extmod/uasyncio: Truncate negative sleeps to 0.

Otherwise a task that continuously awaits on a large negative sleep can
monopolise the scheduler (because its wake time is always less than
everything else in the pairing heap).

Signed-off-by: Damien George <damien@micropython.org>

extmod/uasyncio/core.py

@@ -53,7 +53,7 @@ class SingletonGenerator:
 # Use a SingletonGenerator to do it without allocating on the heap
 def sleep_ms(t, sgen=SingletonGenerator()):
     assert sgen.state is None
-    sgen.state = ticks_add(ticks(), t)
+    sgen.state = ticks_add(ticks(), max(0, t))
     return sgen
 
 

tests/extmod/uasyncio_fair.py

@@ -22,6 +22,7 @@ async def main():
     t1 = asyncio.create_task(task(1, -0.01))
     t2 = asyncio.create_task(task(2, 0.1))
     t3 = asyncio.create_task(task(3, 0.2))
+    t3 = asyncio.create_task(task(4, -100))
     await asyncio.sleep(0.5)
     t1.cancel()
     t2.cancel()

tests/extmod/uasyncio_fair.py.exp

@@ -3,6 +3,7 @@ task start 2
 task work 2
 task start 3
 task work 3
+task start 4
 task work 2
 task work 3
 task work 2