gh-105836: Fix asyncio.run_coroutine_threadsafe leaving underlying cancelled asyncio task running (#141696)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Lib/test/test_asyncio/test_tasks.py

@@ -3680,6 +3680,30 @@ def task_factory(loop, coro):
         (loop, context), kwargs = callback.call_args
         self.assertEqual(context['exception'], exc_context.exception)
 
+    def test_run_coroutine_threadsafe_and_cancel(self):
+        task = None
+        thread_future = None
+        # Use a custom task factory to capture the created Task
+        def task_factory(loop, coro):
+            nonlocal task
+            task = asyncio.Task(coro, loop=loop)
+            return task
+
+        self.addCleanup(self.loop.set_task_factory,
+                        self.loop.get_task_factory())
+
+        async def target():
+            nonlocal thread_future
+            self.loop.set_task_factory(task_factory)
+            thread_future = asyncio.run_coroutine_threadsafe(asyncio.sleep(10), self.loop)
+            await asyncio.sleep(0)
+
+            thread_future.cancel()
+
+        self.loop.run_until_complete(target())
+        self.assertTrue(task.cancelled())
+        self.assertTrue(thread_future.cancelled())
+
 
 class SleepTests(test_utils.TestCase):
     def setUp(self):