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//! Raft leader lease implementation.
//!
//! A leader lease prevents followers from becoming a leader while the current
//! leader's lease is valid. This avoids unnecessary elections when the leader
//! is still reachable.
/// A time-bounded lease held by the Raft leader.
///
/// The lease is defined by:
/// - A `duration_ms` that specifies how long a single grant lasts.
/// - A `granted_at` timestamp (milliseconds since an arbitrary epoch, e.g.
/// Unix epoch) recording when the lease was last renewed.
///
/// The lease is considered valid as long as `ts < granted_at + duration_ms`.
#[derive(Debug, Clone)]
pub struct LeaderLease {
/// Lease duration in milliseconds.
pub duration_ms: u64,
/// Timestamp (ms) when the lease was last granted / renewed.
/// `None` means the lease has never been granted.
pub granted_at: Option<u64>,
}
impl LeaderLease {
/// Create a new `LeaderLease` with the given duration.
///
/// The lease starts in the un-granted state and must be renewed before it
/// can be considered valid.
#[must_use]
pub fn new(duration_ms: u64) -> Self {
Self {
duration_ms,
granted_at: None,
}
}
/// Renew (or initially grant) the lease at timestamp `ts` (milliseconds).
///
/// After calling this, [`Self::is_valid`] will return `true` for any `query_ts`
/// in `[ts, ts + duration_ms)`.
pub fn renew(&mut self, ts: u64) {
self.granted_at = Some(ts);
}
/// Returns `true` if the lease is valid at query timestamp `ts` (ms).
///
/// A lease is valid when it has been granted and `ts < granted_at + duration_ms`.
#[must_use]
pub fn is_valid(&self, ts: u64) -> bool {
match self.granted_at {
None => false,
Some(granted) => {
// Use saturating add to avoid u64 overflow
ts < granted.saturating_add(self.duration_ms)
}
}
}
/// Remaining validity time in milliseconds, or `0` if expired / not granted.
#[must_use]
pub fn remaining_ms(&self, ts: u64) -> u64 {
match self.granted_at {
None => 0,
Some(granted) => {
let expiry = granted.saturating_add(self.duration_ms);
expiry.saturating_sub(ts)
}
}
}
/// Invalidate the lease immediately (e.g., on leader step-down).
pub fn invalidate(&mut self) {
self.granted_at = None;
}
/// Return the expiry timestamp (ms), or `None` if not yet granted.
#[must_use]
pub fn expiry(&self) -> Option<u64> {
self.granted_at.map(|g| g.saturating_add(self.duration_ms))
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_new_lease_is_not_valid() {
let lease = LeaderLease::new(5000);
assert!(!lease.is_valid(0));
assert!(!lease.is_valid(1000));
}
#[test]
fn test_renew_makes_lease_valid() {
let mut lease = LeaderLease::new(5000);
lease.renew(10_000);
assert!(lease.is_valid(10_000));
assert!(lease.is_valid(14_999));
}
#[test]
fn test_lease_expires() {
let mut lease = LeaderLease::new(5000);
lease.renew(10_000);
// Exactly at expiry boundary (10_000 + 5_000 = 15_000) → expired
assert!(!lease.is_valid(15_000));
assert!(!lease.is_valid(16_000));
}
#[test]
fn test_renew_extends_lease() {
let mut lease = LeaderLease::new(3000);
lease.renew(0);
assert!(lease.is_valid(2000));
// Renew again at ts=2000
lease.renew(2000);
assert!(lease.is_valid(4000));
assert!(!lease.is_valid(5001));
}
#[test]
fn test_remaining_ms_before_expiry() {
let mut lease = LeaderLease::new(10_000);
lease.renew(0);
assert_eq!(lease.remaining_ms(3_000), 7_000);
}
#[test]
fn test_remaining_ms_after_expiry() {
let mut lease = LeaderLease::new(5_000);
lease.renew(0);
assert_eq!(lease.remaining_ms(6_000), 0);
}
#[test]
fn test_remaining_ms_not_granted() {
let lease = LeaderLease::new(5_000);
assert_eq!(lease.remaining_ms(0), 0);
}
#[test]
fn test_invalidate_clears_lease() {
let mut lease = LeaderLease::new(5_000);
lease.renew(0);
assert!(lease.is_valid(1_000));
lease.invalidate();
assert!(!lease.is_valid(1_000));
}
#[test]
fn test_expiry_none_before_grant() {
let lease = LeaderLease::new(5_000);
assert!(lease.expiry().is_none());
}
#[test]
fn test_expiry_some_after_grant() {
let mut lease = LeaderLease::new(5_000);
lease.renew(10_000);
assert_eq!(lease.expiry(), Some(15_000));
}
#[test]
fn test_overflow_safe() {
let mut lease = LeaderLease::new(u64::MAX);
lease.renew(1);
// With duration = u64::MAX, expiry = 1 + u64::MAX → saturates at u64::MAX
// is_valid(u64::MAX - 1) should be true (ts < u64::MAX)
assert!(lease.is_valid(u64::MAX - 1));
}
#[test]
fn test_zero_duration_expires_immediately() {
let mut lease = LeaderLease::new(0);
lease.renew(100);
// ts < 100 + 0 = 100: only ts < 100 is valid
assert!(lease.is_valid(99));
assert!(!lease.is_valid(100));
}
}