Add new signal voucher_form_validation

doc/development/api/general.rst

@@ -57,7 +57,7 @@ Vouchers
 """"""""
 
 .. automodule:: pretix.control.signals
-   :members: voucher_form_class, voucher_form_html
+   :members: voucher_form_class, voucher_form_html, voucher_form_validation
 
 Dashboards
 """"""""""

src/pretix/control/forms/vouchers.py

@@ -8,6 +8,7 @@ from django.utils.translation import pgettext_lazy, ugettext_lazy as _
 
 from pretix.base.forms import I18nModelForm
 from pretix.base.models import Item, ItemVariation, Quota, Voucher
+from pretix.control.signals import voucher_form_validation
 
 
 class VoucherForm(I18nModelForm):
@@ -121,6 +122,8 @@ class VoucherForm(I18nModelForm):
         if 'code' in data and Voucher.objects.filter(Q(code=data['code']) & Q(event=self.instance.event) & ~Q(pk=self.instance.pk)).exists():
             raise ValidationError(_('A voucher with this code already exists.'))
 
+        voucher_form_validation.send(sender=self.instance.event, form=self, data=data)
+
         return data
 
     def _clean_quota_needs_checking(self, data):

src/pretix/control/signals.py

@@ -133,6 +133,17 @@ You will receive the default form class (or the class set by a previous plugin)
 As with all plugin signals, the ``sender`` keyword argument will contain the event.
 """
 
+voucher_form_validation = EventPluginSignal(
+    providing_args=['form']
+)
+"""
+This signal allows you to add additional validation to the form that is used for
+creating and modifying vouchers. You will receive the form instance in the ``form``
+argument and the current data state in the ``data`` argument.
+
+As with all plugin signals, the ``sender`` keyword argument will contain the event.
+"""
+
 quota_detail_html = EventPluginSignal(
     providing_args=['quota']
 )