Add DB constraints to check for user balances

controllers/payinvoice.ctrl.go

@@ -79,7 +79,8 @@ func (controller *PayInvoiceController) PayInvoice(c echo.Context) error {
 
 	entry, err := controller.svc.PayInvoice(invoice)
 	if err != nil {
-		c.Logger().Errorf("Failed: %v", err)
+		c.Logger().Errorf("Payment failed: %v", err)
+		// TODO: sentry notification
 		return c.JSON(http.StatusBadRequest, echo.Map{
 			"error":   true,
 			"code":    10,

db/migrations/20220120000700_add_constraints.up.go

@@ -0,0 +1,59 @@
+package migrations
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/uptrace/bun"
+)
+
+func init() {
+	Migrations.MustRegister(func(ctx context.Context, db *bun.DB) error {
+
+		if db.Dialect().Name().String() != "pg" {
+			fmt.Printf("\033[1;31m%s\033[0m", "You are not using PostgreSQL. DB level checks can not be enabled!\n")
+			return nil
+		}
+		sql := `
+			-- make sure transfers happen from one account to another one
+				alter table transaction_entries
+				ADD CONSTRAINT check_not_same_account
+				CHECK (debit_account_id != credit_account_id);
+
+			-- make sure that account balances >= 0 (except for incoming account)
+				CREATE OR REPLACE FUNCTION check_balance()
+					RETURNS TRIGGER AS $$
+				DECLARE
+					sum BIGINT;
+					debit_account_type VARCHAR;
+				BEGIN
+					SELECT INTO debit_account_type type
+					FROM accounts
+					WHERE id = NEW.debit_account_id;
+
+					SELECT INTO  sum SUM(amount)
+					FROM account_ledgers
+					WHERE account_ledgers.account_id = NEW.debit_account_id;
+
+					-- the incoming account can have a negative balance
+					-- all other accounts must have a positive balance
+					IF sum < 0 AND debit_account_type != 'incoming'
+					THEN
+						RAISE EXCEPTION 'invalid balance [user_id:%] [debit_account_id:%] balance [%]',
+						NEW.user_id,
+						NEW.debit_account_id,
+						sum;
+					END IF;
+					RETURN NEW;
+				END;
+				$$ LANGUAGE plpgsql;
+				CREATE TRIGGER check_balance
+				AFTER INSERT OR UPDATE ON transaction_entries
+				FOR EACH ROW EXECUTE PROCEDURE check_balance();
+		`
+		if _, err := db.Exec(sql); err != nil {
+			return err
+		}
+		return nil
+	}, nil)
+}

lib/service/invoices.go

@@ -54,6 +54,9 @@ func (svc *LndhubService) PayInvoice(invoice *models.Invoice) (*models.Transacti
 	if err != nil {
 		return &entry, err
 	}
+
+	// The DB constraints make sure the user actually has enough balance for the transaction
+	// If the user does not have enough balance this call fails
 	_, err = tx.NewInsert().Model(&entry).Exec(context.TODO())
 	if err != nil {
 		tx.Rollback()