fix(prune): use saturating_sub in PruneLimiter::deleted_entries_limit_left (#19535)

Author: Snezhkko

crates/prune/prune/src/limiter.rs

@@ -96,7 +96,7 @@ impl PruneLimiter {
 
     /// Returns the number of deleted entries left before the limit is reached.
     pub fn deleted_entries_limit_left(&self) -> Option<usize> {
-        self.deleted_entries_limit.as_ref().map(|limit| limit.limit - limit.deleted)
+        self.deleted_entries_limit.as_ref().map(|limit| limit.limit.saturating_sub(limit.deleted))
     }
 
     /// Returns the limit on the number of deleted entries (rows in the database).
@@ -411,4 +411,35 @@ mod tests {
         sleep(Duration::new(0, 10_000_000)); // 10 milliseconds
         assert!(limiter.is_limit_reached(), "Limit should be reached when time limit is reached");
     }
+
+    #[test]
+    fn test_deleted_entries_limit_left_saturation_and_normal() {
+        // less than limit → no saturation
+        let mut limiter = PruneLimiter::default().set_deleted_entries_limit(10);
+        limiter.increment_deleted_entries_count_by(3);
+        assert_eq!(limiter.deleted_entries_limit_left(), Some(7));
+
+        // equal to limit → saturates to 0
+        let mut limiter = PruneLimiter::default().set_deleted_entries_limit(3);
+        limiter.increment_deleted_entries_count_by(3);
+        assert_eq!(limiter.deleted_entries_limit_left(), Some(0));
+
+        // overrun past limit → saturates to 0
+        let mut limiter = PruneLimiter::default().set_deleted_entries_limit(10);
+        limiter.increment_deleted_entries_count_by(12);
+        assert_eq!(limiter.deleted_entries_limit_left(), Some(0));
+
+        // lowering limit via set → saturates to 0 if below deleted
+        let mut limiter = PruneLimiter::default().set_deleted_entries_limit(20);
+        limiter.increment_deleted_entries_count_by(15);
+        let limiter = limiter.set_deleted_entries_limit(10);
+        assert_eq!(limiter.deleted_entries_limit_left(), Some(0));
+
+        // lowering limit via floor → saturates to 0 if below deleted
+        let mut limiter = PruneLimiter::default().set_deleted_entries_limit(15);
+        limiter.increment_deleted_entries_count_by(14);
+        let denominator = NonZeroUsize::new(8).unwrap();
+        let limiter = limiter.floor_deleted_entries_limit_to_multiple_of(denominator);
+        assert_eq!(limiter.deleted_entries_limit_left(), Some(0));
+    }
 }