feat(nova): Adds custom Nova scheduler filter for host traits

components/nova/values.yaml

@@ -91,6 +91,7 @@ conf:
         - ServerGroupAntiAffinityFilter
         - ServerGroupAffinityFilter
         - JsonFilter
+        - TraitRequiredFilter
   nova_api_uwsgi:
     uwsgi:
       processes: 2

containers/nova/Dockerfile

@@ -13,3 +13,4 @@ COPY containers/nova/patches /tmp/patches/
 RUN cd /var/lib/openstack/lib/python3.12/site-packages && \
     QUILT_PATCHES=/tmp/patches quilt push -a
 COPY python/nova-understack/ironic_understack  /var/lib/openstack/lib/python3.12/site-packages/nova/virt/ironic_understack
+COPY python/nova-understack/nova_understack_filters  /var/lib/openstack/lib/python3.12/site-packages/nova/scheduler/filters/understack

python/nova-understack/nova_understack_filters/trait_required_filter.py

@@ -0,0 +1,111 @@
+from typing import ClassVar
+
+import nova.conf
+from nova import context as nova_context
+from nova.scheduler import filters
+from nova.scheduler.client import report as report_client
+from oslo_log import log as logging
+
+LOG = logging.getLogger(__name__)
+CONF = nova.conf.CONF
+
+
+class TraitRequiredFilter(filters.BaseHostFilter):
+    """Filter hosts based on custom traits passed via scheduler hints.
+
+    Accepts scheduler hints in the form:
+        trait:CUSTOM_<NAME>=required
+
+    Only hosts whose resource provider reports the requested trait(s)
+    will pass the filter. If no trait hints are provided, all hosts pass.
+
+    Traits are fetched from the Placement API using the resource provider
+    UUID associated with each host.
+
+    Usage:
+        openstack server create ... --hint trait:CUSTOM_CAB_A1_1=required
+    """
+
+    # Scheduler hints can differ per request, so this filter must
+    # run against every host for each request.
+    run_filter_once_per_request = False
+
+    # Cache of {rp_uuid: set(traits)} populated per scheduling pass.
+    # This avoids repeated Placement API calls for the same provider
+    # within a single filter run.
+    _traits_cache: ClassVar[dict[str, set[str]]] = {}
+
+    def host_passes(self, host_state, spec_obj):
+        requested_traits = set()
+
+        hints = spec_obj.scheduler_hints or {}
+        for key, values in hints.items():
+            if not key.startswith("trait:"):
+                continue
+            trait_name = key.split("trait:", 1)[1]
+            if not trait_name.startswith("CUSTOM_"):
+                continue
+            # values is a list; check if 'required' is among them
+            if "required" in values:
+                requested_traits.add(trait_name)
+
+        if not requested_traits:
+            return True
+
+        host_traits = self._get_traits_for_host(host_state)
+        missing = requested_traits - host_traits
+
+        if missing:
+            LOG.debug(
+                "%(host)s fails TraitRequiredFilter: missing %(missing)s",
+                {"host": host_state.host, "missing": missing},
+            )
+            return False
+
+        return True
+
+    def _get_traits_for_host(self, host_state):
+        """Get traits for a host's resource provider from Placement.
+
+        Uses a per-instance cache to avoid repeated API calls within
+        the same scheduling pass.
+        """
+        rp_uuid = host_state.uuid
+        if rp_uuid in self._traits_cache:
+            return self._traits_cache[rp_uuid]
+
+        try:
+            client = report_client.report_client_singleton()
+            context = nova_context.get_admin_context()
+            trait_info = client.get_provider_traits(context, rp_uuid)
+            traits = trait_info.traits
+        except Exception:
+            LOG.warning(
+                "Could not retrieve traits for host %(host)s "
+                "(rp_uuid=%(uuid)s) from Placement API.",
+                {"host": host_state.host, "uuid": rp_uuid},
+            )
+            traits = set()
+
+        self._traits_cache[rp_uuid] = traits
+        return traits
+
+    def filter_all(self, filter_obj_list, spec_obj):
+        """Override to clear the traits cache before each filter pass."""
+        self._traits_cache = {}
+        return super().filter_all(filter_obj_list, spec_obj)
+
+
+def all_filters():
+    """Return all standard Nova filters plus Understack custom filters.
+
+    This function is used as the value for [filter_scheduler]available_filters
+    to work around OpenStack Helm's inability to render MultiStrOpt values
+    (it joins YAML lists into a single comma-separated line).
+
+    By pointing available_filters to this single function path, we avoid
+    needing multiple available_filters lines.
+    """
+    from nova.scheduler.filters import all_filters as nova_all_filters
+
+    return [*nova_all_filters(), TraitRequiredFilter]