Incorrect uasyncio behaviour: tasks are not correctly terminated
(First reported on the forum)
The following snippet:
try:
import uasyncio as asyncio
except ImportError:
import asyncio
async def count():
i = 0
while True:
print(i)
i += 1
await asyncio.sleep(1)
async def main():
asyncio.create_task(count())
await asyncio.sleep(5)
asyncio.run(main())
asyncio.run(main())
Produces the following in CPython:
0
1
2
3
4
0
1
2
3
4
But in MicroPython:
0
1
2
3
4
0
5
1
6
2
7
3
8
4
9
It appears that tasks are not correctly terminated (?).
A workaround that produces the correct output is to run the second task in a second event loop:
try:
import uasyncio as asyncio
except ImportError:
import asyncio
async def count():
i = 0
while True:
print(i)
i += 1
await asyncio.sleep(1)
async def main():
asyncio.create_task(count())
await asyncio.sleep(5)
asyncio.run(main())
asyncio.new_event_loop() # Added line
asyncio.run(main())
Note that running either of the snippets repeatedly causes more confusing output because all previous tasks are still running on the queue.
There is still a cancellation/race issue in uasyncio wait_for
I know this behavior has been reported before, but I am still seeing it. Tracking a strange condition in our software, where cancelling a task soon after creating it did not work, we found the issue in wait_for and some other reported issues, the last one being #7386
I also found this reported in Python, and a test program mentioned there causes also the issue on micropython : https://bugs.python.org/issue42130
Basically, this code :
import uasyncio as asyncio
async def inner():
return
async def with_for_coro():
await asyncio.wait_for(inner(), timeout=100)
await asyncio.sleep(1)
print('End of with_for_coro. Should not be reached!')
async def main():
task = asyncio.create_task(with_for_coro())
await asyncio.sleep(0)
assert not task.done()
task.cancel()
print('Called task.cancel()')
await task # -> You would expect a CancelledError to be raised.
asyncio.run(main())
Running this, no CancelledError is raised when awaiting task in main() and we get this output:
$ micropython cancel.py
Called task.cancel()
End of with_for_coro. Should not be reached!
Now, if we add a small delay before cancelling, changing for instance to 0.1s:
async def main():
task = asyncio.create_task(with_for_coro())
await asyncio.sleep(0.1)
Then we get the expect result :
$ micropython cancel.py
Called task.cancel()
Traceback (most recent call last):
File "cancel.py", line 21, in <module>
File "/usr/lib/micropython/uasyncio/core.py", line 222, in run
File "/usr/lib/micropython/uasyncio/core.py", line 191, in run_until_complete
File "/usr/lib/micropython/uasyncio/core.py", line 176, in run_until_complete
File "cancel.py", line 18, in main
File "/usr/lib/micropython/uasyncio/task.py", line 145, in __next__
File "/usr/lib/micropython/uasyncio/core.py", line 183, in run_until_complete
File "cancel.py", line 9, in with_for_coro
CancelledError:
This was tested using micropython 1.18