uasyncio/core.py: Make current_task raise when there is no task.

[jimmo]

To match CPython it should raise RuntimeError if no loop is currently active. Previously it returned the most recent task that ran.

I compared doing it this way versus setting it to None, but then we need to add the extra line to declare cur_task.

It's +60 bytes, but only +24 bytes if you don't include the error message for the RuntimeError.

Fixes #11530 CC @peterhinch

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:  +120 +0.015% standard[incl +32(data)]
      stm32:   +56 +0.014% PYBV10
        rp2:   +64 +0.020% PICO

[jimmo]

I tried a few other ways to handle the cur_task variable and this was still the smallest.

[dpgeorge]

The problem with this solution is performance of the asyncio loop: it's now adding another exception handler for the try-finally to the very hot path of task scheduling. And also deleting a global for each loop around the scheduler, which can be costly (a dict lookup).

I did a quick performance comparison with and without this PR and it drops performance by about 6%. The test is simply calling sleep_ms in a tight loop:

async def test(r):
    sleep_ms = asyncio.sleep_ms
    for _ in r:
        await sleep_ms(0)

[dpgeorge]

See #13752 for an alternative.

[jimmo]

Thanks @dpgeorge, yep #13752 is exactly the alternative approach I was referring to in the description

I compared doing it this way versus setting it to None, but then we need to add the extra line to declare cur_task.

Happy to go with that so will close this PR.