Improve edge cases in handling of check-in nonces (#3516)

src/pretix/api/serializers/order.py

@@ -284,11 +284,12 @@ class FailedCheckinSerializer(I18nAwareModelSerializer):
     raw_item = serializers.PrimaryKeyRelatedField(queryset=Item.objects.none(), required=False, allow_null=True)
     raw_variation = serializers.PrimaryKeyRelatedField(queryset=ItemVariation.objects.none(), required=False, allow_null=True)
     raw_subevent = serializers.PrimaryKeyRelatedField(queryset=SubEvent.objects.none(), required=False, allow_null=True)
+    nonce = serializers.CharField(required=False, allow_null=True)
 
     class Meta:
         model = Checkin
         fields = ('error_reason', 'error_explanation', 'raw_barcode', 'raw_item', 'raw_variation',
-                  'raw_subevent', 'datetime', 'type', 'position')
+                  'raw_subevent', 'nonce', 'datetime', 'type', 'position')
 
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)

src/pretix/api/views/checkin.py

@@ -164,8 +164,21 @@ class CheckinListViewSet(viewsets.ModelViewSet):
                 secret=serializer.validated_data['raw_barcode']
             ).first()
 
+        clist = self.get_object()
+        if serializer.validated_data.get('nonce'):
+            if kwargs.get('position'):
+                prev = kwargs['position'].all_checkins.filter(nonce=serializer.validated_data['nonce']).first()
+            else:
+                prev = clist.checkins.filter(
+                    nonce=serializer.validated_data['nonce'],
+                    raw_barcode=serializer.validated_data['raw_barcode'],
+                ).first()
+            if prev:
+                # Ignore because nonce is already handled
+                return Response(serializer.data, status=201)
+
         c = serializer.save(
-            list=self.get_object(),
+            list=clist,
             successful=False,
             forced=True,
             force_sent=True,