Add Taler as payment method

It doesn't take payments yet but can be selected during checkout.

app/controllers/spree/admin/payment_methods_controller.rb

@@ -14,6 +14,7 @@ module Spree
         Spree::Gateway::PayPalExpress
         Spree::Gateway::StripeSCA
         Spree::PaymentMethod::Check
+        Spree::PaymentMethod::Taler
       }.freeze
 
       def create

app/models/spree/payment_method/taler.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module Spree
+  class PaymentMethod
+    # GNU Taler is a distributed, open source payment system.
+    # You need a hosted Taler backend server to process payments.
+    #
+    # For testing, you can use the official demo backend:
+    #
+    # - Merchant UX: https://backend.demo.taler.net
+    # - Username: sandbox
+    # - Password: sandbox
+    #
+    # Configure this payment method for testing with:
+    #
+    # - backend_url: https://backend.demo.taler.net/instances/sandbox
+    # - api_key: sandbox
+    class Taler < PaymentMethod
+      preference :backend_url, :string
+      preference :api_key, :password
+
+      # Name of the view to display during checkout
+      def method_type
+        "check" # empty view
+      end
+
+      def external_gateway?
+        true
+      end
+
+      # The backend provides this URL. It can look like this:
+      # https://backend.demo.taler.net/instances/blog/orders/2026..?token=S8Y..&session_id=b0b..
+      def external_payment_url(options)
+        # order = options.fetch(:order)
+        # Taler.create_order(backend_url, api_key, order.total, "OFN Order", "https://ofn.example.net")
+      end
+    end
+  end
+end

app/services/permitted_attributes/payment_method.rb

@@ -11,7 +11,7 @@ module PermittedAttributes
         [:name, :description, :type, :active,
          :environment, :display_on, :tag_list,
          :preferred_enterprise_id, :preferred_server, :preferred_login, :preferred_password,
-         :calculator_type, :preferred_api_key,
+         :calculator_type, :preferred_api_key, :preferred_backend_url,
          :preferred_signature, :preferred_solution, :preferred_landing_page, :preferred_logourl,
          :preferred_test_mode, :calculator_type, { distributor_ids: [] },
          { calculator_attributes: PermittedAttributes::Calculator.attributes }]

config/locales/en.yml

@@ -4344,6 +4344,10 @@ en:
     thumbnail: "Thumbnail"
     back_to_images_list: "Back To Images List"
 
+    # Attributes of the Taler payment gateway
+    backend_url: "Backend URL"
+    api_key: "API key"
+
     # TODO: remove `email` key once we get to Spree 2.0
     email: Email
     # TODO: remove 'account_updated' key once we get to Spree 2.0
@@ -4666,6 +4670,7 @@ en:
           check: "Cash/EFT/etc. (payments for which automatic validation is not required)"
           paypalexpress: "PayPal Express"
           stripesca: "Stripe SCA"
+          taler: "Taler"
       payments:
         source_forms:
           stripe:

spec/controllers/spree/admin/payment_methods_controller_spec.rb

@@ -19,6 +19,7 @@ RSpec.describe Spree::Admin::PaymentMethodsController do
       expect(providers).to eq %w[
         Spree::Gateway::PayPalExpress
         Spree::PaymentMethod::Check
+        Spree::PaymentMethod::Taler
       ]
     end