Data shredder optimizations (#3429)

Co-authored-by: Martin Gross <gross@rami.io>

src/pretix/control/templates/pretixcontrol/base.html

@@ -470,6 +470,8 @@
                         <div class="progress-bar progress-bar-success">
                         </div>
                     </div>
+                    <div class="steps">
+                    </div>
                 </div>
             </div>
         </div>

src/pretix/control/templates/pretixcontrol/shredder/download.html

@@ -8,7 +8,7 @@
         {% trans "Data shredder" %}
     </h1>
     <form action="{% url "control:event.shredder.shred" event=request.event.slug organizer=request.organizer.slug %}"
-          method="post" class="form-horizontal" data-asynctask>
+          method="post" class="form-horizontal" data-asynctask data-asynctask-long>
         {% csrf_token %}
         <fieldset>
             {% if download_on_shred %}
@@ -55,6 +55,12 @@
             </fieldset>
         {% endif %}
         <input type="hidden" name="file" value="{{ file.pk }}">
+        <div class="alert alert-info">
+            {% blocktrans trimmed %}
+                Depending on the amount of data in your event, the following step may take a while to complete.
+                We will inform you via email once it has been completed.
+            {% endblocktrans %}
+        </div>
         <div class="form-group submit-group">
             <button type="submit" class="btn btn-primary btn-save">
                 {% trans "Continue" %}

src/pretix/control/views/shredder.py

@@ -37,10 +37,11 @@ import logging
 from collections import OrderedDict
 from zipfile import ZipFile
 
-from django.shortcuts import get_object_or_404
+from django.contrib import messages
+from django.shortcuts import get_object_or_404, redirect
 from django.urls import reverse
 from django.utils.functional import cached_property
-from django.utils.translation import gettext_lazy as _
+from django.utils.translation import get_language, gettext_lazy as _
 from django.views import View
 from django.views.generic import TemplateView
 
@@ -62,6 +63,16 @@ class ShredderMixin:
             sorted(self.request.event.get_data_shredders().items(), key=lambda s: s[1].verbose_name)
         )
 
+    def dispatch(self, request, *args, **kwargs):
+        try:
+            return super().dispatch(request, *args, **kwargs)
+        except ShredError as e:
+            messages.error(request, str(e))
+            return redirect(reverse('control:event.shredder.start', kwargs={
+                'event': self.request.event.slug,
+                'organizer': self.request.event.organizer.slug
+            }))
+
 
 class StartShredView(RecentAuthenticationRequiredMixin, EventPermissionRequiredMixin, ShredderMixin, TemplateView):
     permission = 'can_change_orders'
@@ -167,4 +178,5 @@ class ShredDoView(RecentAuthenticationRequiredMixin, EventPermissionRequiredMixi
         if request.event.slug != request.POST.get("slug"):
             return self.error(ShredError(_("The slug you entered was not correct.")))
 
-        return self.do(self.request.event.id, request.POST.get("file"), request.POST.get("confirm_code"))
+        return self.do(self.request.event.id, request.POST.get("file"), request.POST.get("confirm_code"),
+                       self.request.user.pk, get_language())