Delay of 0.1 seconds between asyncio MPD commands causes hang #173
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yakshaver2000
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Feb 9, 2023
When two commands were issued exactly 0.1 seconds apart, then `mpd.asyncio.MPDClient.__run` would incorrectly send an "idle" command to the server, and then attempt to parse the server's response to the second command as an "idle" response, causing errors like ``` mpd.base.ProtocolError: Expected key 'volume', got 'repeat' ``` as described by Mic92#195, and hangs as described by Mic92#173. The root of the problem is that wrapping `asyncio.Queue.get` in `asyncio.wait_for` can result in a `TimeoutError` exception even when the queue is not empty, as demonstrated by the following example (tested with python 3.9.2 and 3.11.2): ```python import asyncio TIMEOUT = 0.1 async def get_from_queue(queue): try: await asyncio.wait_for( queue.get(), timeout=TIMEOUT ) except asyncio.exceptions.TimeoutError: # This is counterintuitive: The "get" operation has timed out, # but the queue is not empty! assert not queue.empty() else: # This block is never executed. assert False async def main(): queue = asyncio.Queue() task = asyncio.create_task(get_from_queue(queue)) await asyncio.sleep(TIMEOUT) queue.put_nowait(1) await task asyncio.run(main()) ```
yakshaver2000
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Feb 9, 2023
When two commands were issued exactly 0.1 seconds apart, then `mpd.asyncio.MPDClient.__run` would incorrectly send an "idle" command to the server, and then attempt to parse the server's response to the second command as an "idle" response, causing errors like ``` mpd.base.ProtocolError: Expected key 'volume', got 'repeat' ``` as described by Mic92#195, and hangs as described by Mic92#173. The root of the problem is that wrapping `asyncio.Queue.get` in `asyncio.wait_for` can result in a `TimeoutError` exception even when the queue is not empty, as demonstrated by the following example (tested with python 3.9.2 and 3.11.2): ```python import asyncio TIMEOUT = 0.1 async def get_from_queue(queue): try: await asyncio.wait_for( queue.get(), timeout=TIMEOUT ) except asyncio.exceptions.TimeoutError: # This is counterintuitive: The "get" operation has timed out, # but the queue is not empty! assert not queue.empty() else: # This block is never executed. assert False async def main(): queue = asyncio.Queue() task = asyncio.create_task(get_from_queue(queue)) await asyncio.sleep(TIMEOUT) queue.put_nowait(1) await task asyncio.run(main()) ```
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Consider this snippet:
Running this with Python 3.9.5 and python-mpd2 3.0.4, I get this output:
At this point, the second call to
client.status()never completes, and the program hangs. If the 0.1 second delay is changed to any other value, the problem goes away. It seems like this is some kind of race condition withIMMEDIATE_COMMAND_TIMEOUT.The text was updated successfully, but these errors were encountered: