API: Allow to simulate check-ins (#6360)

* API: Allow to simulate check-ins

* Add missing file

doc/api/resources/checkin.rst

@@ -71,6 +71,8 @@ Checking a ticket in
    :>json object questions: List of questions to be answered for check-in, only set on status ``"incomplete"``.
    :>json object media_policy: Reusable media policy (see documentation on items), only set on status ``"exchange"``.
    :>json object media_type: Reusable media type (see documentation on items), only set on status ``"exchange"``.
+   :>json boolean simulate: Do not actually perform the check-in, only simulate the response. The ``position`` response
+                            object will not reflect the simulated changes.
 
    **Example request**:
 

src/pretix/api/serializers/checkin.py

@@ -90,6 +90,7 @@ class CheckinRPCRedeemInputSerializer(serializers.Serializer):
     answers = serializers.JSONField(required=False, allow_null=True)
     exchange_medium_type = serializers.ChoiceField(required=False, choices=MEDIA_TYPES)
     exchange_medium_identifier = serializers.CharField(required=False)
+    simulate = serializers.BooleanField(default=False, required=False)
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)

src/pretix/api/views/checkin.py

@@ -839,6 +839,11 @@ def _redeem_process(*, checkinlists, raw_barcode, answers_data, datetime, force,
             )
 
             if exchange_medium_identifier:  # other fields are filled, see CheckinRPCRedeemInputSerializer.validate
+                if simulate:
+                    raise CheckInError(
+                        gettext('You cannot simulate a medium exchange.'),
+                        'error'
+                    )
                 with transaction.atomic():
                     # Do exchange and check-in atomically, i.e. both succeed or both fail
                     medium = perform_media_exchange(
@@ -1066,6 +1071,7 @@ class CheckinRPCRedeemView(views.APIView):
             legacy_url_support=False,
             exchange_medium_type=s.validated_data.get('exchange_medium_type'),
             exchange_medium_identifier=s.validated_data.get('exchange_medium_identifier'),
+            simulate=s.validated_data.get('simulate'),
         )
 
 

src/pretix/base/services/checkin.py

@@ -40,7 +40,7 @@ import dateutil
 import dateutil.parser
 from dateutil.tz import datetime_exists
 from django.core.files import File
-from django.db import IntegrityError, transaction
+from django.db import IntegrityError
 from django.db.models import (
     BooleanField, Case, Count, ExpressionWrapper, F, IntegerField, Max, Min,
     OuterRef, Q, Subquery, TextField, Value, When,
@@ -59,6 +59,7 @@ from pretix.base.models import (
 )
 from pretix.base.signals import checkin_created, periodic_task
 from pretix.helpers import OF_SELF
+from pretix.helpers.database import conditional_atomic
 from pretix.helpers.jsonlogic import Logic
 from pretix.helpers.jsonlogic_boolalg import convert_to_dnf
 from pretix.helpers.jsonlogic_query import (
@@ -1043,10 +1044,10 @@ def perform_checkin(op: OrderPosition, clist: CheckinList, given_answers: dict,
         if not simulate:
             _save_answers(op, answers, given_answers)
 
-    with transaction.atomic():
+    with conditional_atomic(not simulate):
         # Lock order positions, if it is an entry. We don't need it for exits, as a race condition wouldn't be problematic
         opqs = OrderPosition.all.select_related("order", "item")
-        if type != Checkin.TYPE_EXIT:
+        if type != Checkin.TYPE_EXIT and not simulate:
             opqs = opqs.select_for_update(of=OF_SELF)
         op = opqs.get(pk=op.pk)
 

src/pretix/helpers/database.py

@@ -288,6 +288,15 @@ def get_deterministic_ordering(model, ordering):
     return ordering
 
 
+@contextlib.contextmanager
+def conditional_atomic(do_atomic, **kwargs):
+    if do_atomic:
+        with transaction.atomic(**kwargs):
+            yield
+    else:
+        yield
+
+
 class IgnoreOnSQLiteMixin:
     # Mixin to allow defining PostgreSQL-specific indexes that will just not be created
     # on SQLite. SQLite is supported for testing only anyways!

src/pretix/testutils/db.py

@@ -0,0 +1,25 @@
+#
+# This file is part of pretix (Community Edition).
+#
+# Copyright (C) 2014-2020  Raphael Michel and contributors
+# Copyright (C) 2020-today pretix GmbH and contributors
+#
+# This program is free software: you can redistribute it and/or modify it under the terms of the GNU Affero General
+# Public License as published by the Free Software Foundation in version 3 of the License.
+#
+# ADDITIONAL TERMS APPLY: Pursuant to Section 7 of the GNU Affero General Public License, additional terms are
+# applicable granting you additional permissions and placing additional restrictions on your usage of this software.
+# Please refer to the pretix LICENSE file to obtain the full terms applicable to this work. If you did not receive
+# this file, see <https://pretix.eu/about/en/license>.
+#
+# This program 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 Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
+# <https://www.gnu.org/licenses/>.
+#
+def readonly_db(execute, sql, params, many, context):
+    if not sql.lower().startswith("select"):
+        raise Exception(f"Should not write anything to the database, but detected query: {sql}")
+    return execute(sql, params, many, context)

src/tests/api/test_checkinrpc.py

@@ -25,6 +25,7 @@ from unittest import mock
 
 import pytest
 from django.core.files.base import ContentFile
+from django.db import connection
 from django.utils.timezone import now
 from django_countries.fields import Country
 from django_scopes import scopes_disabled
@@ -36,6 +37,7 @@ from pretix.api.serializers.item import QuestionSerializer
 from pretix.base.models import (
     Checkin, InvoiceAddress, Item, Order, OrderPosition, ReusableMedium,
 )
+from pretix.testutils.db import readonly_db
 
 # Lots of this code is overlapping with test_checkin.py, and some of it is arguably redundant since it's triggering
 # the same backend code paths (for now). However, this is SUCH a critical part of pretix that we don't want to take
@@ -1739,3 +1741,41 @@ def test_exchange_create_gift_card(token_client, organizer, clist, event, order,
     with scopes_disabled():
         rm = ReusableMedium.objects.get(identifier="0412345")
         assert rm.linked_giftcard.currency == "EUR"
+
+
+@pytest.mark.django_db
+def test_simulate(token_client, organizer, clist, event, order):
+    with scopes_disabled():
+        p = order.positions.first()
+    with connection.execute_wrapper(readonly_db):
+        resp = _redeem(token_client, organizer, clist, p.secret, {"simulate": True})
+    assert resp.status_code == 201
+    assert resp.data['status'] == 'ok'
+    with scopes_disabled():
+        assert not p.checkins.exists()
+
+
+@pytest.mark.django_db
+def test_simulate_no_exchange(token_client, organizer, clist, event, order, item):
+    organizer.settings.reusable_media_type_nfc_uid = True
+    item.media_type = "nfc_uid"
+    item.media_policy = Item.MEDIA_POLICY_NEW
+    item.save()
+    with scopes_disabled():
+        rm = ReusableMedium.objects.create(
+            type="nfc_uid",
+            identifier="12345678",
+            organizer=organizer,
+        )
+    with connection.execute_wrapper(readonly_db):
+        resp = _redeem(token_client, organizer, clist, "z3fsn8jyufm5kpk768q69gkbyr5f4h6w", {
+            "source_type": "barcode",
+            "exchange_medium_type": "nfc_uid",
+            "exchange_medium_identifier": "12345678",
+            "simulate": True,
+        })
+    assert resp.status_code == 400
+    assert resp.data['status'] == 'error'
+    assert resp.data['reason'] == 'error'
+    with scopes_disabled():
+        assert not rm.linked_orderpositions.exists()