Fix #1301 -- Do not export empty files

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