Adding the Taler payment system.

9 files changed, 589 insertions, 10 deletions

diff --git a/doc/modules.rst b/doc/modules.rst
index 745a7bc..a547b7a 100644
--- a/doc/modules.rst
+++ b/doc/modules.rst
@@ -255,7 +255,7 @@ Example::
 This backend does not support fraud detection.
 
 Sofort.com
---------
+----------
 
 .. class:: payments.sofort.SofortProvider(key, id, project_id[, endpoint='https://api.sofort.com/api/xml'])
 
@@ -301,3 +301,30 @@ Example::
 
 
 This backend does not support fraud detection.
+
+Taler
+-----
+
+.. class:: payments.taler.TalerProvider(self, backend_url, address, name, jurisdiction, instance=None)
+
+   This backend implements payments using `Taler <https://taler/net/>`_.
+
+   :param backend_url: Backend's base URL.
+   :param instance: Merchant instance.
+   :param address: Merchant physical address.
+   :param name: Merchant/shop name.
+   :param jurisdiction: Jurisdiction where the merchant operates.
+
+Example::
+
+    PAYMENT_VARIANTS = {
+        'taler': ('payments.taler.TalerProvider',
+            {'backend_url': 'http://backend.test.taler.net/',
+             'address': 'US',
+             'jurisdiction': 'US',
+             'name': 'Taler tester shop'})
+    }
+
+.. This backend does not report fraud to Saleor, as fraud in
+   the classical sense is not possible with Taler.  Naturally,
+   protocol failures are detected and reported (to the wallet).
diff --git a/payments/locale/en/LC_MESSAGES/django.po b/payments/locale/en/LC_MESSAGES/django.po
index d063dda..de8542a 100644
--- a/payments/locale/en/LC_MESSAGES/django.po
+++ b/payments/locale/en/LC_MESSAGES/django.po
@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PACKAGE VERSION\n"
 "Report-Msgid-Bugs-To: \n"
-"POT-Creation-Date: 2017-03-13 13:13+0100\n"
+"POT-Creation-Date: 2017-10-20 14:16+0200\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: FULL NAME <EMAIL@ADDRESS>\n"
 "Language-Team: LANGUAGE <LL@li.org>\n"
@@ -158,11 +158,11 @@ msgstr ""
 msgid "We were unable to complete the transaction. Please try again later."
 msgstr ""
 
-#: payments/cybersource/forms.py:51
+#: payments/cybersource/forms.py:52
 msgid "fingerprint"
 msgstr ""
 
-#: payments/cybersource/forms.py:68 payments/stripe/forms.py:52
+#: payments/cybersource/forms.py:69 payments/stripe/forms.py:52
 msgid "This payment has already been processed."
 msgstr ""
 
@@ -205,6 +205,24 @@ msgstr ""
 msgid "Total payment"
 msgstr ""
 
+#: payments/taler/__init__.py:89
+msgid ""
+"Taler wallet disabled, please go back and either enable the wallet or pick "
+"another payment method.  Please keep the wallet enabled if you pick Taler "
+"again!"
+msgstr ""
+
+#: payments/taler/__init__.py:117
+msgid ""
+"Your wallet seems to have been disabled while processing this payment; your "
+"coins didn't get spent though.  Please go back in the checkout page and "
+"restart the payment with the method  you desire."
+msgstr ""
+
+#: payments/templates/payments/taler_fallback.html:5
+msgid "Payment unsuccessful"
+msgstr ""
+
 #: payments/utils.py:8
 msgid "Month"
 msgstr ""
diff --git a/payments/locale/it/LC_MESSAGES/django.po b/payments/locale/it/LC_MESSAGES/django.po
index bfc3f8c..cb5ca7a 100644
--- a/payments/locale/it/LC_MESSAGES/django.po
+++ b/payments/locale/it/LC_MESSAGES/django.po
@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PACKAGE VERSION\n"
 "Report-Msgid-Bugs-To: \n"
-"POT-Creation-Date: 2017-03-13 13:13+0100\n"
+"POT-Creation-Date: 2017-10-20 14:17+0200\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: Marco Badan <marco.badan@gmail.com>, 2017\n"
 "Language-Team: Italian (https://www.transifex.com/mirumee/teams/34782/it/)\n"
@@ -177,11 +177,11 @@ msgstr ""
 msgid "We were unable to complete the transaction. Please try again later."
 msgstr "Non siamo riusciti a completare la transazione. Riprova più tardi."
 
