[WEB-8332] fix: validate role before guest-cascade, make it atomic (Copilot #9460)

partial_update cascaded the guest project-role downgrade BEFORE the serializer
was validated/saved, using a manual int() cast that 500s on non-integer input.
Reorder: validate the serializer first, then cascade using
serializer.validated_data["role"] inside a transaction.atomic() with save(), so a
bad role returns 400 (not 500) and project roles can't be downgraded when the
member update doesn't persist. Adds a non-integer-role 400 test.

Co-authored-by: Plane AI <noreply@plane.so>

apps/api/plane/app/views/workspace/member.py

@@ -3,6 +3,7 @@
 # See the LICENSE file for details.
 
 # Django imports
+from django.db import transaction
 from django.db.models import Count, Q, OuterRef, Subquery, IntegerField
 from django.utils import timezone
 from django.db.models.functions import Coalesce
@@ -84,10 +85,6 @@ class WorkSpaceMemberViewSet(BaseViewSet):
                 status=status.HTTP_400_BAD_REQUEST,
             )
 
-        # If a user is moved to a guest role he can't have any other role in projects
-        if "role" in request.data and int(request.data.get("role")) == 5:
-            ProjectMember.objects.filter(workspace__slug=slug, member_id=workspace_member.member_id).update(role=5)
-
         # SECURITY: The only field this endpoint is allowed to mutate is ``role``.
         # ``WorkSpaceMemberSerializer`` is declared with ``fields = "__all__"`` and
         # ``DynamicBaseSerializer`` ignores the ``fields=`` kwarg for writes, so
@@ -101,10 +98,22 @@ class WorkSpaceMemberViewSet(BaseViewSet):
 
         serializer = WorkSpaceMemberSerializer(workspace_member, data=allowed_data, partial=True)
 
-        if serializer.is_valid():
+        if not serializer.is_valid():
+            return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
+
+        # Validate the role (via the serializer) BEFORE cascading, and do the
+        # cascade + save atomically. Otherwise a bad role (e.g. non-integer) would
+        # 500 on the manual int() cast, and the guest project-role downgrade could
+        # persist even if the member update never succeeds.
+        with transaction.atomic():
+            # If a user is moved to a guest role they can't hold any other project role.
+            if serializer.validated_data.get("role") == 5:
+                ProjectMember.objects.filter(
+                    workspace__slug=slug, member_id=workspace_member.member_id
+                ).update(role=5)
             serializer.save()
-            return Response(serializer.data, status=status.HTTP_200_OK)
-        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
+
+        return Response(serializer.data, status=status.HTTP_200_OK)
 
     @allow_permission(allowed_roles=[ROLE.ADMIN], level="WORKSPACE")
     def destroy(self, request, slug, pk):

apps/api/plane/tests/contract/app/test_workspace_member_mass_assignment_app.py

@@ -158,3 +158,22 @@ class TestWorkspaceMemberMassAssignment:
         assert response.status_code == status.HTTP_400_BAD_REQUEST
         own_member.refresh_from_db()
         assert own_member.role == 20
+
+    def test_non_integer_role_is_400_not_500(self, attacker_workspace):
+        """A non-integer role must be a validation error (400), not a 500 — and
+        the guest project-role cascade must not run when the update is invalid."""
+        attacker_ws, attacker = attacker_workspace
+        target = _make_user("bad-role-target@plane.so")
+        target_member = _add_member(attacker_ws, target, role=15)
+
+        client = APIClient()
+        client.force_authenticate(user=attacker)
+        response = client.patch(
+            _member_detail_url(attacker_ws.slug, target_member.id),
+            {"role": "not-a-number"},
+            format="json",
+        )
+
+        assert response.status_code == status.HTTP_400_BAD_REQUEST
+        target_member.refresh_from_db()
+        assert target_member.role == 15