API: Allow to simulate check-ins

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

@@ -837,6 +837,11 @@ 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(
@@ -1064,6 +1069,7 @@ 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'),
         )