Add simple locking mechanism

src/pretix/base/datasync/datasync.py

@@ -28,6 +28,7 @@ from functools import cached_property
 from itertools import groupby
 
 import sentry_sdk
+from django.db import transaction, DatabaseError
 from django.db.models import F, Window
 from django.db.models.functions import RowNumber
 from django.dispatch import receiver
@@ -41,6 +42,7 @@ from pretix.base.logentrytype_registry import make_link
 from pretix.base.models.datasync import OrderSyncQueue, OrderSyncResult
 from pretix.base.signals import EventPluginRegistry, periodic_task
 from pretix.celery_app import app
+from pretix.helpers import OF_SELF
 
 logger = logging.getLogger(__name__)
 
@@ -56,7 +58,6 @@ sync_targets = EventPluginRegistry({"identifier": lambda o: o.identifier})
 def sync_event_to_target(event, target_cls, queued_orders):
     with scope(organizer=event.organizer):
         with target_cls(event=event) as p:
-            # TODO: should I somehow lock the queued orders or events, to avoid syncing them twice at the same time?
             p.sync_queued_orders(queued_orders)
 
 
@@ -71,11 +72,10 @@ def sync_all():
                 partition_by=[F("event_id")],
                 order_by="not_before",
             ))
-            .select_related("order")
             .prefetch_related("event")
             [:1000]
         )
-        grouped = groupby(sorted(queue, key=lambda q: (q.sync_provider, q.order.event.pk)), lambda q: (q.sync_provider, q.order.event))
+        grouped = groupby(sorted(queue, key=lambda q: (q.sync_provider, q.event.pk)), lambda q: (q.sync_provider, q.event))
         for (target, event), queued_orders in grouped:
             target_cls, meta = sync_targets.get(identifier=target, active_in=event)
 
@@ -207,7 +207,17 @@ class OutboundSyncProvider:
         raise NotImplementedError
 
     def sync_queued_orders(self, queued_orders):
-        for sq in queued_orders:
+        for queue_item in queued_orders:
+            with transaction.atomic():
+                try:
+                    sq = (
+                        OrderSyncQueue.objects
+                        .select_for_update(of=OF_SELF, nowait=True)
+                        .select_related("order")
+                        .get(pk=queue_item.pk)
+                    )
+                except DatabaseError:
+                    continue
                 try:
                     mapped_objects = self.sync_order(sq.order)
                 except UnrecoverableSyncError as e: