realm_reactivation: Migrate old Confirmations to the new data format.

cf74d7d140 changed what .content_object on
these Confirmations is, but old Confirmations still need to be migrated
to that to make sense.

zerver/migrations/0401_migrate_old_realm_reactivation_links.py

@@ -0,0 +1,79 @@
+from django.db import migrations
+from django.db.backends.postgresql.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+
+
+def fix_old_realm_reactivation_confirmations(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    """
+    Migration 0400_realmreactivationstatus changed REALM_REACTIVATION Confirmation
+    to have a RealmReactivationStatus instance as .content_object. Now we need to migrate
+    pre-existing REALM_REACTIVATION Confirmations to follow this format.
+
+    The process is a bit fiddly because Confirmation.content_object is a GenericForeignKey,
+    which can't be directly accessed in migration code, so changing it involves manually
+    updating the .object_id and .content_type attributes underpinning it.
+
+    For these old Confirmation we don't have a mechanism for tracking which have been used,
+    so it's safest to just revoke them all. If any users need a realm reactivation link, it
+    can just be re-generated.
+    """
+    REALM_REACTIVATION = 8
+
+    RealmReactivationStatus = apps.get_model("zerver", "RealmReactivationStatus")
+    Realm = apps.get_model("zerver", "Realm")
+    Confirmation = apps.get_model("confirmation", "Confirmation")
+    ContentType = apps.get_model("contenttypes", "ContentType")
+
+    if not Confirmation.objects.exists():
+        # No Confirmations so nothing to do, and the database may actually
+        # no be provisioned yet, which would make the code below break.
+        return
+
+    # .content_type of these old Confirmation will be changed to this.
+    realm_reactivation_status_content_type = ContentType.objects.get(
+        model="realmreactivationstatus", app_label="zerver"
+    )
+
+    for confirmation in Confirmation.objects.filter(type=REALM_REACTIVATION):
+        if confirmation.content_type_id == realm_reactivation_status_content_type.id:
+            # This Confirmation is already in the new format.
+            continue
+
+        assert confirmation.content_type.model == "realm"
+        realm_object_id = confirmation.object_id
+
+        # Sanity check that the realm exists.
+        try:
+            Realm.objects.get(id=realm_object_id)
+        except Realm.DoesNotExist:
+            print(
+                f"Confirmation {confirmation.id} is tied to realm_id {realm_object_id} which doesn't exist. "
+                "This is unexpected! Skipping migrating it."
+            )
+            continue
+
+        # We create the object with STATUS_REVOKED.
+        new_content_object = RealmReactivationStatus(realm_id=realm_object_id, status=2)
+        new_content_object.save()
+
+        # Now we can finally change the .content_object. This is done by setting
+        # .content_type to the correct ContentType as mentioned above and the object_id
+        # to the id of the RealmReactivationStatus instance that's supposed to be
+        # the content_object. This works because .content_object is dynamically
+        # derived by django from the .content_type and object_id values.
+        confirmation.content_type_id = realm_reactivation_status_content_type
+        confirmation.object_id = new_content_object.id
+        confirmation.save()
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0400_realmreactivationstatus"),
+    ]
+
+    operations = [
+        migrations.RunPython(fix_old_realm_reactivation_confirmations, elidable=True),
+    ]