do_delete_user: Add migration to fix bugged UserProfiles.

do_delete_users had two bugs:
1. Creating the replacement dummy users
with active=True
2. Creating the replacement dummy users with email domain set to
realm.uri, which may not be a valid email domain.
Prior commits fixed the bugs, and this migration fixes the pre-existing
objects.

zerver/migrations/0373_fix_deleteduser_dummies.py

@@ -0,0 +1,86 @@
+from typing import Any
+
+from django.conf import settings
+from django.core.exceptions import ValidationError
+from django.core.validators import validate_email
+from django.db import migrations
+from django.db.backends.postgresql.schema import DatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def host_for_subdomain(subdomain: str) -> str:
+    if subdomain == "":
+        return settings.EXTERNAL_HOST
+    default_host = f"{subdomain}.{settings.EXTERNAL_HOST}"
+    return settings.REALM_HOSTS.get(subdomain, default_host)
+
+
+def get_fake_email_domain(realm: Any) -> str:
+    """
+    Taken from zerver.models. Adjusted to work in a migration without changing
+    behavior.
+    """
+    try:
+        # Check that realm.host can be used to form valid email addresses.
+        realm_host = host_for_subdomain(realm.string_id)
+        validate_email(f"bot@{realm_host}")
+        return realm_host
+    except ValidationError:
+        pass
+
+    try:
+        # Check that the fake email domain can be used to form valid email addresses.
+        validate_email("bot@" + settings.FAKE_EMAIL_DOMAIN)
+    except ValidationError:
+        raise Exception(
+            settings.FAKE_EMAIL_DOMAIN + " is not a valid domain. "
+            "Consider setting the FAKE_EMAIL_DOMAIN setting."
+        )
+
+    return settings.FAKE_EMAIL_DOMAIN
+
+
+def fix_dummy_users(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
+    """
+    do_delete_users had two bugs:
+    1. Creating the replacement dummy users with active=True
+    2. Creating the replacement dummy users with email domain set to realm.uri,
+    which may not be a valid email domain.
+    Prior commits fixed the bugs, and this migration fixes the pre-existing objects.
+    """
+
+    UserProfile = apps.get_model("zerver", "UserProfile")
+    Subscription = apps.get_model("zerver", "Subscription")
+    users_to_fix = UserProfile.objects.filter(
+        is_mirror_dummy=True, is_active=True, delivery_email__regex=r"^deleteduser\d+@.+"
+    )
+
+    update_fields = ["is_active"]
+    for user_profile in users_to_fix:
+        user_profile.is_active = False
+        try:
+            validate_email(user_profile.delivery_email)
+        except ValidationError:
+            user_profile.delivery_email = (
+                f"deleteduser{user_profile.id}@{get_fake_email_domain(user_profile.realm)}"
+            )
+
+            update_fields.append("delivery_email")
+
+    UserProfile.objects.bulk_update(users_to_fix, update_fields)
+    # The denormalized is_user_active field needs to be updated too.
+    Subscription.objects.filter(user_profile__in=users_to_fix).update(is_user_active=False)
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0372_realmemoji_unique_realm_emoji_when_false_deactivated"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            fix_dummy_users,
+            reverse_code=migrations.RunPython.noop,
+        )
+    ]