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asyncio.subprocess blocked event loop unexpectedly on MacOS #158287
Description
Bug report
Bug description:
After upgrading to 3.14.7 & macOS 27, this bug has happened.
When pausing the subprocess created by asyncio.create_subprocess_exec, it will blocked the whole event loop on MacOS.
Here is a minimal test code:
import asyncio
import psutil
async def main():
proc = await asyncio.create_subprocess_exec(
"sleep",
"1000",
)
psutil.Process(proc.pid).suspend()
print("child pid:", proc.pid, flush=True)
count = 0
while True:
await asyncio.sleep(1)
print(f"alive {count}", flush=True)
count += 1
asyncio.run(main())Expectly, it should print:
child pid: xxxxx
alive 0
alive 1
...However, it just prints child pid: xxxxx, without any alive.
This issue isn't caused by psutil because the same result was got with kill -STOP xxxxx.
This runs well on Windows, but not on MacOS. This didn't happen on the previous version of 3.14.
CPython versions tested on:
3.14
Operating systems tested on:
macOS, Windows
Linked PRs
- gh-158287: Fix blocked asyncio event loop on macOS when child process is stopped #158962
- [3.15] gh-158287: Fix blocked asyncio event loop on macOS when child process is stopped (GH-158962) #160087
- [3.14] gh-158287: Fix blocked asyncio event loop on macOS when child process is stopped (GH-158962) #160088
Activity
picnixz commentedon Sep 27, 2026
cc @kumaraditya303
picnixz commentedon Sep 27, 2026
@starstreammm Do you mean that on Python 3.14.6 it works well? or is it on 3.13 that it works well? (this is to confirm whether it's a release blocker or not)
starstreammm commentedon Sep 27, 2026
I updated my Python version yesterday. It had been quite a while since my last update, but I can confirm that I was already using 3.14 after the update. This issue didn't happen on that version. But I don't remember which version I used before exactly.
picnixz commentedon Sep 27, 2026
I'm going to let Kumar have a look at this issue and decide whether this is a release-blocker. At least, it's a regression AFAICT.
picnixz commentedon Sep 27, 2026
Can you bisect the faulty commit first? @deadlovelll
kumaraditya303 commentedon Oct 5, 2026
This seems like a macos kernel bug to me, dotnet is also hitting this quirk in dotnet/runtime#133930
I think the fix is to exclude
CLD_STOPPEDmanually here to avoid blocking inwaitpid.pythongh-158287: Add timeout to test_stopped_child_does_not_block_eve…
pythongh-158287: Clarify user-visible behavior in NEWS entry
pythongh-158287: Simplify CLD_STOPPED handling in _ThreadedChildWatcher
pythongh-158287: Use watchdog timer in test_stopped_child_does_not_bl…
pythongh-158287: Remove redundant inner ChildProcessError in _Threade…
gh-158287: Fix blocked asyncio event loop on macOS when child process…
[3.15] gh-158287: Fix blocked asyncio event loop on macOS when child …
[3.14] gh-158287: Fix blocked asyncio event loop on macOS when child …