-#: payments/cybersource/forms.py:51
+#: payments/cybersource/forms.py:52
 msgid "fingerprint"
 msgstr "fingerprint"
 
-#: payments/cybersource/forms.py:68 payments/stripe/forms.py:52
+#: payments/cybersource/forms.py:69 payments/stripe/forms.py:52
 msgid "This payment has already been processed."
 msgstr "Questo pagamento è già stato processato."
 
@@ -226,6 +226,24 @@ msgstr "controllo frode"
 msgid "Total payment"
 msgstr "Totale pagamento"
 
+#: payments/taler/__init__.py:89
+msgid ""
+"Taler wallet disabled, please go back and either enable the wallet or pick "
+"another payment method.  Please keep the wallet enabled if you pick Taler "
+"again!"
+msgstr ""
+
+#: payments/taler/__init__.py:117
+msgid ""
+"Your wallet seems to have been disabled while processing this payment; your "
+"coins didn't get spent though.  Please go back in the checkout page and "
+"restart the payment with the method  you desire."
+msgstr ""
+
+#: payments/templates/payments/taler_fallback.html:5
+msgid "Payment unsuccessful"
+msgstr ""
+
 #: payments/utils.py:8
 msgid "Month"
 msgstr "Mese"
diff --git a/payments/locale/ru/LC_MESSAGES/django.po b/payments/locale/ru/LC_MESSAGES/django.po
index 31d44bc..d9b2e3e 100644
--- a/payments/locale/ru/LC_MESSAGES/django.po
+++ b/payments/locale/ru/LC_MESSAGES/django.po
@@ -8,7 +8,7 @@ msgid ""
 msgstr ""
 "Project-Id-Version: PACKAGE VERSION\n"
 "Report-Msgid-Bugs-To: \n"
-"POT-Creation-Date: 2017-03-13 13:12+0100\n"
+"POT-Creation-Date: 2017-10-12 06:57-0500\n"
 "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n"
 "Last-Translator: Weronika Terpilowska <weronika@mirumee.com>, 2017\n"
 "Language-Team: Russian (https://www.transifex.com/mirumee/teams/34782/ru/)\n"
@@ -174,11 +174,11 @@ msgstr ""
 msgid "We were unable to complete the transaction. Please try again later."
 msgstr "Мы не смогли провести транзакцию. Пожалуйста, попробуйте позже."
 
-#: payments/cybersource/forms.py:51
+#: payments/cybersource/forms.py:52
 msgid "fingerprint"
 msgstr "отпечаток пальца"
 
-#: payments/cybersource/forms.py:68 payments/stripe/forms.py:52
+#: payments/cybersource/forms.py:69 payments/stripe/forms.py:52
 msgid "This payment has already been processed."
 msgstr "Этот платеж уже был обработан."
 
