API: Fix crash on distributing a fee over tax rates with zero value (Z#23196669) (#5226)

doc/api/resources/orders.rst

@@ -1059,9 +1059,10 @@ Creating orders
         prices. Note that this will not include other fees and is calculated once during order generation and will not
         be respected automatically when the order changes later.)
       * ``_split_taxes_like_products`` (Optional convenience flag. If set to ``true``, your ``tax_rule`` will be ignored
-        and the fee will be taxed like the products in the order. If the products have multiple tax rates, multiple fees
-        will be generated with weights adjusted to the net price of the products. Note that this will be calculated once
-        during order generation and is not respected automatically when the order changes later.)
+        and the fee will be taxed like the products in the order *unless* the total amount of the positions is zero.
+        If the products have multiple tax rates, multiple fees will be generated with weights adjusted to the net price
+        of the products. Note that this will be calculated once during order generation and is not respected automatically
+        when the order changes later.)
 
    * ``force`` (optional). If set to ``true``, quotas will be ignored.
    * ``send_email`` (optional). If set to ``true``, the same emails will be sent as for a regular order, regardless of

src/pretix/api/serializers/order.py

@@ -1600,7 +1600,7 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
                                                   self.context['event'].currency)
             is_split_taxes = fee_data.pop('_split_taxes_like_products', False)
 
-            if is_split_taxes:
+            if is_split_taxes and order.total:
                 d = defaultdict(lambda: Decimal('0.00'))
                 trz = TaxRule.zero()
                 for p in pos_map.values():

src/tests/api/test_order_create.py

@@ -961,6 +961,42 @@ def test_order_create_fee_as_percentage(token_client, organizer, event, item, qu
         assert o.total == Decimal('25.30')
 
 
+@pytest.mark.django_db
+def test_order_create_fee_as_percentage_with_zero(token_client, organizer, event, item, quota, question):
+    with scopes_disabled():
+        voucher = event.vouchers.create(price_mode="set", value=Decimal("0.00"))
+    res = copy.deepcopy(ORDER_CREATE_PAYLOAD)
+    res['fees'][0]['_treat_value_as_percentage'] = True
+    res['fees'][0]['_split_taxes_like_products'] = True
+    res['fees'][0]['value'] = '10.00'
+    res['positions'][0]['item'] = item.pk
+    res['positions'][0]['answers'][0]['question'] = question.pk
+    res['positions'][0]['voucher'] = voucher.code
+    del res['positions'][0]['price']
+
+    res['simulate'] = True
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orders/'.format(
+            organizer.slug, event.slug
+        ), format='json', data=res
+    )
+    assert resp.status_code == 201
+    assert resp.data["total"] == "0.00"
+
+    res['simulate'] = False
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orders/'.format(
+            organizer.slug, event.slug
+        ), format='json', data=res
+    )
+    assert resp.status_code == 201
+    with scopes_disabled():
+        o = Order.objects.get(code=resp.data['code'])
+        fee = o.fees.first()
+        assert fee.value == Decimal('0.00')
+        assert o.total == Decimal('0.00')
+
+
 @pytest.mark.django_db
 def test_order_create_fee_with_auto_tax(token_client, organizer, event, item, quota, question, taxrule):
     res = copy.deepcopy(ORDER_CREATE_PAYLOAD)