loopin: fail swap when htlc amount is incorrect

Previously the swap would get stuck in a state where it wouldn't ever
progress because the server rejected the htlc.

loopdb/swapstate.go

@@ -60,6 +60,10 @@ const (
 	// StateInvoiceSettled means that the swap invoice has been paid by the
 	// server.
 	StateInvoiceSettled SwapState = 9
+
+	// StateFailIncorrectHtlcAmt indicates that the amount of an externally
+	// published loop in htlc didn't match the swap amount.
+	StateFailIncorrectHtlcAmt SwapState = 10
 )
 
 // SwapStateType defines the types of swap states that exist. Every swap state
@@ -127,6 +131,9 @@ func (s SwapState) String() string {
 	case StateInvoiceSettled:
 		return "InvoiceSettled"
 
+	case StateFailIncorrectHtlcAmt:
+		return "IncorrectHtlcAmt"
+
 	default:
 		return "Unknown"
 	}

loopin.go

@@ -363,6 +363,13 @@ func (s *loopInSwap) executeSwap(globalCtx context.Context) error {
 		return err
 	}
 
+	// Verify that the confirmed (external) htlc value matches the swap
+	// amount. Otherwise fail the swap immediately.
+	if htlcValue != s.LoopInContract.AmountRequested {
+		s.setState(loopdb.StateFailIncorrectHtlcAmt)
+		return s.persistAndAnnounceState(globalCtx)
+	}
+
 	// TODO: Add miner fee of htlc tx to swap cost balance.
 
 	// The server is expected to see the htlc on-chain and knowing that it

loopin_test.go

@@ -209,6 +209,20 @@ func testLoopInTimeout(t *testing.T,
 		Tx: &htlcTx,
 	}
 
+	// Assert that the swap is failed in case of an invalid amount.
+	invalidAmt := externalValue != 0 && externalValue != int64(req.Amount)
+	if invalidAmt {
+		ctx.assertState(loopdb.StateFailIncorrectHtlcAmt)
+		ctx.store.assertLoopInState(loopdb.StateFailIncorrectHtlcAmt)
+
+		err = <-errChan
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		return
+	}
+
 	// Client starts listening for spend of htlc.
 	<-ctx.lnd.RegisterSpendChannel
 
@@ -380,6 +394,7 @@ func testLoopInResume(t *testing.T, state loopdb.SwapState, expired bool) {
 
 		htlcTx.AddTxOut(&wire.TxOut{
 			PkScript: htlc.PkScript,
+			Value:    int64(contract.AmountRequested),
 		})
 	}