why do we check that tick is not equal to both `a` and `b` when polling Ready？

[goldwind-ting]

Hi, I have a question: when polling returns Ready, why do we check that tick is not equal to both a and b? Wouldn't checking tick != b be enough? Since tick only changes inside react, a change from the old value should already mean an event occurred.

async-io/src/reactor.rs

Line 620 in d31570e

if state[*dir].tick != a && state[*dir].tick != b {

[iamlockon]

@goldwind-ting
I am not sure about the answer, but I think it's because we want to prevent returning Poll::Ready while the reactor tick is still the same as last poll. To have ticks to be Some, it means the future has been polled at least once, and if we are still in the same reactor tick since last poll, it's likely a spurious wake-up.