chore: forbid API based flat fee line manipulation if ubp fs is active

cmd/server/main.go

@@ -161,6 +161,7 @@ func main() {
 			AppStripe:                   app.AppRegistry.Stripe,
 			AppCustomInvoicing:          app.AppRegistry.CustomInvoicing,
 			Billing:                     app.Billing,
+			BillingFeatureSwitches:      conf.Billing.FeatureSwitches,
 			Customer:                    app.Customer,
 			DebugConnector:              debugConnector,
 			ErrorHandler:                errorsx.NewSlogHandler(logger),

openmeter/billing/httpdriver/handler.go

@@ -6,6 +6,7 @@ import (
 	"log/slog"
 	"net/http"
 
+	"github.com/openmeterio/openmeter/app/config"
 	appstripe "github.com/openmeterio/openmeter/openmeter/app/stripe"
 	"github.com/openmeterio/openmeter/openmeter/billing"
 	"github.com/openmeterio/openmeter/openmeter/namespace/namespacedriver"
@@ -52,6 +53,7 @@ type CustomerOverrideHandler interface {
 type handler struct {
 	service          billing.Service
 	namespaceDecoder namespacedriver.NamespaceDecoder
+	featureSwitches  config.BillingFeatureSwitchesConfiguration
 	options          []httptransport.HandlerOption
 }
 
@@ -67,6 +69,7 @@ func (h *handler) resolveNamespace(ctx context.Context) (string, error) {
 func New(
 	logger *slog.Logger,
 	namespaceDecoder namespacedriver.NamespaceDecoder,
+	featureSwitches config.BillingFeatureSwitchesConfiguration,
 	service billing.Service,
 	stripeAppService appstripe.Service,
 	options ...httptransport.HandlerOption,
@@ -75,5 +78,6 @@ func New(
 		service:          service,
 		namespaceDecoder: namespaceDecoder,
 		options:          options,
+		featureSwitches:  featureSwitches,
 	}
 }

openmeter/billing/httpdriver/invoiceline.go

@@ -69,6 +69,16 @@ func (h *handler) CreatePendingLine() CreatePendingLineHandler {
 				}
 			}
 
+			if h.featureSwitches.UseUsageBasedFlatFeeLines {
+				for _, line := range lineEntities {
+					if line.Type == billing.InvoiceLineTypeFee {
+						return CreatePendingLineRequest{}, billing.ValidationError{
+							Err: fmt.Errorf("creating flat fee lines is not supported, please use usage based lines instead"),
+						}
+					}
+				}
+			}
+
 			return CreatePendingLineRequest{
 				Customer: customer.CustomerID{
 					Namespace: ns,
@@ -942,6 +952,12 @@ func (h *handler) mergeInvoiceLinesFromAPI(ctx context.Context, invoice *billing
 				return billing.LineChildren{}, fmt.Errorf("failed to create new line: %w", err)
 			}
 
+			if h.featureSwitches.UseUsageBasedFlatFeeLines && newLine.Type == billing.InvoiceLineTypeFee {
+				return billing.LineChildren{}, billing.ValidationError{
+					Err: fmt.Errorf("creating flat fee lines is not supported, please use usage based lines instead"),
+				}
+			}
+
 			if invoice.Status != billing.InvoiceStatusGathering {
 				newLine, err = h.service.SnapshotLineQuantity(ctx, billing.SnapshotLineQuantityInput{
 					Invoice: invoice,
@@ -962,6 +978,12 @@ func (h *handler) mergeInvoiceLinesFromAPI(ctx context.Context, invoice *billing
 			return billing.LineChildren{}, fmt.Errorf("failed to merge line: %w", err)
 		}
 
+		if h.featureSwitches.UseUsageBasedFlatFeeLines && changed && mergedLine.Type == billing.InvoiceLineTypeFee {
+			return billing.LineChildren{}, billing.ValidationError{
+				Err: fmt.Errorf("updating flat fee lines is not supported, please use usage based lines instead"),
+			}
+		}
+
 		if changed && invoice.Status != billing.InvoiceStatusGathering {
 			mergedLine, err = h.service.SnapshotLineQuantity(ctx, billing.SnapshotLineQuantityInput{
 				Invoice: invoice,

openmeter/server/router/router.go

@@ -12,6 +12,7 @@ import (
 	"github.com/getkin/kin-openapi/openapi3filter"
 
 	"github.com/openmeterio/openmeter/api"
+	"github.com/openmeterio/openmeter/app/config"
 	"github.com/openmeterio/openmeter/openmeter/app"
 	appcustominvoicing "github.com/openmeterio/openmeter/openmeter/app/custominvoicing"
 	appcustominvoicinghttpdriver "github.com/openmeterio/openmeter/openmeter/app/custominvoicing/httpdriver"
@@ -86,6 +87,7 @@ type Config struct {
 	AppStripe                   appstripe.Service
 	AppCustomInvoicing          appcustominvoicing.SyncService
 	Billing                     billing.Service
+	BillingFeatureSwitches      config.BillingFeatureSwitchesConfiguration
 	Customer                    customer.Service
 	DebugConnector              debug.DebugConnector
 	EntitlementConnector        entitlement.Connector
@@ -344,6 +346,7 @@ func NewRouter(config Config) (*Router, error) {
 	router.billingHandler = billinghttpdriver.New(
 		config.Logger,
 		staticNamespaceDecoder,
+		config.BillingFeatureSwitches,
 		config.Billing,
 		config.AppStripe,
 		httptransport.WithErrorHandler(config.ErrorHandler),