Deterministic scheduling

[mandolaerik]

It would be nice with a documented way to make scheduling deterministic when running in guest mode. I have a use case with no I/O, where we value deterministic behavior.

Example: The following program sometimes prints 0 1 2 3 4, and sometimes prints 4 3 2 1 0.

from queue import SimpleQueue, Empty
import trio

class Loop:
    def __init__(self):
        self.q = SimpleQueue()
    def process_work(self):
        while True:
            try:
                f = self.q.get_nowait()
            except Empty:
                return
            f()

loop = Loop()

async def task(i):
    print(i)
async def run_nursery():
    async with trio.open_nursery() as nursery:
        for i in range(5):
            nursery.start_soon(task, i)

trio.lowlevel.start_guest_run(
    run_nursery,
    run_sync_soon_threadsafe=loop.q.put,
    done_callback=lambda _: None)
loop.process_work()

I have a hideously crude workaround to suppress the problem for now:

trio._core._run._r.random = lambda: 1

but it would be nice with a supported way to do this.

[Zac-HD]

This is a big design question, discussed in #32.