@@ -223,6 +223,20 @@ msgstr "проверка транзакции"
 msgid "Total payment"
 msgstr "Общая сумма"
 
+#: payments/taler/__init__.py:92
+msgid ""
+"Taler wallet disabled, please go back and either enable the wallet or pick "
+"another payment method.  Please keep the wallet enabled if you pick Taler "
+"again!"
+msgstr ""
+
+#: payments/taler/__init__.py:120
+msgid ""
+"Your wallet seems to have been disabled while processing this payment; your "
+"coins didn't get spent though.  Please go back in the checkout page and "
+"restart the payment with the method  you desire."
+msgstr ""
+
 #: payments/utils.py:8
 msgid "Month"
 msgstr "Месяц"
diff --git a/payments/taler/__init__.py b/payments/taler/__init__.py
new file mode 100644
index 0000000..191150c
--- /dev/null
+++ b/payments/taler/__init__.py
@@ -0,0 +1,156 @@
+#  This file is part of DJANGO-PAYMENTS
+#  (C) 2017 Taler Systems SA
+#
+#  This library is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2.1 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library; if not, write to the Free Software
+#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+#
+#  @author Marcello Stanisci
+
+from django.utils.translation import ugettext as _
+from django.template.loader import render_to_string
+import json
+from django.shortcuts import redirect
+from ..core import BasicProvider, get_base_url
+from .. import RedirectNeeded, PaymentStatus
+import re
+from django.http import HttpResponse, JsonResponse
+import requests
+from urllib.parse import urljoin
+import logging
+from django.conf import settings
+from .amount import Amount, BadAmount
+
+logger = logging.getLogger(__name__)
+
+class TalerProvider(BasicProvider):
+    '''
+    GNU Taler payment provider
+    '''
+
+    def __init__(self, backend_url, address, name, jurisdiction, instance=None):
+
+        # The backend URL is served by the C backend, it is
+        # used to sign data coming from the frontend and to
+        # communicate with the exchange.
+        # Its URL must end with a slash, like 'http://backend.demo.taler.net/'.
+        self.backend_url = backend_url
+        # Token which identifies this frontend to the backend.  In fact,
+        # any backend can support multiple frontends, and 'None' is the
+        # default one.
+        self.instance = instance
+        # Physical address and jurisdiction, and shop name
+        self.address = address
+        self.name = name
+        self.jurisdiction = jurisdiction
+        super(TalerProvider, self)
+
+    # This function gets called when the user chooses the
+    # payment method to use.  It will redirect the user to
+    # the page returning 402+contract_url.
+    def get_form(self, payment, data=None):
+        raise RedirectNeeded(self.get_return_url(payment))
+
+    # Design note: in django-payments, there is usually ONE URL that
+    # processes the payment, thus decisions are taken on the basis of
+    # the _state_ of the payment, rather than a particular endpoint.
+    # For example, it is perfectly normal having fulfillment and pay
+    # URL being the same thing.
+
+    def process_data(self, payment, request):
+
+        # Very first branch taken.  It returns the 402 status
+        # plus the contract generation URL.
+        if payment.status == PaymentStatus.WAITING:
+            wallet_not_found_msg = _('Taler wallet disabled.  Please' \
+                ' either enable the wallet or pick another payment method.')
+            data = render_to_string('payments/taler_fallback.html',
+                                   {'msg': wallet_not_found_msg})
+            response = HttpResponse(data, status=402)
+
+            response['X-Taler-Contract-Url'] = self.get_return_url(payment)
+            payment.change_status(PaymentStatus.INPUT)
+            return response
+
+        # Listen for contract generation.
+        if payment.status == PaymentStatus.INPUT:
+            try:
+                total_amount = Amount.parse('%s:%0.2f' % (payment.currency, payment.total)).dump()
+                order = {
+                    'summary': payment.message,
+                    'nonce': request.GET.get('nonce'),
