cmd: allow max miner fee to be specified

cmd/loop/loopout.go

@@ -63,6 +63,20 @@ var loopOutCommand = cli.Command{
 				"satoshis, if not specified, a default max " +
 				"fee will be used",
 		},
+		cli.Float64Flag{
+			Name: "max_sweep_fee_percent",
+			Usage: "the max on-chain miner fee used for sweeping " +
+				"HTLCs, expressed as a percent (0 to 1.0) to " +
+				"be deducted from the total swap amount. If " +
+				"not set, will use the default value of " +
+				"max_sweep_fee instead.",
+		},
+		cli.Uint64Flag{
+			Name: "max_sweep_fee",
+			Usage: "the max on-chain miner fee (in satoshis) " +
+				"used for sweeping HTLCs. If not set, will " +
+				"use the prepay amount as the default value.",
+		},
 		cli.BoolFlag{
 			Name: "fast",
 			Usage: "Indicate you want to swap immediately, " +
@@ -176,6 +190,25 @@ func loopOut(ctx *cli.Context) error {
 			ctx.Int64("max_swap_routing_fee"),
 		)
 	}
+
+	// Calculate max miner fee. The prepay amount will be used if not set.
+	maxMinerFee, err := calculateMaxMinerFee(ctx, amt)
+	if err != nil {
+		return err
+	}
+	if maxMinerFee != 0 {
+		// Override the default if maxMinerFee is set.
+		limits.maxMinerFee = maxMinerFee
+	}
+
+	// Check that the maxMinerFee is sane. The prepay amount should also be
+	// greater then the minMinerFee.
+	if limits.maxMinerFee < limits.minMinerFee {
+		return fmt.Errorf("max sweep fee is too small, must be "+
+			"greater than %d sats", limits.minMinerFee,
+		)
+	}
+
 	err = displayOutDetails(
 		limits, warning, quoteReq, quote, ctx.Bool("verbose"),
 	)
@@ -213,3 +246,43 @@ func loopOut(ctx *cli.Context) error {
 
 	return nil
 }
+
+// calculateMaxMinerFee validates the max miner fee set by cli is sane.
+func calculateMaxMinerFee(ctx *cli.Context,
+	amt btcutil.Amount) (btcutil.Amount, error) {
+
+	useSatoshi := ctx.IsSet("max_sweep_fee")
+	usePercent := ctx.IsSet("max_sweep_fee_percent")
+
+	switch {
+	case useSatoshi && usePercent:
+		// Max sweep fee can only be set either in the format of
+		// satoshi or percent.
+		return 0, fmt.Errorf("either max_sweep_fee or " +
+			"max_sweep_fee_percent should be set, not both",
+		)
+
+	case useSatoshi:
+		fee := btcutil.Amount(ctx.Uint64("max_sweep_fee"))
+		if fee >= amt {
+			return 0, fmt.Errorf("max_sweep_fee (%d) cannot be "+
+				"greater than total swap amount (%d)", fee, amt,
+			)
+		}
+		return btcutil.Amount(ctx.Uint64("max_sweep_fee")), nil
+
+	case usePercent:
+		// Validate percent is sane.
+		percent := ctx.Float64("max_sweep_fee_percent")
+		if percent < 0 || percent >= 1 {
+			return 0, fmt.Errorf("max_sweep_fee_percent is "+
+				"invalid, must be in range(0, 1), "+
+				"instead got %f", percent,
+			)
+		}
+		return amt.MulF64(percent), nil
+	}
+
+	// Return zero if neither is set.
+	return 0, nil
+}

cmd/loop/main.go

@@ -243,6 +243,10 @@ type outLimits struct {
 	maxMinerFee         btcutil.Amount
 	maxSwapFee          btcutil.Amount
 	maxPrepayAmt        btcutil.Amount
+
+	// minMinerFee guards the min value can be used when specifying the max
+	// miner fee.
+	minMinerFee btcutil.Amount
 }
 
 func getOutLimits(amt btcutil.Amount,
@@ -258,10 +262,9 @@ func getOutLimits(amt btcutil.Amount,
 		maxSwapRoutingFee:   maxSwapRoutingFee,
 		maxPrepayRoutingFee: maxPrepayRoutingFee,
 
-		// Apply a multiplier to the estimated miner fee, to not get
-		// the swap canceled because fees increased in the mean time.
-		maxMinerFee: btcutil.Amount(quote.HtlcSweepFeeSat) * 100,
-
+		// maxMinerFee is default to the prepay amount.
+		maxMinerFee:  maxPrepayAmt,
+		minMinerFee:  btcutil.Amount(quote.HtlcSweepFeeSat),
 		maxSwapFee:   btcutil.Amount(quote.SwapFeeSat),
 		maxPrepayAmt: maxPrepayAmt,
 	}

release_notes.md

@@ -15,6 +15,14 @@ This file tracks release notes for the loop client.
 ## Next release
 
 #### New Features
+* Two new flags, `--max_sweep_fee` and `--max_sweep_fee_percent`, are added to
+  `loop out`. Users now can specify the max on-chain fee to be paid when
+  sweeping the HTLCs. `max_sweep_fee` specifies the max fee can be used in
+  satoshis, default to the prepay amount. `max_sweep_fee_percent` specifies a
+  portion based on the total swap amount. Note that these flags cannot be used
+  together. When neither is specified, the default value of `max_sweep_fee` will
+  be used. Be cautious when setting the max fee as a low value might cause
+  forfeiting the prepay.
 
 #### Breaking Changes