[wip] Add rule disabling to migration, add uninstall hook

src/pretix/plugins/sendmail/apps.py

@@ -40,3 +40,6 @@ class SendMailApp(AppConfig):
     def ready(self):
         from . import signals  # NOQA
         from . import tasks  # NOQA
+
+    def uninstalled(self, event):
+        event.sendmail_rules.update(enabled=False)

src/pretix/plugins/sendmail/migrations/0013_activate_plugin_for_events_with_scheduled_mails.py

@@ -5,15 +5,24 @@ from django.db.models import Exists, OuterRef
 def activate_plugin(apps, schema_editor):
     Event = apps.get_model("pretixbase", "Event")
     ScheduledMail = apps.get_model("sendmail", "ScheduledMail")
+    Rule = apps.get_model("sendmail", "Rule")
+
     events = (
         Event.objects
         .exclude(plugins__icontains="pretix.plugins.sendmail")
         .filter(
             Exists(
-                ScheduledMail.objects.filter(event=OuterRef('pk')).exclude(state=ScheduledMail.STATE_COMPLETED)
+                ScheduledMail.objects.filter(event=OuterRef('pk'), rule__enabled=True).exclude(state='completed')
             )
         )
     )
+
+    only_completed_rules = Rule.objects.exclude(event__plugins__icontains="pretix.plugins.sendmail",
+                                                enabled=False).filter(
+        ~Exists(ScheduledMail.objects.filter(rule=OuterRef('pk')).exclude(state='completed'))
+    )
+    only_completed_rules.update(enabled=False)
+
     for event in events:
         event.enable_plugin('pretix.plugins.sendmail')
         event.save(update_fields=['plugins'])

src/pretix/plugins/sendmail/signals.py

@@ -150,8 +150,6 @@ class SendmailPluginRuleLogEntryType(EventLogEntryType):
 
 @receiver(periodic_task)
 def sendmail_run_rules(sender, **kwargs):
-    if "pretix.plugins.sendmail" not in sender.get_plugins():
-        return  # do not send scheduled mails if this plugin is disabled
     with scopes_disabled():
         mails = ScheduledMail.objects.all()