ trait ErrorHandlers extends CommonHandlers {
         // if we were in the process of closing and already received a closing sig from the counterparty, it's always better to use that
         handleMutualClose(bestUnpublishedClosingTx, Left(negotiating))
       // NB: we publish the commitment even if we have nothing at stake (in a dataloss situation our peer will send us an error just for that)
-      case hasCommitments: ChannelDataWithCommitments => spendLocalCurrent(hasCommitments)
+      case hasCommitments: ChannelDataWithCommitments =>
+        if (e.toAscii == "internal error") {
+          // It seems like lnd sends this error whenever something wrong happens on their side, regardless of whether
+          // the channel actually needs to be closed. We ignore it to avoid paying the cost of a channel force-close,
+          // it's up to them to broadcast their commitment if they wish.
+          log.warning("ignoring remote 'internal error', probably coming from lnd")
+          stay() sending Warning(d.channelId, "ignoring your 'internal error' to avoid an unnecessary force-close")
+        } else {
+          spendLocalCurrent(hasCommitments)
+        }
       // When there is no commitment yet, we just go to CLOSED state in case an error occurs.
       case _: ChannelDataWithoutCommitments => goto(CLOSED)
       case _: TransientChannelData => goto(CLOSED)

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

@@ -3342,6 +3342,14 @@ class NormalStateSpec extends TestKitBaseClass with FixtureAnyFunSuiteLike with
     alice2blockchain.expectNoMessage(1 second)
   }
 
+  test("recv Error (ignored internal error from lnd)") { f =>
+    import f._
+
+    alice ! Error(channelId(alice), "internal error")
+    alice2bob.expectMsgType[Warning]
+    alice2blockchain.expectNoMessage(100 millis)
+  }
+
   def testErrorAnchorOutputsWithHtlcs(f: FixtureParam): Unit = {
     import f._