test: Add failing test for set_config() blocking concurrent get()

ldclient/testing/test_ldclient_singleton.py

@@ -1,7 +1,12 @@
 import json
+import threading
+import time
+
+import pytest
 
 import ldclient
 from ldclient import _reset_client
+from ldclient.client import LDClient
 from ldclient.config import Config
 from ldclient.testing.http_util import BasicResponse, start_server
 from ldclient.testing.stub_util import make_put_event, stream_content
@@ -73,3 +78,53 @@ def test_set_config():
                 assert r.headers['Authorization'] == sdk_key
             finally:
                 _reset_client()
+
+
+@pytest.mark.xfail(strict=True, reason="set_config() holds the global write lock across the old client's close()")
+def test_set_config_does_not_block_concurrent_get():
+    """
+    INVARIANT: reconfiguring the shared client does not stall unrelated threads that are only
+    reading it. ldclient.get() is on the hot path of every flag evaluation in an application.
+
+    set_config() holds the global write lock (ldclient/__init__.py) across both the construction
+    of the replacement client and old_client.close(). ReadWriteLock.lock() holds the underlying
+    mutex for its whole duration, so every concurrent ldclient.get() - which takes a read lock -
+    blocks until both of those finish.
+
+    That couples a network-dependent shutdown to a lock every evaluating thread needs. A close()
+    that stalls stalls the entire application, not just the thread that called set_config().
+    """
+    _reset_client()
+    close_duration = 3.0
+    real_close = LDClient.close
+
+    def slow_close(self):
+        time.sleep(close_duration)
+        real_close(self)
+
+    try:
+        ldclient.set_config(Config(sdk_key, offline=True))
+        ldclient.get()
+
+        LDClient.close = slow_close  # type: ignore[method-assign]
+
+        reconfiguring = threading.Event()
+
+        def reconfigure():
+            reconfiguring.set()
+            ldclient.set_config(Config(sdk_key, offline=True))
+
+        threading.Thread(target=reconfigure, name='reconfigure', daemon=True).start()
+        assert reconfiguring.wait(5)
+        time.sleep(0.2)  # let set_config get inside the write lock
+
+        started = time.time()
+        ldclient.get()
+        blocked_for = time.time() - started
+
+        assert blocked_for < close_duration / 2, (
+            "ldclient.get() blocked for %.1fs while set_config() was closing the previous client" % blocked_for
+        )
+    finally:
+        LDClient.close = real_close  # type: ignore[method-assign]
+        _reset_client()