+                    'amount': total_amount,
+                    'products': [{'description': payment.description,
+                                  'quantity': 1,
+                                  'product_id': 0,
+                                  'price': total_amount}],
+                    'fulfillment_url': self.get_return_url(payment),
+                    'pay_url': self.get_return_url(payment),
+                    'merchant': {
+                        'instance': self.instance,
+                        'address': self.address,
+                        'name': self.name,
+                        'jurisdiction': self.jurisdiction},
+                    'extra': {}}
+            except BadAmount as e:
+                logger.error('Malformed amount: %s' % e.faulty_str)
+                data = {'error': _('Internal error generating contract'),
+                                   'detail': _('Could not parse amount')}
+                return JsonResponse(data, status=500)
+
+            try:
+                r = requests.post(urljoin(self.backend_url, 'proposal'),
+                                  json={'order': order})
+            except requests.RequestException as e:
+                logger.error(e)
+                return JsonResponse({'error': 'Internal server error',
+                    'detail': 'Could not reach the backend'}, status=500)
+            if r.status_code == 200:
+                payment.change_status(PaymentStatus.PREAUTH)
+            return JsonResponse(r.json(), status=r.status_code)
+
+        # This is responsible to both execute the payment and receive it.
+        # When the wallet attempts to GET it, it returns the 402 which
+        # executes the payment, whereas POSTing to it triggers the /pay behaviour.
+        if payment.status == PaymentStatus.PREAUTH:
+            if request.method == 'POST':
+                try:
+                    r = requests.post(urljoin(self.backend_url, 'pay'),
+                                      json=json.loads(request.body.decode('utf-8')))
+                except requests.RequestException as e:
+                    logger.error(e)
+                    return JsonResponse({'error': 'Internal server error',
+                        'detail': 'Could not reach the backend'}, status=500)
+                if r.status_code == 200:
+                    payment.change_status(PaymentStatus.CONFIRMED)
+                return JsonResponse(r.json(), status=r.status_code)
+
+            if request.method == 'GET':
+                wallet_not_found_msg = _('Taler wallet disabled.  Please' \
+                    ' either enable the wallet or pick another payment method.')
+                data = render_to_string('payments/taler_fallback.html',
+                                       {'msg': wallet_not_found_msg})
+                response = HttpResponse(data, status=402)
+                response['X-Taler-Contract-Url'] = self.get_return_url(payment)
+                response['X-Taler-Offer-Url'] = get_base_url()
+                return response 
+
+        # Taken when the payment has gone through, redirect to persistent
+        # fulfillment page.
+        if payment.status == PaymentStatus.CONFIRMED:
+            # returns absolute url
+            return redirect(payment.get_success_url())
+
+        # This should _never_ happen
+        return JsonResponse({'error': 'Internal server error',
+            'detail': 'Unknown payment status!'}, status=500)
diff --git a/payments/taler/amount.py b/payments/taler/amount.py
new file mode 100644
index 0000000..5bd2354
--- /dev/null
+++ b/payments/taler/amount.py
@@ -0,0 +1,120 @@
+#  This file is part of TALER
+#  (C) 2017 Taler Systems SA
+#
+#  This library is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2.1 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library; if not, write to the Free Software
+#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+#
+#  @author Marcello Stanisci
+#  @version 0.0
+#  @repository https://git.taler.net/copylib.git/
+#  This code is "copylib", it is versioned under the Git repository
+#  mentioned above, and it is meant to be manually copied into any project
