Don't force-close on remote error

When using zero-fee commitments, we don't force-close when receiving an
`error` from our peer: if they want to force-close the channel, they can
publish their commitment instead of forcing us to publish ours. It is
especially true when the commit tx doesn't pay any fees, because the
publisher will pay the entire fees for the force-close.

Note that for wallet peers, we could introduce a mechanism where they
send us their signed commit tx in the error message if they don't have
any wallet input to pay the fees, and we could be nice and publish it
while paying the fees from our main output (which isn't delayed since
it is the remote commit from our point of view).

Author: t-bast

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

@@ -160,6 +160,12 @@ trait ErrorHandlers extends CommonHandlers {
           // The channel closing is retried on every reconnect of the channel, until it succeeds.
           log.warning("ignoring remote 'link failed to shutdown', probably coming from lnd")
           stay() sending Warning(d.channelId, "ignoring your 'link failed to shutdown' to avoid an unnecessary force-close")
+        } else if (hasCommitments.commitments.latest.commitmentFormat == ZeroFeeCommitmentFormat) {
+          // When using v3 transactions, we want to avoid using our wallet inputs as much as possible.
+          // It is much more convenient if our peer publishes their commitment transaction if they're having an issue,
+          // because we can use our main output for pay the fees of the commitment transaction directly (if necessary).
+          log.warning("ignoring remote error: waiting for remote commit tx to be published")
+          stay() sending Warning(d.channelId, "ignoring your error: please publish your commitment if you want to force-close the channel")
         } else {
           spendLocalCurrent(hasCommitments, maxClosingFeerateOverride_opt = None)
         }

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

@@ -648,7 +648,7 @@ trait ChannelStateTestsBase extends Assertions with Eventually {
   }
 
   def localClose(s: TestFSMRef[ChannelState, ChannelData, Channel], s2blockchain: TestProbe, htlcSuccessCount: Int = 0, htlcTimeoutCount: Int = 0): (LocalCommitPublished, PublishedForceCloseTxs) = {
-    s ! Error(ByteVector32.Zeroes, "oops")
+    s ! CMD_FORCECLOSE(ActorRef.noSender, None, None)
     eventually(assert(s.stateName == CLOSING))
     val closingState = s.stateData.asInstanceOf[DATA_CLOSING]
     assert(closingState.localCommitPublished.isDefined)