[backport] Allow blocking launch of federated tracker. (#10414) (#10425)

plugin/example/custom_obj.cc

@@ -69,7 +69,7 @@ class MyLogistic : public ObjFunction {
 
   void SaveConfig(Json* p_out) const override {
     auto& out = *p_out;
-    out["name"] = String("my_logistic");
+    out["name"] = String("mylogistic");
     out["my_logistic_param"] = ToJson(param_);
   }
 

python-package/xgboost/federated.py

@@ -65,9 +65,19 @@ def run_federated_server(  # pylint: disable=too-many-arguments
     server_key_path: Optional[str] = None,
     server_cert_path: Optional[str] = None,
     client_cert_path: Optional[str] = None,
+    blocking: bool = True,
     timeout: int = 300,
-) -> Dict[str, Any]:
-    """See :py:class:`~xgboost.federated.FederatedTracker` for more info."""
+) -> Optional[Dict[str, Any]]:
+    """See :py:class:`~xgboost.federated.FederatedTracker` for more info.
+
+    Parameters
+    ----------
+    blocking :
+        Block the server until the training is finished. If set to False, the function
+        launches an additional thread and returns the worker arguments. The default is
+        True and a higher level framework is responsible for setting worker parameters.
+
+    """
     args: Dict[str, Any] = {"n_workers": n_workers}
     secure = all(
         path is not None
@@ -78,6 +88,10 @@ def run_federated_server(  # pylint: disable=too-many-arguments
     )
     tracker.start()
 
+    if blocking:
+        tracker.wait_for()
+        return None
+
     thread = Thread(target=tracker.wait_for)
     thread.daemon = True
     thread.start()

tests/python/test_collective.py

@@ -63,7 +63,7 @@ def test_federated_communicator():
     world_size = 2
     tracker = multiprocessing.Process(
         target=federated.run_federated_server,
-        kwargs={"port": port, "n_workers": world_size},
+        kwargs={"port": port, "n_workers": world_size, "blocking": False},
     )
     tracker.start()
     if not tracker.is_alive():