+#  which might need it.
+
+class BadAmount(Exception):
+    def __init__(self, faulty_str):
+        self.faulty_str = faulty_str
+
+class Amount:
+    # How many "fraction" units make one "value" unit of currency
+    # (Taler requires 10^8).  Do not change this 'constant'.
+    @staticmethod
+    def FRACTION():
+        return 10 ** 8
+
+    @staticmethod
+    def MAX_VALUE():
+        return (2 ** 53) - 1
+
+    def __init__(self, currency, value=0, fraction=0):
+        # type: (str, int, int) -> Amount
+        assert(value >= 0 and fraction >= 0)
+        self.value = value
+        self.fraction = fraction
+        self.currency = currency
+        self.__normalize()
+        assert(self.value <= Amount.MAX_VALUE())
+
+    # Normalize amount
+    def __normalize(self):
+        if self.fraction >= Amount.FRACTION():
+            self.value += int(self.fraction / Amount.FRACTION())
+            self.fraction = self.fraction % Amount.FRACTION()
+
+    # Parse a string matching the format "A:B.C"
+    # instantiating an amount object.
+    @classmethod
+    def parse(cls, amount_str):
+        exp = '^\s*([-_*A-Za-z0-9]+):([0-9]+)\.([0-9]+)\s*$'
+        import re
+        parsed = re.search(exp, amount_str)
+        if not parsed:
+            raise BadAmount(amount_str)
+        value = int(parsed.group(2))
+        fraction = 0
+        for i, digit in enumerate(parsed.group(3)):
+            fraction += int(int(digit) * (Amount.FRACTION() / 10 ** (i+1)))
+        return cls(parsed.group(1), value, fraction)
+
+    # Comare two amounts, return:
+    # -1 if a < b
+    # 0 if a == b
+    # 1 if a > b
+    @staticmethod
+    def cmp(a, b):
+        assert a.currency == b.currency
+        if a.value == b.value:
+            if a.fraction < b.fraction:
+                return -1
+            if a.fraction > b.fraction:
+                return 1
+            return 0
+        if a.value < b.value:
+            return -1
+        return 1
+
+    # Add the given amount to this one
+    def add(self, a):
+        assert self.currency == a.currency
+        self.value += a.value
+        self.fraction += a.fraction
+        self.__normalize()
+
+    # Subtract passed amount from this one
+    def subtract(self, a):
+        assert self.currency == a.currency
+        if self.fraction < a.fraction:
+            self.fraction += Amount.FRACTION()
+            self.value -= 1
+        if self.value < a.value:
+            raise ValueError('self is lesser than amount to be subtracted')
+        self.value -= a.value
+        self.fraction -= a.fraction
+
+    # Dump string from this amount, will put 'ndigits' numbers
+    # after the dot.
+    def stringify(self, ndigits):
+        assert ndigits > 0
+        ret = '%s:%s.' % (self.currency, str(self.value))
+        f = self.fraction
+        for i in range(0, ndigits):
+            ret += str(int(f / (Amount.FRACTION() / 10)))
+            f = (f * 10) % (Amount.FRACTION())
+        return ret
+
+    # Dump the Taler-compliant 'dict' amount
+    def dump(self):
+        return dict(value=self.value,
+                    fraction=self.fraction,
+                    currency=self.currency)
diff --git a/payments/taler/test_amount.py b/payments/taler/test_amount.py
new file mode 100644
index 0000000..bd59c74
--- /dev/null
+++ b/payments/taler/test_amount.py
@@ -0,0 +1,63 @@
+#  This file is part of TALER
+#  (C) 2017 Taler Systems SA
+#
+#  This library is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2.1 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library; if not, write to the Free Software
+#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+#
+#  @author Marcello Stanisci
+#  @version 0.0
+#  @repository https://git.taler.net/copylib.git/
+#  This code is "copylib", it is versioned under the Git repository
+#  mentioned above, and it is meant to be manually copied into any project
+#  which might need it.
+
+from __future__ import unicode_literals
+from .amount import Amount, BadAmount
+from unittest import TestCase
+import json
+from mock import MagicMock
+
+class TestAmount(TestCase):
+    def setUp(self):
+        self.amount = Amount('TESTKUDOS')
