migration: Migrate messages tied to removed bots.

This cleans up any messages that might have been exchanged with
`NEW_USER_BOT` or `FEEDBACK_BOT` (cross-realm bots that were last
used, as far as we know, years ago) that have been completely removed
from the codebase.

Details on the algorithm are in the migration code itself.

Fixes #13583.

zerver/migrations/0273_migrate_old_bot_messages.py

@@ -0,0 +1,88 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.conf import settings
+from django.db import migrations
+from django.db.backends.postgresql_psycopg2.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+from typing import Any
+
+def fix_messages(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    """Conceptually, this migration cleans up the old NEW_USER_BOT and FEEDBACK_BOT
+    UserProfile objects (their implementations were removed long ago).
+
+    We do this by:
+    * Changing their sent messages to have been sent by NOTIFICATION_BOT.
+    * Changing their 1:1 PMs to be PMs with NOTIFICATION_BOT and deleting their
+      PM recipient object.
+    * Deleting any Huddles that involve them (zulipchat.com data suggests there are none,
+      so this is mainly out of caution) and Recipient objects (which will cascade to
+      associated Subscription, Message, and UserMessage objects if they exist).
+    * Deleting their UserProfile objects.
+
+    The end result if these users are completely removed, with any
+    messages that might have been associated with them transferred
+    to NOTIFICATION_BOT to preserve history.
+    """
+    UserProfile = apps.get_model('zerver', 'UserProfile')
+    Huddle = apps.get_model('zerver', 'Huddle')
+    Subscription = apps.get_model('zerver', 'Subscription')
+    Recipient = apps.get_model('zerver', 'Recipient')
+    RECIPIENT_HUDDLE = 3
+    Message = apps.get_model('zerver', 'Message')
+    Realm = apps.get_model('zerver', 'Realm')
+
+    try:
+        internal_realm = Realm.objects.get(string_id=settings.SYSTEM_BOT_REALM)
+    except Realm.DoesNotExist:
+        # Server not initialized, or no system bot realm. Either way, we shouldn't do anything.
+        return
+
+    def get_bot_by_delivery_email(email: str) -> Any:
+        return UserProfile.objects.select_related().get(delivery_email__iexact=email.strip(),
+                                                        realm=internal_realm)
+
+    notification_bot = get_bot_by_delivery_email(settings.NOTIFICATION_BOT)
+
+    def fix_messages_by_bot(bot_profile: Any) -> None:
+        Message.objects.filter(sender=bot_profile).update(sender=notification_bot)
+        Message.objects.filter(recipient=bot_profile.recipient).update(recipient=notification_bot.recipient)
+
+    def clean_up_bot(bot_profile: Any) -> None:
+        huddle_recipient_ids = Subscription.objects \
+            .filter(user_profile_id=bot_profile.id, recipient__type=RECIPIENT_HUDDLE) \
+            .values_list('recipient_id', flat=True)
+        Huddle.objects.filter(recipient_id__in=huddle_recipient_ids).delete()
+        Recipient.objects.filter(id__in=huddle_recipient_ids).delete()
+
+        personal_recipient_id = bot_profile.recipient_id
+        bot_profile.delete()
+        Recipient.objects.filter(id=personal_recipient_id).delete()
+
+    new_user_bot_email = getattr(settings, 'NEW_USER_BOT', 'new-user-bot@zulip.com')
+    try:
+        new_user_bot = get_bot_by_delivery_email(new_user_bot_email)
+        fix_messages_by_bot(new_user_bot)
+        clean_up_bot(new_user_bot)
+    except UserProfile.DoesNotExist:
+        pass
+
+    feedback_bot_email = getattr(settings, 'FEEDBACK_BOT', 'feedback@zulip.com')
+    try:
+        feedback_bot = get_bot_by_delivery_email(feedback_bot_email)
+        fix_messages_by_bot(feedback_bot)
+        clean_up_bot(feedback_bot)
+    except UserProfile.DoesNotExist:
+        pass
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0272_realm_default_code_block_language'),
+    ]
+
+    operations = [
+        migrations.RunPython(fix_messages,
+                             reverse_code=migrations.RunPython.noop),
+    ]