cmd/loop: warn user if fee estimation fails

cmd/loop/loopin.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 
 	"github.com/btcsuite/btcutil"
+	"github.com/lightninglabs/loop"
 	"github.com/lightninglabs/loop/looprpc"
 	"github.com/lightninglabs/loop/swap"
 	"github.com/lightningnetwork/lnd/routing/route"
@@ -76,6 +77,18 @@ func loopIn(ctx *cli.Context) error {
 		return err
 	}
 
+	// For loop in, the fee estimation is handed to lnd which tries to
+	// construct a real transaction to sample realistic fees to pay to the
+	// HTLC. If the wallet doesn't have enough funds to create this TX, we
+	// know it won't have enough to pay the real transaction either. It
+	// makes sense to abort the loop in this case.
+	if !external && quote.MinerFee == int64(loop.MinerFeeEstimationFailed) {
+		return fmt.Errorf("miner fee estimation not " +
+			"possible, lnd has insufficient funds to " +
+			"create a sample transaction for selected " +
+			"amount")
+	}
+
 	limits := getInLimits(amt, quote)
 	err = displayLimits(swap.TypeIn, amt, limits, external, "")
 	if err != nil {

cmd/loop/main.go

@@ -159,9 +159,8 @@ func displayLimits(swapType swap.Type, amt btcutil.Amount, l *limits,
 			"wallet.\n\n")
 	}
 
-	fmt.Printf("Max swap fees for %d Loop %v: %d\n",
-		amt, swapType, totalSuccessMax,
-	)
+	fmt.Printf("Max swap fees for %d Loop %v: %d\n", amt, swapType,
+		totalSuccessMax)
 
 	if warning != "" {
 		fmt.Println(warning)

cmd/loop/quote.go

@@ -2,8 +2,11 @@ package main
 
 import (
 	"context"
+	"fmt"
+	"os"
 	"time"
 
+	"github.com/lightninglabs/loop"
 	"github.com/lightninglabs/loop/looprpc"
 	"github.com/urfave/cli"
 )
@@ -59,6 +62,18 @@ func quoteIn(ctx *cli.Context) error {
 		return err
 	}
 
+	// For loop in, the fee estimation is handed to lnd which tries to
+	// construct a real transaction to sample realistic fees to pay to the
+	// HTLC. If the wallet doesn't have enough funds to create this TX, we
+	// don't want to fail the quote. But the user should still be informed
+	// why the fee shows as -1.
+	if quoteResp.MinerFee == int64(loop.MinerFeeEstimationFailed) {
+		_, _ = fmt.Fprintf(os.Stderr, "Warning: Miner fee estimation "+
+			"not possible, lnd has insufficient funds to "+
+			"create a sample transaction for selected "+
+			"amount.\n")
+	}
+
 	printRespJSON(quoteResp)
 	return nil
 }