[Feature] Fix Model Serving User Credentials threading scenarios (#907)

## What changes are proposed in this pull request?

Previously in order to get invokers tokens, we always looked at the
current thread's thread data. however this is not always guaranteed as
langchain is internally multi threading calls. Therefore this PR only
looks at the main thread's data to get the credentials. The Main Thread
is guaranteed to have invokers credentials as we set them initially in
the scoring server.

## How is this tested?

Added unit tests

---------

Signed-off-by: aravind-segu <[email protected]>
Co-authored-by: Renaud Hartert <[email protected]>

Author: aravind-segu

NEXT_CHANGELOG.md

@@ -13,5 +13,6 @@
 * Update Jobs ListRuns API to support paginated responses ([#890](https://github.com/databricks/databricks-sdk-py/pull/890))
 * Introduce automated tagging ([#888](https://github.com/databricks/databricks-sdk-py/pull/888))
 * Update Jobs GetJob API to support paginated responses ([#869](https://github.com/databricks/databricks-sdk-py/pull/869)).
+* Update On Behalf Of User Authentication in Multithreaded applications ([#907](https://github.com/databricks/databricks-sdk-py/pull/907))
 
 ### API Changes

databricks/sdk/credentials_provider.py

@@ -853,8 +853,8 @@ def _get_model_dependency_oauth_token(self, should_retry=True) -> str:
         return self.current_token
 
     def _get_invokers_token(self):
-        current_thread = threading.current_thread()
-        thread_data = current_thread.__dict__
+        main_thread = threading.main_thread()
+        thread_data = main_thread.__dict__
         invokers_token = None
         if "invokers_token" in thread_data:
             invokers_token = thread_data["invokers_token"]

tests/test_model_serving_auth.py

@@ -198,6 +198,26 @@ def test_agent_user_credentials(monkeypatch, mocker):
     assert cfg.host == "x"
     assert headers.get("Authorization") == f"Bearer {invokers_token_val}"
 
+    # Test invokers token in child thread
+
+    successful_authentication_event = threading.Event()
+
+    def authenticate():
+        try:
+            cfg = Config(credentials_strategy=ModelServingUserCredentials())
+            headers = cfg.authenticate()
+            assert cfg.host == "x"
+            assert headers.get("Authorization") == f"Bearer databricks_invokers_token_v2"
+            successful_authentication_event.set()
+        except Exception:
+            successful_authentication_event.clear()
+
+    thread = threading.Thread(target=authenticate)
+
+    thread.start()
+    thread.join()
+    assert successful_authentication_event.is_set()
+
 
 # If this credential strategy is being used in a non model serving environments then use default credential strategy instead
 def test_agent_user_credentials_in_non_model_serving_environments(monkeypatch):