René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Asynchronous multiprocessing.Pipe reads throw when using uvloop","articleBody":"## Summary\n\nWhen using an `asyncio.Event()` with `loop.add_reader()` to asynchronously read from a `multiprocessing.Pipe`, the call to `recv_bytes()` throws:\n\n```\nProcess reader:\nTraceback (most recent call last):\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/multiprocessing/process.py\", line 313, in _bootstrap\n    self.run()\n    ~~~~~~~~^^\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/multiprocessing/process.py\", line 108, in run\n    self._target(*self._args, **self._kwargs)\n    ~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/james/code/connection_test.py\", line 25, in reader_wrapper\n    uvloop.run(async_reader(read_pipe))\n    ~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^\n  File \"/home/james/code/.venv/lib/python3.13/site-packages/uvloop/__init__.py\", line 109, in run\n    return __asyncio.run(\n           ~~~~~~~~~~~~~^\n        wrapper(),\n        ^^^^^^^^^^\n    ...\u003c2 lines\u003e...\n        **run_kwargs\n        ^^^^^^^^^^^^\n    )\n    ^\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/asyncio/runners.py\", line 195, in run\n    return runner.run(main)\n           ~~~~~~~~~~^^^^^^\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/asyncio/runners.py\", line 118, in run\n    return self._loop.run_until_complete(task)\n           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^\n  File \"uvloop/loop.pyx\", line 1518, in uvloop.loop.Loop.run_until_complete\n  File \"/home/james/code/.venv/lib/python3.13/site-packages/uvloop/__init__.py\", line 61, in wrapper\n    return await main\n           ^^^^^^^^^^\n  File \"/home/james/code/connection_test.py\", line 20, in async_reader\n    message = read_pipe.recv_bytes()\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/multiprocessing/connection.py\", line 216, in recv_bytes\n    buf = self._recv_bytes(maxlength)\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/multiprocessing/connection.py\", line 437, in _recv_bytes\n    return self._recv(size)\n           ~~~~~~~~~~^^^^^^\n  File \"/home/james/.pyenv/versions/3.13.5/lib/python3.13/multiprocessing/connection.py\", line 395, in _recv\n    chunk = read(handle, remaining)\nBlockingIOError: [Errno 11] Resource temporarily unavailable\n```\n\nThis does not occur when running with `asyncio`\n\n## Environment\n\n    Python: 3.13.5\n    uvloop: 0.21.0\n\n## Reproduction\n\n```python\nimport asyncio\nimport uvloop\nfrom multiprocessing import Process\nfrom multiprocessing.connection import Connection, Pipe, wait\n\ndef writer(write_pipe: Connection):\n    message = b'!' * 65536\n    while True:\n        write_pipe.send_bytes(message)\n\nasync def async_reader(read_pipe: Connection):\n    message_available = asyncio.Event()\n    asyncio.get_running_loop().add_reader(\n        read_pipe.fileno(), message_available.set\n    )\n\n    while True:\n        await message_available.wait()\n        while read_pipe.poll():\n            message = read_pipe.recv_bytes()\n            print('Got message')\n        message_available.clear()\n\ndef reader_wrapper(read_pipe: Connection):\n    uvloop.run(async_reader(read_pipe))\n\nif __name__ == '__main__':\n    read_pipe, write_pipe = Pipe(duplex=False)\n\n    reader_process = Process(\n        name=\"reader\", target=reader_wrapper, args=(read_pipe,)\n    )\n    reader_process.start()\n\n    writer_process = Process(\n        name=\"writer\", target=writer, args=(write_pipe,)\n    )\n    writer_process.start()\n\n    wait([reader_process.sentinel, writer_process.sentinel])\n\n```","author":{"url":"https://github.com/judilsteve","@type":"Person","name":"judilsteve"},"datePublished":"2025-08-15T02:16:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/686/uvloop/issues/686"}