[Bug] Fix for create_child_builds (#8399)

* Fix for create_child_builds

- Account for concurrency between multiple worker processes
- Ensure db commits are atomic
- Add random delays between build creation

* Check for existing build order

* Initially force  task off to background worker

* Revert force_async change

src/backend/InvenTree/build/serializers.py

@@ -180,6 +180,7 @@ class BuildSerializer(NotesFieldMixin, DataImportExportSerializerMixin, InvenTre
 
         return reference
 
+    @transaction.atomic
     def create(self, validated_data):
         """Save the Build object."""
 
@@ -192,7 +193,7 @@ class BuildSerializer(NotesFieldMixin, DataImportExportSerializerMixin, InvenTre
             InvenTree.tasks.offload_task(
                 build.tasks.create_child_builds,
                 build_order.pk,
-                group='build',
+                group='build'
             )
 
         return build_order

src/backend/InvenTree/build/tasks.py

@@ -1,11 +1,15 @@
 """Background task definitions for the BuildOrder app."""
 
 import logging
+import random
+import time
+
 from datetime import timedelta
 from decimal import Decimal
 
 from django.contrib.auth.models import User
 from django.template.loader import render_to_string
+from django.db import transaction
 from django.utils.translation import gettext_lazy as _
 
 from allauth.account.models import EmailAddress
@@ -198,9 +202,28 @@ def create_child_builds(build_id: int) -> None:
 
     assembly_items = build_order.part.get_bom_items().filter(sub_part__assembly=True)
 
+    # Random delay, to reduce likelihood of race conditions from multiple build orders being created simultaneously
+    time.sleep(random.random())
+
+    with transaction.atomic():
+        # Atomic transaction to ensure that all child build orders are created together, or not at all
+        # This is critical to prevent duplicate child build orders being created (e.g. if the task is re-run)
+
+        sub_build_ids = []
+
         for item in assembly_items:
             quantity = item.quantity * build_order.quantity
 
+
+            # Check if the child build order has already been created
+            if build_models.Build.objects.filter(
+                part=item.sub_part,
+                parent=build_order,
+                quantity=quantity,
+                status__in=BuildStatusGroups.ACTIVE_CODES
+            ).exists():
+                continue
+
             sub_order = build_models.Build.objects.create(
                 part=item.sub_part,
                 quantity=quantity,
@@ -213,10 +236,13 @@ def create_child_builds(build_id: int) -> None:
                 responsible=build_order.responsible,
             )
 
+            sub_build_ids.append(sub_order.pk)
+
+        for pk in sub_build_ids:
             # Offload the child build order creation to the background task queue
             InvenTree.tasks.offload_task(
                 create_child_builds,
-            sub_order.pk,
+                pk,
                 group='build'
             )