Avoid running all suites at the same time at scheduling start

This could lead to a high system load. Instead, we start as soon as the
current timestamp is fully divisible by the execution interval.

v2/robotmk/src/bin/scheduler/scheduling/scheduler.rs

@@ -4,10 +4,11 @@ use crate::internal_config::{GlobalConfig, Suite};
 use crate::logging::log_and_return_error;
 
 use anyhow::{bail, Result};
+use chrono::Utc;
 use log::error;
 use std::time::Duration;
 use tokio::task::{spawn_blocking, JoinSet};
-use tokio::time::interval;
+use tokio::time::{interval_at, Instant};
 use tokio_util::sync::CancellationToken;
 
 #[tokio::main]
@@ -35,7 +36,17 @@ pub async fn run_suites_and_cleanup(global_config: &GlobalConfig, suites: &[Suit
 }
 
 async fn run_suite_scheduler(suite: Suite) {
-    let mut clock = interval(Duration::from_secs(suite.execution_interval_seconds));
+    // It is debatable whether MissedTickBehavior::Burst (the default) is correct. In practice, as
+    // long as timeout * number of attempts is shorter than the execution interval, it shouldn't
+    // make a difference anyway.  However, in case we consider changing this, note that using
+    // `MissedTickBehavior::Delay` leads to a strange sort of lag on Windows (as if we added ~10 ms
+    // to the scheduling interval). See also:
+    // https://www.reddit.com/r/rust/comments/13yymkh/weird_tokiotimeinterval_tick_behavior/
+    // https://github.com/tokio-rs/tokio/issues/5021
+    let mut clock = interval_at(
+        compute_start_time(suite.execution_interval_seconds),
+        Duration::from_secs(suite.execution_interval_seconds),
+    );
     loop {
         let suite = suite.clone();
         tokio::select! {
@@ -47,7 +58,7 @@ async fn run_suite_scheduler(suite: Suite) {
 }
 
 async fn run_cleanup_job(cancellation_token: CancellationToken, suites: Vec<Suite>) {
-    let mut clock = interval(Duration::from_secs(300));
+    let mut clock = interval_at(compute_start_time(300), Duration::from_secs(300));
     loop {
         let suites = suites.clone();
         tokio::select! {
@@ -57,3 +68,34 @@ async fn run_cleanup_job(cancellation_token: CancellationToken, suites: Vec<Suit
         spawn_blocking(move || cleanup_working_directories(suites.iter()));
     }
 }
+
+fn compute_start_time(execution_interval_secs: u64) -> Instant {
+    let now = Instant::now();
+    now.checked_add(Duration::from_millis(compute_start_time_offset_millis(
+        Utc::now().timestamp_millis() as u64,
+        execution_interval_secs * 1000,
+    )))
+    .unwrap_or(now)
+}
+
+fn compute_start_time_offset_millis(now_millis: u64, execution_interval_millis: u64) -> u64 {
+    execution_interval_millis - now_millis % execution_interval_millis
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_compute_start_time_offset_millis() {
+        let now_millis: u64 = 1701942935796;
+        let five_min_interval_millis = 5 * 60 * 1000;
+        let expected_offset = 264204;
+        assert_eq!(
+            compute_start_time_offset_millis(now_millis, five_min_interval_millis),
+            expected_offset
+        );
+        assert_eq!((now_millis + expected_offset) % five_min_interval_millis, 0);
+        assert!(expected_offset <= five_min_interval_millis);
+    }
+}