Fixes: rng warning & flakey example test (#263)

* fix: retries example flaky test

This test was flaky as the retry backoff _started_ at 3600ms, instead of
capping at 3600ms. This meant that we could have clients picking dead
servers for a lot longer, all potentially stuck in long backoff loops.
This fixes that, and also fixes the commented jitter.

* chore: fix build warning on rng

`rng` is now only used on `tokio_unstable`, so fix the build warning.

Author: Benjscho

examples/cluster/src/lib.rs

@@ -159,8 +159,8 @@ pub fn machine_attrition(
                 break;
             }
 
-            // let mut jitter = (Duration::from_secs(0)..mean_delay).gen_range(&mut rng);
-            tokio::time::sleep(mean_delay).await;
+            let jitter = cluster.rng().random_range(0..mean_delay.as_millis());
+            tokio::time::sleep(Duration::from_millis(jitter as u64)).await;
 
             let machines = cluster.get_machines();
 
@@ -256,7 +256,7 @@ impl Client {
 
             attempt += 1;
 
-            let backoff = std::cmp::max(init_backoff * 2u64.pow(attempt), 3600);
+            let backoff = std::cmp::min(init_backoff * 2u64.pow(attempt), 3600);
             let jitter = self.cluster.rng().random_range(0..backoff);
 
             tokio::time::sleep(Duration::from_millis(jitter)).await;

examples/cluster/tests/cluster.rs

@@ -12,7 +12,7 @@ fn retries() {
 
     let test_duration = Duration::from_secs(10);
 
-    let mut cluster = Cluster::new(50, None);
+    let mut cluster = Cluster::new(50, Some(8877673959163467592));
 
     let mut sim = turmoil::Builder::new()
         .simulation_duration(Duration::from_secs(360))

src/rt.rs

@@ -38,6 +38,7 @@ pub struct Config {
     /// Rng used to seed the tokio runtime.
     ///
     /// Only used if the `tokio_unstable` cfg flag is set.
+    #[cfg_attr(not(tokio_unstable), expect(dead_code))]
     pub rng: Option<SmallRng>,
 }