fix(instances): two owners added at once keep a VM to one owner

With one owner per VM, the VM page and a user's page counted the VM's
owners and then added one in separate steps, so two owners added at the
same moment both found the VM without one, and it got two. The check
and the insert now run in one transaction that holds the ownership lock
(PostgreSQL's advisory lock; SQLite's IMMEDIATE transactions already
serialize them): the second waits for the first and is refused. The
audit row is written after the transaction, so a failure keeps its row.

instances/views.py

@@ -18,7 +18,7 @@ from appsettings.models import AppSettings
 from appsettings.settings import app_settings
 from computes.models import Compute
 from computes.validators import validate_hostname, validate_login
-from computes.utils import libvirt_compute_lock, libvirt_instance_lock, user_quota_lock
+from computes.utils import libvirt_compute_lock, libvirt_instance_lock, lock_ownership, user_quota_lock
 from django.conf import settings
 from django.contrib import messages
 from django.contrib.auth.decorators import permission_required
@@ -26,6 +26,7 @@ from django.contrib.auth.models import User
 from django.core import signing
 from django.core.exceptions import PermissionDenied, ValidationError
 from django.http import Http404, HttpResponse, JsonResponse
+from django.db import transaction
 from django.db.models import Prefetch, prefetch_related_objects
 from django.shortcuts import get_object_or_404, redirect, render
 from django.urls import reverse
@@ -2077,22 +2078,22 @@ def add_owner(request, pk):
         raise Http404
     user = get_object_or_404(User, pk=user_id)
 
-    check_inst = 0
-
-    if app_settings.ALLOW_INSTANCE_MULTIPLE_OWNER == "False":
-        check_inst = UserInstance.objects.filter(instance=instance).count()
-
     action = f"add owner {user.username}"
-    if check_inst > 0:
-        failed_vm(request, instance, action, "only one owner is allowed")
-        messages.error(
-            request, _("Only one owner is allowed and the one already added")
-        )
-    else:
-        with audit_vm(request, instance, action) as entry:
+    # the check and the insert in one step: two owners added at once must not
+    # both find the VM without one; the row is written after the transaction
+    with audit_vm(request, instance, action) as entry, transaction.atomic():
+        lock_ownership()
+        taken = (app_settings.ALLOW_INSTANCE_MULTIPLE_OWNER == "False"
+                 and UserInstance.objects.filter(instance=instance).exists())
+        if taken:
+            entry.skip = True
+        else:
             entry.skip = not UserInstance.objects.get_or_create(instance=instance, user=user)[1]
-        if entry.skip:
-            messages.warning(request, _("%(user)s is already an owner") % {"user": user})
+    if taken:
+        failed_vm(request, instance, action, "only one owner is allowed")
+        messages.error(request, _("Only one owner is allowed and the one already added"))
+    elif entry.skip:
+        messages.warning(request, _("%(user)s is already an owner") % {"user": user})
     return _back(request, pk, "users")