utils: use an exclusive lock on model's table in safe_get_or_create (#60658)

src/authentic2/utils/models.py

@@ -14,24 +14,8 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import math
-import random
-import time
-
 from django.conf import settings
-from django.db import connection
-
-
-def poisson_random(frequency):
-    '''Generate random numbers following a poisson distribution'''
-    return -math.log(1.0 - random.random()) / frequency
-
-
-SAFE_GET_OR_CREATE_RETRIES = 3
-
-
-class ConcurrencyError(Exception):
-    pass
+from django.db import connection, transaction
 
 
 def safe_get_or_create(model, defaults=None, **kwargs):
@@ -39,19 +23,11 @@ def safe_get_or_create(model, defaults=None, **kwargs):
         getattr(settings, 'TESTING', False) or not connection.in_atomic_block
     ), 'safe_get_or_create cannot be used in inside a transaction'
 
-    defaults = defaults or {}
-    exception = None
-    for dummy in range(SAFE_GET_OR_CREATE_RETRIES):
-        try:
-            instance, created = model.objects.get_or_create(defaults=defaults, **kwargs)
-        except model.MultipleObjectsReturned as e:
-            exception = e
-            time.sleep(max(poisson_random(1), 0.5))
-            continue
-
-        if created and model.objects.filter(**kwargs).exclude(pk=instance.pk).exists():
-            instance.delete()
-            time.sleep(max(poisson_random(1), 0.5))
-            continue
-        return instance, created
-    raise exception
+    try:
+        return model.objects.get(**kwargs), False
+    except model.DoesNotExist:
+        pass
+    with transaction.atomic():
+        with connection.cursor() as cur:
+            cur.execute('LOCK TABLE "%s" IN EXCLUSIVE MODE' % model._meta.db_table)
+        return model.objects.get_or_create(defaults=defaults, **kwargs)

tests/test_utils_models.py

@@ -17,7 +17,6 @@
 
 import threading
 
-from django.core.exceptions import MultipleObjectsReturned
 from django.db import connection
 
 from authentic2.custom_user.models import User
@@ -48,8 +47,7 @@ def test_safe_get_or_create(transactional_db, concurrency):
         threads[-1].start()
     for thread in threads:
         thread.join()
+    assert not exceptions
     assert len(users) == 1
     assert User.objects.count() == 1
-    assert all(isinstance(exception, MultipleObjectsReturned) for exception in exceptions)
-    assert len(exceptions) < (0.5 * concurrency)  # 50% of failure is 'ok-ish' with a lot of concurrency
     users[0].delete()