fix: revise the logic of exponential strategy (#71)

1. fix the bug #38 in exponential
strategy
2. revise the logic about 'max_delay is None' scenario

Co-authored-by: yunju.lly <[email protected]>

src/exponential.rs

@@ -139,36 +139,40 @@ impl Iterator for ExponentialBackoff {
         }
         self.attempts += 1;
 
-        match self.current_delay {
+        let mut tmp_cur = match self.current_delay {
             None => {
                 // If current_delay is None, it's must be the first time to retry.
-                let mut cur = self.min_delay;
-                self.current_delay = Some(cur);
-
-                // If jitter is enabled, add random jitter based on min delay.
-                if self.jitter {
-                    cur += self.min_delay.mul_f32(fastrand::f32());
-                }
-
-                Some(cur)
+                self.current_delay = Some(self.min_delay);
+                self.min_delay
             }
             Some(mut cur) => {
                 // If current delay larger than max delay, we should stop increment anymore.
                 if let Some(max_delay) = self.max_delay {
                     if cur < max_delay {
-                        cur = cur.mul_f32(self.factor);
+                        cur = saturating_mul(cur, self.factor);
                     }
+                    if cur > max_delay {
+                        cur = max_delay;
+                    }
+                } else {
+                    cur = saturating_mul(cur, self.factor);
                 }
                 self.current_delay = Some(cur);
-
-                // If jitter is enabled, add random jitter based on min delay.
-                if self.jitter {
-                    cur += self.min_delay.mul_f32(fastrand::f32());
-                }
-
-                Some(cur)
+                cur
             }
+        };
+        // If jitter is enabled, add random jitter based on min delay.
+        if self.jitter {
+            tmp_cur = tmp_cur.saturating_add(self.min_delay.mul_f32(fastrand::f32()));
         }
+        Some(tmp_cur)
+    }
+}
+
+pub(crate) fn saturating_mul(d: Duration, rhs: f32) -> Duration {
+    match Duration::try_from_secs_f32(rhs * d.as_secs_f32()) {
+        Ok(v) => v,
+        Err(_) => Duration::MAX,
     }
 }
 
@@ -231,7 +235,7 @@ mod tests {
     }
 
     #[test]
-    fn test_exponential_max_delay() {
+    fn test_exponential_max_delay_with_default() {
         let mut exp = ExponentialBuilder::default()
             .with_max_delay(Duration::from_secs(2))
             .build();
@@ -242,6 +246,56 @@ mod tests {
         assert_eq!(None, exp.next());
     }
 
+    #[test]
+    fn test_exponential_max_delay_without_default_1() {
+        let mut exp = ExponentialBuilder {
+            jitter: false,
+            factor: 10_000_000_000_f32,
+            min_delay: Duration::from_secs(1),
+            max_delay: None,
+            max_times: None,
+        }
+        .build();
+
+        assert_eq!(Some(Duration::from_secs(1)), exp.next());
+        assert_eq!(Some(Duration::from_secs(10_000_000_000)), exp.next());
+        assert_eq!(Some(Duration::MAX), exp.next());
+        assert_eq!(Some(Duration::MAX), exp.next());
+    }
+
+    #[test]
+    fn test_exponential_max_delay_without_default_2() {
+        let mut exp = ExponentialBuilder {
+            jitter: true,
+            factor: 10_000_000_000_f32,
+            min_delay: Duration::from_secs(10_000_000_000),
+            max_delay: None,
+            max_times: Some(2),
+        }
+        .build();
+        let v = exp.next().expect("value must valid");
+        assert!(v >= Duration::from_secs(10_000_000_000), "current: {v:?}");
+        assert!(v < Duration::from_secs(20_000_000_000), "current: {v:?}");
+        assert_eq!(Some(Duration::MAX), exp.next());
+        assert_eq!(None, exp.next());
+    }
+
+    #[test]
+    fn test_exponential_max_delay_without_default_3() {
+        let mut exp = ExponentialBuilder {
+            jitter: false,
+            factor: 10_000_000_000_f32,
+            min_delay: Duration::from_secs(10_000_000_000),
+            max_delay: Some(Duration::from_secs(60_000_000_000)),
+            max_times: Some(3),
+        }
+        .build();
+        assert_eq!(Some(Duration::from_secs(10_000_000_000)), exp.next());
+        assert_eq!(Some(Duration::from_secs(60_000_000_000)), exp.next());
+        assert_eq!(Some(Duration::from_secs(60_000_000_000)), exp.next());
+        assert_eq!(None, exp.next());
+    }
+
     #[test]
     fn test_exponential_max_times() {
         let mut exp = ExponentialBuilder::default().with_max_times(1).build();