Allow more decimal places for tax rates (#6092)

* Allow more decimal places for tax rates

* Fix normalization

* More tax rate handling

* Add uncommitted tests

* Rebase migration

* Update src/pretix/base/migrations/0304_tax_rate_decimals.py

Co-authored-by: robbi5 <richt@rami.io>

---------

Co-authored-by: robbi5 <richt@rami.io>

src/pretix/api/serializers/__init__.py

@@ -20,6 +20,7 @@
 # <https://www.gnu.org/licenses/>.
 #
 import json
+import re
 
 from django.db.models import prefetch_related_objects
 from rest_framework import serializers
@@ -135,3 +136,30 @@ class SalesChannelMigrationMixin:
         else:
             value["sales_channels"] = value["limit_sales_channels"]
         return value
+
+
+class CompatDecimalField(serializers.DecimalField):
+    """
+    Historically, pretix recorded tax rates as decimals with two places. Today, pretix supports tax rates with up to
+    four places. Since our API outputs decimals with the stored precision, this would have changed the API output from
+    "19.00" to "19.0000" without warning. While this is semantically the same thing, we need to assume some pretix API
+    users might run into trouble, either because they treat the value as a string and then map something
+    (e.g. ``if tax_rate == "19.00"``) or process it with a language where this is a significant difference. For example,
+    while in Python ``Decimal("19.00") == Decimal("19.0000")`` is true, in Java
+    ``(new BigDecimal("19.00")).equals(new BigDecimal("19.0000"))`` is false and only
+    ``(new BigDecimal("19.00")).compareTo(new BigDecimal("19.0000")) == 0`` is true.
+
+    Therefore, we stay backwards compatible by outputting two decimal places *as long as the trailing digits are zero-valued.
+    """
+
+    regex = re.compile(r"^([0-9]+\.[0-9]{2})0+$")
+
+    def to_representation(self, value):
+        if self.localize:
+            raise ValueError("localization not supported")
+        value = super().to_representation(value)
+        if value and "." not in value:
+            return f"{value}.00"
+        if m := self.regex.match(value):
+            return m.group(1)
+        return value