♻️(backend) move lock in create_for_owner action in the serializer

For the create_for_owner action, all the db operation are made in the
serializer. But the lock of the table was acquired in the viewsets, lot
of operation are made between the lock is made and the insert in db. We
move the lock operation closer to the insert in the database. We wrap it
in a transaction to release the lock once the commit made.

src/backend/core/api/serializers.py

@@ -7,6 +7,7 @@ from base64 import b64decode
 from os.path import splitext
 
 from django.conf import settings
+from django.db import connection, transaction
 from django.db.models import Q
 from django.utils.functional import lazy
 from django.utils.text import slugify
@@ -505,11 +506,19 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
                 {"content": ["Could not convert content"]}
             ) from err
 
-        document = models.Document.add_root(
-            title=validated_data["title"],
-            content=document_content,
-            creator=user,
-        )
+        with transaction.atomic():
+            # locks the table to ensure safe concurrent access
+            with connection.cursor() as cursor:
+                cursor.execute(
+                    f'LOCK TABLE "{models.Document._meta.db_table}" '  # noqa: SLF001
+                    "IN SHARE ROW EXCLUSIVE MODE;"
+                )
+
+            document = models.Document.add_root(
+                title=validated_data["title"],
+                content=document_content,
+                creator=user,
+            )
 
         if user:
             # Associate the document with the pre-existing user

src/backend/core/api/viewsets.py

@@ -887,19 +887,11 @@ class DocumentViewSet(
         permission_classes=[],
         url_path="create-for-owner",
     )
-    @transaction.atomic
     def create_for_owner(self, request):
         """
         Create a document on behalf of a specified owner (pre-existing user or invited).
         """
 
-        # locks the table to ensure safe concurrent access
-        with connection.cursor() as cursor:
-            cursor.execute(
-                f'LOCK TABLE "{models.Document._meta.db_table}" '  # noqa: SLF001
-                "IN SHARE ROW EXCLUSIVE MODE;"
-            )
-
         # Deserialize and validate the data
         serializer = serializers.ServerCreateDocumentSerializer(data=request.data)
         if not serializer.is_valid():