Fix #1301 -- Do not export empty files

src/pretix/base/exporters/answers.py

@@ -40,6 +40,7 @@ class AnswerFilesExporter(BaseExporter):
         if form_data.get('questions'):
             qs = qs.filter(question__in=form_data['questions'])
         with tempfile.TemporaryDirectory() as d:
+            any = False
             with ZipFile(os.path.join(d, 'tmp.zip'), 'w') as zipf:
                 for i in qs:
                     if i.file:
@@ -51,9 +52,12 @@ class AnswerFilesExporter(BaseExporter):
                             i.question.pk,
                             os.path.basename(i.file.name).split('.', 1)[1]
                         )
+                        any = True
                         zipf.writestr(fname, i.file.read())
                         i.file.close()
 
+            if not any:
+                return None
             with open(os.path.join(d, 'tmp.zip'), 'rb') as zipf:
                 return '{}_answers.zip'.format(self.event.slug), 'application/zip', zipf.read()
 

src/pretix/base/exporters/invoices.py

@@ -46,6 +46,7 @@ class InvoiceExporter(BaseExporter):
             qs = qs.filter(date__lte=date_value)
 
         with tempfile.TemporaryDirectory() as d:
+            any = False
             with ZipFile(os.path.join(d, 'tmp.zip'), 'w') as zipf:
                 for i in qs:
                     try:
@@ -54,14 +55,19 @@ class InvoiceExporter(BaseExporter):
                             i.refresh_from_db()
                         i.file.open('rb')
                         zipf.writestr('{}.pdf'.format(i.number), i.file.read())
+                        any = True
                         i.file.close()
                     except FileNotFoundError:
                         invoice_pdf_task.apply(args=(i.pk,))
                         i.refresh_from_db()
                         i.file.open('rb')
                         zipf.writestr('{}.pdf'.format(i.number), i.file.read())
+                        any = True
                         i.file.close()
 
+            if not any:
+                return None
+
             with open(os.path.join(d, 'tmp.zip'), 'rb') as zipf:
                 return '{}_invoices.zip'.format(self.event.slug), 'application/zip', zipf.read()
 

src/pretix/base/services/export.py

@@ -2,15 +2,20 @@ from typing import Any, Dict
 
 from django.core.files.base import ContentFile
 from django.utils.timezone import override
+from django.utils.translation import ugettext
 
-from pretix.base.i18n import language
+from pretix.base.i18n import LazyLocaleException, language
 from pretix.base.models import CachedFile, Event, cachedfile_name
 from pretix.base.services.tasks import ProfiledTask
 from pretix.base.signals import register_data_exporters
 from pretix.celery_app import app
 
 
-@app.task(base=ProfiledTask)
+class ExportError(LazyLocaleException):
+    pass
+
+
+@app.task(base=ProfiledTask, throws=(ExportError,))
 def export(event: str, fileid: str, provider: str, form_data: Dict[str, Any]) -> None:
     event = Event.objects.get(id=event)
     file = CachedFile.objects.get(id=fileid)
@@ -19,7 +24,12 @@ def export(event: str, fileid: str, provider: str, form_data: Dict[str, Any]) ->
         for receiver, response in responses:
             ex = response(event)
             if ex.identifier == provider:
-                file.filename, file.type, data = ex.render(form_data)
+                d = ex.render(form_data)
+                if d is None:
+                    raise ExportError(
+                        ugettext('Your export did not contain any data.')
+                    )
+                file.filename, file.type, data = d
                 file.file.save(cachedfile_name(file, file.filename), ContentFile(data))
                 file.save()
     return file.pk

src/pretix/control/views/orders.py

@@ -1655,6 +1655,7 @@ class ExportMixin:
 
 class ExportDoView(EventPermissionRequiredMixin, ExportMixin, AsyncAction, View):
     permission = 'can_view_orders'
+    known_errortypes = ['ExportError']
     task = export
 
     def get_success_message(self, value):