Prevent some race conditions

src/pretix/base/models/base.py

@@ -3,6 +3,7 @@ import uuid
 
 from django.contrib.contenttypes.models import ContentType
 from django.db import models
+from django.db.models.constants import LOOKUP_SEP
 from django.db.models.signals import post_delete
 from django.dispatch import receiver
 from django.utils.crypto import get_random_string
@@ -100,3 +101,49 @@ class LoggedModel(models.Model, LoggingMixin):
         return LogEntry.objects.filter(
             content_type=ContentType.objects.get_for_model(type(self)), object_id=self.pk
         ).select_related('user', 'event', 'oauth_application', 'api_token', 'device')
+
+
+class LockModel:
+    def refresh_for_update(self, fields=None, using=None, **kwargs):
+        """
+        Like refresh_from_db(), but with select_for_update().
+        See also https://code.djangoproject.com/ticket/28344
+        """
+        if fields is not None:
+            if not fields:
+                return
+            if any(LOOKUP_SEP in f for f in fields):
+                raise ValueError(
+                    'Found "%s" in fields argument. Relations and transforms '
+                    'are not allowed in fields.' % LOOKUP_SEP)
+
+        hints = {'instance': self}
+        db_instance_qs = self.__class__._base_manager.db_manager(using, hints=hints).filter(pk=self.pk).select_for_update(**kwargs)
+
+        # Use provided fields, if not set then reload all non-deferred fields.
+        deferred_fields = self.get_deferred_fields()
+        if fields is not None:
+            fields = list(fields)
+            db_instance_qs = db_instance_qs.only(*fields)
+        elif deferred_fields:
+            fields = [f.attname for f in self._meta.concrete_fields
+                      if f.attname not in deferred_fields]
+            db_instance_qs = db_instance_qs.only(*fields)
+
+        db_instance = db_instance_qs.get()
+        non_loaded_fields = db_instance.get_deferred_fields()
+        for field in self._meta.concrete_fields:
+            if field.attname in non_loaded_fields:
+                # This field wasn't refreshed - skip ahead.
+                continue
+            setattr(self, field.attname, getattr(db_instance, field.attname))
+            # Clear cached foreign keys.
+            if field.is_relation and field.is_cached(self):
+                field.delete_cached_value(self)
+
+        # Clear cached relations.
+        for field in self._meta.related_objects:
+            if field.is_cached(self):
+                field.delete_cached_value(self)
+
+        self._state.db = db_instance._state.db

src/pretix/base/models/orders.py

@@ -31,7 +31,7 @@ from pretix.base.i18n import language
 from pretix.base.models import User
 from pretix.base.reldate import RelativeDateWrapper
 
-from .base import LoggedModel
+from .base import LockModel, LoggedModel
 from .event import Event, SubEvent
 from .items import Item, ItemVariation, Question, QuestionOption, Quota
 
@@ -47,7 +47,7 @@ def generate_position_secret():
     return get_random_string(length=settings.ENTROPY['ticket_secret'], allowed_chars='abcdefghjkmnpqrstuvwxyz23456789')
 
 
-class Order(LoggedModel):
+class Order(LockModel, LoggedModel):
     """
     An order is created when a user clicks 'buy' on his cart. It holds
     several OrderPositions and is connected to a user. It has an
@@ -903,7 +903,7 @@ class OrderPayment(models.Model):
         if not force and can_be_paid is not True:
             raise Quota.QuotaExceededException(can_be_paid)
         self.order.status = Order.STATUS_PAID
-        self.order.save()
+        self.order.save(update_fields=['status'])
 
         self.order.log_action('pretix.event.order.paid', {
             'provider': self.provider,