+
+    def test_parse_and_cmp(self):
+        a = self.amount.parse('TESTKUDOS:0.0')
+        self.assertEqual(Amount.cmp(self.amount, a), 0)
+        b = self.amount.parse('TESTKUDOS:0.1')
+        self.assertEqual(Amount.cmp(Amount('TESTKUDOS', fraction=10000000), b), 0)
+        c = self.amount.parse('TESTKUDOS:3.3')
+        self.assertEqual(Amount.cmp(Amount('TESTKUDOS', 3, 30000000), c), 0)
+        self.assertEqual(Amount.cmp(a, b), -1)
+        self.assertEqual(Amount.cmp(c, b), 1)
+        with self.assertRaises(BadAmount):
+            Amount.parse(':3')
+
+    def test_add_and_dump(self):
+        mocky = MagicMock()
+        self.amount.add(Amount('TESTKUDOS', 9, 10**8))
+        mocky(**self.amount.dump())
+        mocky.assert_called_with(currency='TESTKUDOS', value=10, fraction=0)
+
+    def test_subtraction(self):
+        with self.assertRaises(ValueError):
+            self.amount.subtract(Amount('TESTKUDOS', fraction=1))
+        a = Amount('TESTKUDOS', 2)
+        a.subtract(Amount('TESTKUDOS', 1, 99999999))
+        self.assertEqual(Amount.cmp(a, Amount('TESTKUDOS', fraction=1)), 0)
+
+    def test_stringify(self):
+        self.assertEqual(self.amount.stringify(3), 'TESTKUDOS:0.000')
+        self.amount.add(Amount('TESTKUDOS', 2, 100))
+        self.assertEqual(self.amount.stringify(6), 'TESTKUDOS:2.000001')
diff --git a/payments/taler/test_taler.py b/payments/taler/test_taler.py
new file mode 100644
index 0000000..e2a5ce2
--- /dev/null
+++ b/payments/taler/test_taler.py
@@ -0,0 +1,155 @@
+#  This file is part of DJANGO-PAYMENTS
+#  (C) 2017 Taler Systems SA
+#
+#  This library is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU Lesser General Public
+#  License as published by the Free Software Foundation; either
+#  version 2.1 of the License, or (at your option) any later version.
+#
+#  This library is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+#  Lesser General Public License for more details.
+#
+#  You should have received a copy of the GNU Lesser General Public
+#  License along with this library; if not, write to the Free Software
+#  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+#
+#  @author Marcello Stanisci
+
+from __future__ import unicode_literals
+import json
+from unittest import TestCase
+from . import TalerProvider
+from mock import patch, MagicMock, Mock
+from .. import RedirectNeeded, PaymentStatus
+from decimal import Decimal
+
+class Payment(Mock):
+    status = PaymentStatus.WAITING
+    message = 'mocked payment'
+    total = Decimal(10)
+    description = 'mocked payment object'
+    instance = 'mock instance'
+    address = 'mock address'
+    name = 'mock name'
+    jurisdiction = 'mock jurisdiction'
+    currency = 'MOCK'
+
+    def get_process_url(self):
+        return 'mock-process-url'
+
+    def get_success_url(self):
+        return 'http://example.com/mock-success'
+
+    def change_status(self, status):
+        self.status = status
+
+class TestTalerProvider(TestCase):
+
+    def setUp(self):
+        self.payment = Payment()
+        self.provider = TalerProvider(backend_url='http://mocked_backend_url',
+            instance='mock_instance', address='mock address',
+            name='mock name', jurisdiction='mock jurisdiction')
+
+    # This tests the very first redirect, then one that
+    # redirects to the first 402 page.
+    def test_provider_redirects_to_contract_url(self):
+        with self.assertRaises(RedirectNeeded):
+            self.provider.get_form(self.payment)
+
+    # Test whether a contract generation url is returned
+    # along with the 402 status code.
+    def test_provider_contract_generation_header(self):
+        response = self.provider.process_data(self.payment, MagicMock())
+        self.assertTrue(response.get('X-Taler-Contract-Url'))
+        self.assertFalse(response.get('X-Taler-Offer-Url'))
+        self.assertEqual(response.status_code, 402)
+        self.assertEqual(self.payment.status, PaymentStatus.INPUT)
+
+    # Test whether the frontend logic interacts well with backend.
+    @patch('requests.post')
+    def test_provider_contract_generation(self, mocked_post):
