test: Add failing test for thread leak when construction fails

ldclient/testing/test_ldclient_construction_failure.py

@@ -0,0 +1,69 @@
+"""
+Tests for what happens to already-started background threads when LDClient construction fails
+partway through.
+"""
+
+import threading
+
+import pytest
+
+from ldclient.client import Config, LDClient
+from ldclient.interfaces import UpdateProcessor
+
+unreachable_uri = "http://fake"
+
+
+class FailingUpdateProcessor(UpdateProcessor):
+    """
+    A data source that fails on start(). Reaching this is realistic: update_processor_class is a
+    documented configuration hook, and the built-in data sources do real work in start().
+    """
+
+    def __init__(self, config, store, ready):
+        pass
+
+    def start(self):
+        raise Exception("deliberate failure while starting the data source")
+
+    def stop(self):
+        pass
+
+    def initialized(self):
+        return False
+
+
+def ldclient_threads() -> set:
+    return {t.name for t in threading.enumerate() if t.name.startswith('ldclient.')}
+
+
+@pytest.mark.xfail(strict=True, reason="a failure partway through __start_up leaves already-started components running with no way to reach them")
+def test_failed_construction_does_not_leak_background_threads():
+    """
+    INVARIANT: if the constructor raises, it leaves nothing running. A caller that never receives
+    a client object has no way to release anything.
+
+    __start_up() creates the event processor - which starts a dispatcher thread, a pool of flush
+    workers and two repeating timer threads - and only then starts the data system. If the data
+    system fails to start, the exception propagates out of the constructor, the caller gets no
+    object, and there is no handle on which to call close(). Every thread the event processor
+    started stays running, along with its HTTP connection pool.
+
+    They are daemon threads, so this does not prevent process exit, but it does leak steadily in
+    any application that retries client construction, and postfork() re-runs this same path.
+    """
+    before = ldclient_threads()
+
+    config = Config(
+        sdk_key='SDK_KEY',
+        base_uri=unreachable_uri,
+        events_uri=unreachable_uri,
+        stream_uri=unreachable_uri,
+        update_processor_class=FailingUpdateProcessor,
+        diagnostic_opt_out=True,
+    )
+
+    with pytest.raises(Exception):
+        LDClient(config=config, start_wait=0)
+
+    leaked = ldclient_threads() - before
+    assert not leaked, "construction failed but left these threads running: %s" % sorted(leaked)