`call_soon_threadsafe` callbacks from executor no longer run before `await run_in_executor()` returns in 3.14.3

[sairon]

Bug report

Bug description:

Python 3.14.3 introduced a behavior change of how run_in_executor interacts with call_soon_threadsafe. Previously, callbacks scheduled via call_soon_threadsafe from within an executor job were guaranteed to run before await run_in_executor() returned.

This seems to be a direct cause of changes from gh-141696, as monkey-patching the asyncio.futures._chain_future reverts to the old behavior.

Reproducer:

import asyncio
import sys
import threading
from concurrent.futures import ThreadPoolExecutor

print(f"Python version: {sys.version}")


async def test_timing():
 loop = asyncio.get_running_loop()
 executor = ThreadPoolExecutor(max_workers=1)
 results = []


 def final_callback():
 results.append("final_callback")

 def intermediate_callback():
 results.append("intermediate_callback")
 loop.call_soon(final_callback)

 def thread_work():
 results.append("thread_work")
 loop.call_soon_threadsafe(intermediate_callback)

 await loop.run_in_executor(executor, thread_work)
 results.append("executor_done")

 await asyncio.sleep(0)
 results.append("after_sleep_0")

 executor.shutdown(wait=False)

 final_ran_before_sleep = (
 "final_callback" in results
 and results.index("final_callback") < results.index("after_sleep_0")
 )
 return final_ran_before_sleep, results


async def main():
 for i in range(20):
 success, results = await test_timing()
 print(f" Iteration {i+1} {'PASS' if success else 'FAIL'}: {results}")


asyncio.run(main())

3.14.2:

Python version: 3.14.2 (main, Feb 5 2026, 13:08:22) [GCC 15.2.1 20260103]
 Iteration 1 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 (rest is the same)

3.14.3:

Python version: 3.14.3 (main, Feb 5 2026, 12:15:05) [GCC 15.2.1 20260103]
 Iteration 1 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 2 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 3 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 4 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 5 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 6 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 7 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 8 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 9 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 10 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 11 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 12 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 13 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 14 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 15 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 16 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 17 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 18 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 19 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']
 Iteration 20 FAIL: ['thread_work', 'executor_done', 'intermediate_callback', 'after_sleep_0']

3.14.3, with asyncio.sleep(1e-15), all callbacks run but order changes:

Python version: 3.14.3 (main, Feb 5 2026, 12:15:05) [GCC 15.2.1 20260103]
 Iteration 1 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 2 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 3 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 4 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 5 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 6 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 7 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 8 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 9 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 10 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 11 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 12 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 13 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 14 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 15 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 16 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 17 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 18 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']
 Iteration 19 PASS: ['thread_work', 'intermediate_callback', 'final_callback', 'executor_done', 'after_sleep_0']
 Iteration 20 PASS: ['thread_work', 'executor_done', 'intermediate_callback', 'final_callback', 'after_sleep_0']

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[kumaraditya303]

Previously, callbacks scheduled via call_soon_threadsafe from within an executor job were guaranteed to run before await run_in_executor() returned.

Hmm, AFAIK this was never guaranteed, it just happened to work like that on previous versions. If you change the call_soon to call_later with small delay such as 0.001 in intermediate_callback then you would see this behaviour on older versions as well.

[kumaraditya303]

I'll close this as this is being fixed on home-assistant side.