lnd: add max fee rate check to closechannel rpc

rpcserver.go

@@ -2878,6 +2878,15 @@ func (r *rpcServer) CloseChannel(in *lnrpc.CloseChannelRequest,
 		maxFee := chainfee.SatPerKVByte(
 			in.MaxFeePerVbyte * 1000,
 		).FeePerKWeight()
+
+		// In case the max fee was specified, we check if it's less than
+		// the initial fee rate and abort if it is.
+		if maxFee != 0 && maxFee < feeRate {
+			return fmt.Errorf("max_fee_per_vbyte (%v) is less "+
+				"than the required fee rate (%v)", maxFee,
+				feeRate)
+		}
+
 		updateChan, errChan = r.server.htlcSwitch.CloseLink(
 			chanPoint, contractcourt.CloseRegular, feeRate,
 			maxFee, deliveryScript,
@@ -2909,7 +2918,9 @@ out:
 		case err := <-errChan:
 			rpcsLog.Errorf("[closechannel] unable to close "+
 				"ChannelPoint(%v): %v", chanPoint, err)
+
 			return err
+
 		case closingUpdate := <-updateChan:
 			rpcClosingUpdate, err := createRPCCloseUpdate(
 				closingUpdate,
@@ -2948,6 +2959,7 @@ out:
 					"txid(%v)", h)
 				break out
 			}
+
 		case <-r.quit:
 			return nil
 		}