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)
 
@@ -207,54 +207,64 @@ class OutboundSyncProvider:
         raise NotImplementedError
 
     def sync_queued_orders(self, queued_orders):
-        for sq in queued_orders:
-            try:
-                mapped_objects = self.sync_order(sq.order)
-            except UnrecoverableSyncError as e:
-                logger.warning(
-                    f"Could not sync order {sq.order.code} to {type(self).__name__}",
-                    exc_info=True,
-                )
-                sq.order.log_action(e.log_action_type, {
-                    "provider": self.identifier,
-                    "error": e.messages,
-                    "full_message": e.full_message,
-                })
-                sq.delete()
-            except RecoverableSyncError as e:
-                sq.failed_attempts += 1
-                sq.not_before = self.next_retry_date(sq)
-                logger.info(
-                    f"Could not sync order {sq.order.code} to {type(self).__name__} (transient error, attempt #{sq.failed_attempts})",
-                    exc_info=True,
-                )
-                if sq.failed_attempts >= self.max_attempts:
-                    sentry_sdk.capture_exception(e)
-                    sq.order.log_action("pretix.event.order.data_sync.failed.exceeded", {
+        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:
+                    logger.warning(
+                        f"Could not sync order {sq.order.code} to {type(self).__name__}",
+                        exc_info=True,
+                    )
+                    sq.order.log_action(e.log_action_type, {
                         "provider": self.identifier,
                         "error": e.messages,
                         "full_message": e.full_message,
                     })
                     sq.delete()
+                except RecoverableSyncError as e:
+                    sq.failed_attempts += 1
+                    sq.not_before = self.next_retry_date(sq)
+                    logger.info(
+                        f"Could not sync order {sq.order.code} to {type(self).__name__} (transient error, attempt #{sq.failed_attempts})",
+                        exc_info=True,
+                    )
+                    if sq.failed_attempts >= self.max_attempts:
+                        sentry_sdk.capture_exception(e)
+                        sq.order.log_action("pretix.event.order.data_sync.failed.exceeded", {
+                            "provider": self.identifier,
+                            "error": e.messages,
+                            "full_message": e.full_message,
+                        })
+                        sq.delete()
+                    else:
+                        sq.save()
+                except Exception as e:
+                    logger.exception(
+                        f"Could not sync order {sq.order.code} to {type(self).__name__} (unhandled exception)"
+                    )
+                    sentry_sdk.capture_exception(e)
+                    sq.order.log_action("pretix.event.order.data_sync.failed.internal", {
+                        "provider": self.identifier,
+                        "error": [],
+                        "full_message": str(e),
+                    })
+                    sq.delete()
                 else:
-                    sq.save()
-            except Exception as e:
-                logger.exception(
-                    f"Could not sync order {sq.order.code} to {type(self).__name__} (unhandled exception)"
-                )
-                sentry_sdk.capture_exception(e)
-                sq.order.log_action("pretix.event.order.data_sync.failed.internal", {
-                    "provider": self.identifier,
-                    "error": [],
-                    "full_message": str(e),
-                })
-                sq.delete()
-            else:
-                sq.order.log_action("pretix.event.order.data_sync.success", {
-                    "provider": self.identifier,
-                    "objects": mapped_objects
-                })
-                sq.delete()
+                    sq.order.log_action("pretix.event.order.data_sync.success", {
+                        "provider": self.identifier,
+                        "objects": mapped_objects
+                    })
+                    sq.delete()
 
     @cached_property
     def data_fields(self):