[BugFix] Avoid calling KV connector layer APIs when metadata is unset (#28253)

Signed-off-by: David Ben-David <davidb@pliops.com>
Co-authored-by: David Ben-David <davidb@pliops.com>
Co-authored-by: Mark McLoughlin <markmc@redhat.com>

vllm/attention/layer.py

@@ -837,6 +837,8 @@ def wait_for_kv_layer_from_connector(layer_name: str):
         return
 
     connector = get_kv_transfer_group()
+    if not connector.has_connector_metadata():
+        return
 
     forward_context: ForwardContext = get_forward_context()
     attn_metadata = forward_context.attn_metadata
@@ -854,6 +856,8 @@ def maybe_save_kv_layer_to_connector(
         return
 
     connector = get_kv_transfer_group()
+    if not connector.has_connector_metadata():
+        return
 
     forward_context: ForwardContext = get_forward_context()
     attn_metadata = forward_context.attn_metadata

vllm/distributed/kv_transfer/kv_connector/v1/base.py

@@ -204,11 +204,18 @@ class KVConnectorBase_V1(ABC):
         Returns:
             ConnectorMetadata: the connector metadata.
         """
-
         # Should only be called while set to valid metadata.
         assert self._connector_metadata is not None
         return self._connector_metadata
 
+    def has_connector_metadata(self) -> bool:
+        """Check whether the connector metadata is currently set.
+
+        Returns:
+            bool: True if connector metadata exists, False otherwise.
+        """
+        return self._connector_metadata is not None
+
     def register_kv_caches(self, kv_caches: dict[str, torch.Tensor]):
         """
         Initialize with the KV caches. Useful for pre-registering the

vllm/distributed/kv_transfer/kv_connector/v1/multi_connector.py

@@ -171,16 +171,22 @@ class MultiConnector(KVConnectorBase_V1):
     # We must override the base class method here because we need to bind
     # the metadata to each connector in the order of the connectors in the
     # MultiKVConnectorMetadata.
+    #
+    # Note: Call the base class method to ensure metadata is also set on the
+    # MultiConnector instance itself; otherwise, `has_connector_metadata()` will
+    # always return False.
     def bind_connector_metadata(self, connector_metadata: KVConnectorMetadata) -> None:
         assert isinstance(connector_metadata, MultiKVConnectorMetadata)
         if connector_metadata.extra_async_saves:
             self._extra_async_saves.update(connector_metadata.extra_async_saves)
         for c, cm in zip(self._connectors, connector_metadata.metadata):
             c.bind_connector_metadata(cm)
+        super().bind_connector_metadata(connector_metadata)
 
     def clear_connector_metadata(self) -> None:
         for c in self._connectors:
             c.clear_connector_metadata()
+        super().clear_connector_metadata()
 
     def shutdown(self):
         exception: Exception | None = None