Event cancellation: Add safety and security checks (#5565)

* Event cancellation: Add safety and security checks

When cancelling an event, a large sum of money might be refunded
instantly. This PR adds safety features around this by

- doing a dry-run first that shows a preview of the expected refund sum

- sending a confirmation mode via email for any automatic refunds of more than 100 currency units

- keeping a more detailed log of the settings this was executed with

* Update src/pretix/control/views/orders.py

Co-authored-by: luelista <weller@rami.io>

---------

Co-authored-by: luelista <weller@rami.io>

src/pretix/control/views/orders.py

@@ -42,6 +42,7 @@ from datetime import datetime, time, timedelta
 from decimal import Decimal, DecimalException
 from urllib.parse import quote, urlencode
 
+from celery.result import AsyncResult
 from django import forms
 from django.conf import settings
 from django.contrib import messages
@@ -122,8 +123,8 @@ from pretix.control.forms.filter import (
     RefundFilterForm,
 )
 from pretix.control.forms.orders import (
-    CancelForm, CommentForm, DenyForm, EventCancelForm, ExporterForm,
-    ExtendForm, MarkPaidForm, OrderContactForm, OrderFeeAddForm,
+    CancelForm, CommentForm, DenyForm, EventCancelConfirmForm, EventCancelForm,
+    ExporterForm, ExtendForm, MarkPaidForm, OrderContactForm, OrderFeeAddForm,
     OrderFeeAddFormset, OrderFeeChangeForm, OrderLocaleForm, OrderMailForm,
     OrderPositionAddForm, OrderPositionAddFormset, OrderPositionChangeForm,
     OrderPositionMailForm, OrderRefundForm, OtherOperationsForm,
@@ -2975,10 +2976,99 @@ class EventCancel(EventPermissionRequiredMixin, AsyncAction, FormView):
             send_waitinglist_subject=form.cleaned_data.get('send_waitinglist_subject').data,
             send_waitinglist_message=form.cleaned_data.get('send_waitinglist_message').data,
             user=self.request.user.pk,
+            dry_run=settings.HAS_CELERY,
+        )
+
+    def get_context_data(self, **kwargs):
+        return super().get_context_data(
+            dry_run_supported=settings.HAS_CELERY,
         )
 
     def get_success_message(self, value):
-        if value == 0:
+        if value["dry_run"]:
+            return None
+        elif value["failed"] == 0:
+            return _('All orders have been canceled.')
+        else:
+            return _('The orders have been canceled. An error occurred with {count} orders, please '
+                     'check all uncanceled orders.').format(count=value)
+
+    def get_success_url(self, value):
+        if settings.HAS_CELERY:
+          return reverse('control:event.cancel.confirm', kwargs={
+              'organizer': self.request.organizer.slug,
+              'event': self.request.event.slug,
+              'task': value["id"],
+          })
+        else:
+          return reverse('control:event.cancel', kwargs={
+              'organizer': self.request.organizer.slug,
+              'event': self.request.event.slug,
+          })
+
+    def get_error_url(self):
+        return reverse('control:event.cancel', kwargs={
+            'organizer': self.request.organizer.slug,
+            'event': self.request.event.slug,
+        })
+
+    def get_error_message(self, exception):
+        if isinstance(exception, str):
+            return exception
+        return super().get_error_message(exception)
+
+    def form_invalid(self, form):
+        messages.error(self.request, _('Your input was not valid.'))
+        return super().form_invalid(form)
+
+
+class EventCancelConfirm(EventPermissionRequiredMixin, AsyncAction, FormView):
+    template_name = 'pretixcontrol/orders/cancel_confirm.html'
+    permission = 'can_change_orders'
+    form_class = EventCancelConfirmForm
+    task = cancel_event
+    known_errortypes = ['OrderError']
+
+    @cached_property
+    def dryrun_result(self):
+        res = AsyncResult(self.kwargs.get("task"))
+        if not res.ready():
+            raise Http404()
+        if not res.successful():
+            raise Http404()
+        data = res.info
+        if not data.get("dry_run"):
+            raise Http404()
+        if data.get("args")[0] != self.request.event.pk:
+            raise Http404()
+        return data
+
+    def get(self, request, *args, **kwargs):
+        if 'async_id' in request.GET and settings.HAS_CELERY:
+            return self.get_result(request)
+        return FormView.get(self, request, *args, **kwargs)
+
+    def get_form_kwargs(self):
+        k = super().get_form_kwargs()
+        k['confirmation_code'] = self.dryrun_result["confirmation_code"]
+        return k
+
+    def form_valid(self, form):
+        return self.do(
+            *self.dryrun_result["args"],
+            **{
+                **self.dryrun_result["kwargs"],
+                "dry_run": False,
+            },
+        )
+
+    def get_context_data(self, **kwargs):
+        return super().get_context_data(
+            dryrun_result=self.dryrun_result,
+        )
+
+    def get_success_message(self, value):
+        if value["failed"] == 0:
             return _('All orders have been canceled.')
         else:
             return _('The orders have been canceled. An error occurred with {count} orders, please '