cmd/commands: add validation for MPP parameters

Adds validation to ensure that the combination of max_parts and
max_shard_size_msat parameters can accommodate the full payment amount
before attempting the payment. This prevents payments from entering a path
finding loop that would eventually timeout when parameters are incompatible.

cmd/commands/cmd_payments.go

@@ -26,6 +26,8 @@ import (
 	"github.com/lightningnetwork/lnd/routing/route"
 	"github.com/urfave/cli"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	"google.golang.org/grpc/status"
 )
 
 const (
@@ -488,10 +490,47 @@ func routerRPCSendPayment(ctx context.Context, payConn grpc.ClientConnInterface,
 	return routerrpc.NewRouterClient(payConn).SendPaymentV2(ctx, req)
 }
 
+// maybeValidateMPPParams validates that the MPP parameters are compatible with the
+// payment amount. It returns an error if the parameters don't allow the full
+// payment amount to be sent.
+// maybeValidateMPPParams could be enhanced with additional checks
+func maybeValidateMPPParams(amt lnwire.MilliSatoshi, maxParts uint32,
+	maxShardSize lnwire.MilliSatoshi) error {
+	// Add validation for negative or zero amounts
+	if amt <= 0 {
+		return fmt.Errorf("payment amount must be positive, got %v", amt)
+	}
+
+	// Early return if MPP is not being used
+	if maxParts == 0 || maxShardSize == 0 {
+		return nil
+	}
+
+	// Validate maxShardSize
+	if maxShardSize <= 0 {
+		return fmt.Errorf("max_shard_size must be positive, got %v", maxShardSize)
+	}
+
+	// Validate maxParts
+	if maxParts <= 0 {
+		return fmt.Errorf("max_parts must be positive, got %v", maxParts)
+	}
+
+	// Calculate maximum possible amount
+	maxPossibleAmount := lnwire.MilliSatoshi(maxParts) * maxShardSize
+
+	if amt > maxPossibleAmount {
+		return fmt.Errorf("payment amount %v exceeds maximum possible "+
+			"amount %v with max_parts=%v and max_shard_size_msat=%v",
+			amt, maxPossibleAmount, maxParts, maxShardSize)
+	}
+
+	return nil
+}
+
 func SendPaymentRequest(ctx *cli.Context, req *routerrpc.SendPaymentRequest,
 	lnConn, paymentConn grpc.ClientConnInterface,
 	callSendPayment SendPaymentFn) error {
-
 	ctxc := getContext()
 
 	lnClient := lnrpc.NewLightningClient(lnConn)
@@ -586,7 +625,13 @@ func SendPaymentRequest(ctx *cli.Context, req *routerrpc.SendPaymentRequest,
 		if invoiceAmt != 0 {
 			amt = invoiceAmt
 		}
-
+		err = maybeValidateMPPParams(
+			lnwire.MilliSatoshi(amt*1000), req.MaxParts, lnwire.MilliSatoshi(req.MaxShardSizeMsat),
+		)
+		if err != nil {
+			return status.Errorf(codes.InvalidArgument,
+				"invalid MPP parameters: %v", err)
+		}
 		// Calculate fee limit based on the determined amount.
 		feeLimit, err = retrieveFeeLimit(ctx, amt)
 		if err != nil {
@@ -603,6 +648,13 @@ func SendPaymentRequest(ctx *cli.Context, req *routerrpc.SendPaymentRequest,
 		}
 	} else {
 		var err error
+		err = maybeValidateMPPParams(
+			lnwire.MilliSatoshi(req.Amt*1000), req.MaxParts, lnwire.MilliSatoshi(req.MaxShardSizeMsat),
+		)
+		if err != nil {
+			return status.Errorf(codes.InvalidArgument,
+				"invalid MPP parameters: %v", err)
+		}
 		feeLimit, err = retrieveFeeLimit(ctx, req.Amt)
 		if err != nil {
 			return err