fix(admin): a default VM that is not exactly one VM is refused on the form

NEW_USER_DEFAULT_INSTANCES names VMs, and a name may repeat across hosts.
A name with no VM, or with one on two hosts, made creating any user a 500.
The create form now looks the names up and says which one is not exactly
one VM; the user and its default VMs are saved together.

admin/forms.py

@@ -1,4 +1,5 @@
 from django import forms
+from django.conf import settings
 from django.contrib.auth import password_validation
 from django.contrib.auth.forms import AdminPasswordChangeForm
 from django.contrib.auth.models import Group, User
@@ -7,6 +8,7 @@ from django.utils.translation import gettext_lazy as _
 
 from accounts.forms import UniqueEmailMixin
 from accounts.models import UserAttributes
+from instances.models import Instance
 
 from .models import Permission
 
@@ -85,6 +87,17 @@ class UserCreateForm(UserForm):
             content_type__app_label="accounts", codename="change_password"
         )
 
+    def clean(self):
+        # the VMs a new user owns, by name: a name may repeat across hosts
+        cleaned = super().clean()
+        self.default_instances = []
+        for name in settings.NEW_USER_DEFAULT_INSTANCES:
+            found = list(Instance.objects.filter(name=name)[:2])
+            if len(found) != 1:
+                raise ValidationError(_("NEW_USER_DEFAULT_INSTANCES: %(name)s is not exactly one VM") % {"name": name})
+            self.default_instances += found
+        return cleaned
+
     def _post_clean(self):
         # The validators in AUTH_PASSWORD_VALIDATORS, as on the password change
         # forms; after the instance has the form's values (username, email).

admin/tests/test_audit_rows.py

@@ -1,7 +1,7 @@
 """Changes to users and groups write rows on the Logs page."""
 
 from django.contrib.auth.models import Group, Permission, User
-from django.test import TestCase, override_settings
+from django.test import TestCase
 from django.urls import reverse
 
 from logs.models import Logs
@@ -77,14 +77,6 @@ class UserRowTests(TestCase):
         ])
         self.assertFalse(Logs.objects.filter(message__contains="New-carol-pass-1").exists())
 
-    @override_settings(NEW_USER_DEFAULT_INSTANCES=["no-such-vm"])
-    def test_a_failing_default_vm_leaves_no_user(self):
-        from instances.models import Instance
-        with self.assertRaises(Instance.DoesNotExist):
-            self.create()
-        self.assertFalse(User.objects.filter(username="carol").exists())
-        self.assertEqual(rows(), [("User carol: add: DoesNotExist", False)])
-
     def test_the_row_has_actor_and_ip(self):
         self.client.post(
             reverse("admin:user_create"), {"username": "carol", "password": "Carol-pass-2026", **ATTRIBUTES},

admin/tests/test_user_create.py

@@ -6,6 +6,10 @@ from django.db import connection
 from django.test import TestCase, override_settings
 from django.urls import reverse
 
+from accounts.models import UserInstance
+from computes.models import Compute
+from instances.models import Instance
+
 RAW = "Raw-Passw0rd-in-the-form"
 LIMITS = {"max_instances": 1, "max_cpus": 1, "max_memory": 1024, "max_disk_size": 4}
 
@@ -63,3 +67,30 @@ class UserCreateTests(TestCase):
         )
         self.assertEqual(response.status_code, 200)
         self.assertContains(response, "too similar to the username")
+
+
+class DefaultInstancesTests(TestCase):
+    """NEW_USER_DEFAULT_INSTANCES names VMs, and a name may repeat across hosts."""
+
+    def setUp(self):
+        self.client.force_login(User.objects.create_superuser("uc-admin", "uc@example.com", "pw"))
+        one, two = (Compute.objects.create(name=name, hostname="localhost", login="", password="", details="", type=4)
+                    for name in ("uc-one", "uc-two"))
+        self.web = Instance.objects.create(compute=one, name="web", uuid="00000000-0000-0000-0000-000000000001")
+        for compute, uuid in ((one, "00000000-0000-0000-0000-000000000002"), (two, "00000000-0000-0000-0000-000000000003")):
+            Instance.objects.create(compute=compute, name="db", uuid=uuid)
+
+    def create(self):
+        return self.client.post(reverse("admin:user_create"), {"username": "newbie", "password": RAW, **LIMITS})
+
+    @override_settings(NEW_USER_DEFAULT_INSTANCES=["web"])
+    def test_the_new_user_owns_the_named_vm(self):
+        self.assertRedirects(self.create(), reverse("admin:user_list"))
+        self.assertEqual([o.instance for o in UserInstance.objects.filter(user__username="newbie")], [self.web])
+
+    def test_a_name_that_is_not_one_vm_is_refused_on_the_form(self):
+        for name in ("no-such-vm", "db"):
+            with self.subTest(name), override_settings(NEW_USER_DEFAULT_INSTANCES=["web", name]):
+                response = self.create()
+                self.assertContains(response, f"NEW_USER_DEFAULT_INSTANCES: {name} is not exactly one VM")
+                self.assertFalse(User.objects.filter(username="newbie").exists())

admin/views.py

@@ -127,7 +127,8 @@ def user_create(request):
             attributes = attributes_form.save(commit=False)
             attributes.user = user
             attributes.save()
-            add_default_instances(user)
+            UserInstance.objects.bulk_create(UserInstance(user=user, instance=instance)
+                                             for instance in user_form.default_instances)
         return redirect("admin:user_list")
 
     return render(
@@ -346,14 +347,3 @@ def tombstone_delete(request, pk):
          "consequence": _("Its owners no longer get the VM back if its UUID shows up again.")},
     )
 
-
-def add_default_instances(user):
-    """
-    Adds instances listed in NEW_USER_DEFAULT_INSTANCES to user
-    """
-    existing_instances = UserInstance.objects.filter(user=user)
-    if not existing_instances:
-        for instance_name in settings.NEW_USER_DEFAULT_INSTANCES:
-            instance = Instance.objects.get(name=instance_name)
-            user_instance = UserInstance(user=user, instance=instance)
-            user_instance.save()