+        request = MagicMock()
+        request.GET = {'nonce': 'nonce098'}
+        data = MagicMock()
+        data.return_value = {'not': 'changed'}
+        post = MagicMock()
+        post.status_code = 200
+        post.json = data
+        mocked_post.return_value = post
+        self.payment.change_status(PaymentStatus.INPUT)
+        r = self.provider.process_data(self.payment, request)
+        total_amount = {
+            'value': 10,
+            'fraction': 0,
+            'currency': self.payment.currency}
+        expected_order = {'order': {
+            'summary': self.payment.message,
+            'nonce': 'nonce098',
+            'amount': total_amount,
+            'products': [{
+                'description': self.payment.description,
+                'quantity': 1,
+                'product_id': 0,
+                'price': total_amount}],
+            'fulfillment_url': 'https://example.com/mock-process-url',
+            'pay_url': 'https://example.com/mock-process-url',
+            'merchant': {
+                'instance': self.provider.instance,
+                'address': self.provider.address,
+                'name': self.provider.name,
+                'jurisdiction': self.provider.jurisdiction},
+                'extra': {}}}
+        mocked_post.assert_called_with('http://mocked_backend_url/proposal',
+                                       json=expected_order)
+        self.assertEqual(self.payment.status, PaymentStatus.PREAUTH)
+        self.assertEqual(r.status_code, 200)
+        self.assertEqual(json.dumps(data.return_value), r.content.decode('utf-8'))
+
+    # Testing second returned 402, the one returned by GETting the
+    # process url having the PREAUTH status.  This logic is triggered
+    # rigth after the user confirms the payment.
+    def test_provider_trigger_payment(self):
+        request = MagicMock()
+        request.method = 'GET'
+        self.payment.change_status(PaymentStatus.PREAUTH)
+        r = self.provider.process_data(self.payment, request)
+        self.assertEqual(r.status_code, 402)
+        self.assertTrue(r.get('X-Taler-Contract-Url'))
+        self.assertTrue(r.get('X-Taler-Offer-Url'))
+
+    # Testing coins receiving logic.
+    @patch('requests.post')
+    def test_provider_receive_payment(self, mocked_post):
+        data = MagicMock()
+        data.return_value = {'not': 'changed'}
+        post = MagicMock()
+        post.status_code = 200
+        post.json = data
+        mocked_post.return_value = post
+        self.payment.change_status(PaymentStatus.PREAUTH)
+        request = MagicMock()
+        request.method = 'POST'
+        request.body = b'{"mock": "coins"}'
+        r = self.provider.process_data(self.payment, request)
+        self.assertEqual(r.status_code, 200)
+        self.assertEqual(self.payment.status, PaymentStatus.CONFIRMED)
+        mocked_post.assert_called_with('http://mocked_backend_url/pay',
+            json={'mock': 'coins'})
+        self.assertEqual(json.dumps(data.return_value), r.content.decode('utf-8'))
+
+    # Test whether successful payment gets redirected to
+    # "success url"
+    @patch('payments.taler.redirect')
+    def test_provider_success_payment(self, mocked_redirect):
+        self.payment.change_status(PaymentStatus.CONFIRMED)
+        self.provider.process_data(self.payment, MagicMock())
+        mocked_redirect.assert_called_with('http://example.com/mock-success')
+
+    # Test unknown payment status, should never happen
+    def test_provider_unknown_payment_status(self):
+        self.payment.change_status("does not exist")
+        r = self.provider.process_data(self.payment, MagicMock())
+        self.assertEqual(r.status_code, 500)
diff --git a/payments/templates/payments/taler_fallback.html b/payments/templates/payments/taler_fallback.html
new file mode 100644
index 0000000..e84170a
--- /dev/null
+++ b/payments/templates/payments/taler_fallback.html
@@ -0,0 +1,8 @@
+<!DOCTYPE html>
+{% load i18n %}
+{% get_current_language as LANGUAGE_CODE %}
+
+<html lang="{{ LANGUAGE_CODE }}">
+  <title>{% trans "Payment unsuccessful" %}</title>
+  <body>{{ msg }}</body>
+</html>