PYTHON-4147: Silence noisy thread.start() RuntimeError at shutdown (#1486)

doc/changelog.rst

@@ -29,6 +29,9 @@ PyMongo 4.7 brings a number of improvements including:
 
 .. _orjson: https://github.com/ijl/orjson
 
+- Fixed a bug appearing in Python 3.12 where "RuntimeError: can't create new thread at interpreter shutdown"
+  could be written to stderr when a MongoClient's thread starts as the python interpreter is shutting down.
+
 Changes in Version 4.6.1
 ------------------------
 

pymongo/periodic_executor.py

@@ -16,6 +16,7 @@
 
 from __future__ import annotations
 
+import sys
 import threading
 import time
 import weakref
@@ -91,7 +92,15 @@ class PeriodicExecutor:
             thread.daemon = True
             self._thread = weakref.proxy(thread)
             _register_executor(self)
-            thread.start()
+            # Mitigation to RuntimeError firing when thread starts on shutdown
+            # https://github.com/python/cpython/issues/114570
+            try:
+                thread.start()
+            except RuntimeError as e:
+                if "interpreter shutdown" in str(e) or sys.is_finalizing():
+                    self._thread = None
+                    return
+                raise
 
     def close(self, dummy: Any = None) -> None:
         """Stop. To restart, call open().

test/test_monitor.py

@@ -16,6 +16,7 @@
 from __future__ import annotations
 
 import gc
+import subprocess
 import sys
 from functools import partial
 
@@ -79,6 +80,17 @@ class TestMonitor(IntegrationTest):
         for executor in executors:
             wait_until(lambda: executor._stopped, f"closed executor: {executor._name}", timeout=5)
 
+    def test_no_thread_start_runtime_err_on_shutdown(self):
+        """Test we silence noisy runtime errors fired when the MongoClient spawns a new thread
+        on process shutdown."""
+        command = [sys.executable, "-c", "'from pymongo import MongoClient; c = MongoClient()'"]
+        completed_process: subprocess.CompletedProcess = subprocess.run(
+            " ".join(command), shell=True, capture_output=True
+        )
+
+        self.assertFalse(completed_process.stderr)
+        self.assertFalse(completed_process.stdout)
+
 
 if __name__ == "__main__":
     unittest.main()