confirmation: Replace RealmCreationKey - use Confirmation instead.

Fixes #20028.

There's no reason to have a special `RealmCreationKey` class - the
`Confirmation` system already does this job.

This is somewhat complicated by the need to write a migration for
`RealmCreationKey`->`Confirmation` for pre-existing, valid objects, to
avoid breaking realm creation links that haven't been used yet.

confirmation/migrations/0016_realmcreationkey_to_realmcreationstatus.py

@@ -0,0 +1,112 @@
+# Generated by Django 5.2.3 on 2025-07-04 20:33
+
+from datetime import timedelta
+
+from django.conf import settings
+from django.db import migrations, transaction
+from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+from django.db.migrations.state import StateApps
+from django.db.models import Max
+
+
+def migrate_realmcreationkey_to_realmcreationstatus(
+    apps: StateApps, schema_editor: BaseDatabaseSchemaEditor
+) -> None:
+    """
+    This migration is for switching from using a separate RealmCreationKey class
+    for realm creation keys to just using the Confirmation system.
+
+    The aim is to iterate through all the existing RealmCreationKey and create
+    a corresponding Confirmation+RealmCreationStatus.
+
+    For validity of these objects, we only need to worry about RealmCreationKeys
+    expired due to time. This is taken care of by making sure we set expiry_date
+    on the Confirmation we're creating.
+    The way the RealmCreationKey system worked was to .delete() the RealmCreationKey
+    objects when they were used - so those simply no longer exist and we don't need to
+    worry about this case.
+    """
+    CAN_CREATE_REALM = 11
+
+    Confirmation = apps.get_model("confirmation", "Confirmation")
+    ContentType = apps.get_model("contenttypes", "ContentType")
+    RealmCreationKey = apps.get_model("confirmation", "RealmCreationKey")
+    RealmCreationStatus = apps.get_model("zerver", "RealmCreationStatus")
+    realm_creation_status_content_type, created = ContentType.objects.get_or_create(
+        model="realmcreationstatus", app_label="zerver"
+    )
+
+    BATCH_SIZE = 10000
+    max_id = RealmCreationKey.objects.aggregate(Max("id"))["id__max"]
+    if max_id is None:
+        # Nothing to do.
+        return
+
+    lower_bound = 1
+    while lower_bound <= max_id + (BATCH_SIZE / 2):
+        upper_bound = lower_bound + BATCH_SIZE - 1
+        creation_keys = RealmCreationKey.objects.filter(id__range=(lower_bound, upper_bound))
+        creation_statuses_to_create = []
+        confirmations_to_create = []
+        for creation_key in creation_keys:
+            # These keys were generated in the same way as keys for Confirmation objects,
+            # so we can copy them over without breaking anything.
+            key = creation_key.creation_key
+            date_created = creation_key.date_created
+            presume_email_valid = creation_key.presume_email_valid
+
+            creation_status = RealmCreationStatus(
+                status=0, date_created=date_created, presume_email_valid=presume_email_valid
+            )
+            confirmation = Confirmation(
+                content_type=realm_creation_status_content_type,
+                type=CAN_CREATE_REALM,
+                confirmation_key=key,
+                date_sent=date_created,
+                expiry_date=date_created
+                + timedelta(days=settings.CAN_CREATE_REALM_LINK_VALIDITY_DAYS),
+            )
+
+            # To attach the Confirmations to RealmCreationStatus objects we need to set the
+            # confirmation.object_id to their respective ids. But we haven't saved
+            # the RealmCreationStatus objs to the database yet - so we don't have ids.
+            #
+            # After we .bulk_create() them, their .id attributes will be populated.
+            # So for now we just link the RealmCreationStatus to the Confirmation
+            # via a temporary ._object attribute - which we'll use later to set
+            # the .object_id as intended.
+            confirmation._object = creation_status
+
+            creation_statuses_to_create.append(creation_status)
+            confirmations_to_create.append(confirmation)
+        with transaction.atomic():
+            RealmCreationStatus.objects.bulk_create(creation_statuses_to_create)
+
+            # Now the objects in creation_statuses_to_create have had their .id
+            # attrs populated. For every confirmation, confirmation._object
+            # points to its corresponding RealmCreationStatus - so now we can
+            # set the confirmation.object_id values and clear out the temporary
+            # ._object attr.
+            for confirmation in confirmations_to_create:
+                confirmation.object_id = confirmation._object.id
+                delattr(confirmation, "_object")
+            Confirmation.objects.bulk_create(confirmations_to_create)
+
+        lower_bound += BATCH_SIZE
+
+
+class Migration(migrations.Migration):
+    atomic = False
+
+    dependencies = [
+        ("confirmation", "0015_alter_confirmation_object_id"),
+        ("zerver", "0743_realmcreationstatus"),
+    ]
+
+    operations = [
+        migrations.RunPython(
+            migrate_realmcreationkey_to_realmcreationstatus,
+            reverse_code=migrations.RunPython.noop,
+            elidable=True,
+        ),
+    ]

confirmation/migrations/0017_delete_realmcreationkey.py

@@ -0,0 +1,15 @@
+# Generated by Django 5.2.3 on 2025-07-04 19:08
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("confirmation", "0016_realmcreationkey_to_realmcreationstatus"),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name="RealmCreationKey",
+        ),
+    ]