rt: event_interval panics when set to 0

[mattiapitossi]

Motivation

The following configuration fails:

    let rt = Builder::new_multi_thread()
        .enable_all()
        .event_interval(0)
        .build()
        .unwrap();

with

thread 'tokio-runtime-worker' (8988985) panicked at src/runtime/scheduler/multi_thread/worker.rs:740:12:
attempt to calculate the remainder with a divisor of zero

Due to this part of the code in the multi_threaded worker:

   fn maintenance(&self, mut core: Box<Core>) -> Box<Core> {
        if core.tick % self.worker.handle.shared.config.event_interval == 0 {

It seems that event_interval=0 also does not work with current_thread runtime, for example with channel notification that are never handled in the block_on loop:

 fn block_on<F: Future>(self, future: F) -> F::Output {
...
   for _ in 0..handle.shared.config.event_interval {
... handles notify and run the task

While I'm not sure if Tokio should support event_interval to be 0, at the current moment it panics on the multi_thread runtime and it loops forever in the current_thread (please let me know if this requires an issue).

Solution

I'm proposing a similar solution that was applied to the global_queue_interval to avoid unintended behavior when this value is zero.

[Darksonn]

Thanks.