rpcserver: Clarify failure closing offline channel

htlcswitch/switch.go

@@ -1284,8 +1284,8 @@ func (s *Switch) htlcForwarder() {
 			chanID := lnwire.NewChanIDFromOutPoint(req.ChanPoint)
 			link, ok := s.linkIndex[chanID]
 			if !ok {
-				req.Err <- errors.Errorf("channel with "+
-					"chan_id=%x not found", chanID[:])
+				req.Err <- errors.Errorf("no peer for channel with "+
+					"chan_id=%x", chanID[:])
 				continue
 			}
 

lnwire/channel_id.go

@@ -22,6 +22,8 @@ const (
 // within the network. The ChannelID is computed using the outpoint of the
 // funding transaction (the txid, and output index). Given a funding output the
 // ChannelID can be calculated by XOR'ing the big-endian serialization of the
+// txid and the big-endian serialization of the output index, truncated to
+// 2 bytes.
 type ChannelID [32]byte
 
 // ConnectionWideID is an all-zero ChannelID, which is used to represent a

rpcserver.go

@@ -1071,6 +1071,16 @@ func (r *rpcServer) CloseChannel(in *lnrpc.CloseChannelRequest,
 				}
 			})
 	} else {
+		// If the link is not known by the switch, we cannot gracefully close
+		// the channel.
+		channelID := lnwire.NewChanIDFromOutPoint(chanPoint)
+		if _, err := r.server.htlcSwitch.GetLink(channelID); err != nil {
+			rpcsLog.Debugf("Trying to non-force close offline channel with "+
+				"chan_point=%v", chanPoint)
+			return fmt.Errorf("unable to gracefully close channel while peer "+
+				"is offline (try force closing it instead): %v", err)
+		}
+
 		// Based on the passed fee related parameters, we'll determine
 		// an appropriate fee rate for the cooperative closure
 		// transaction.