lease: randomize expiry on initial refresh call

Randomize the very first expiry on lease recovery
to prevent recovered leases from expiring all at
the same time.

Address etcd-io#8096.

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

lease/lessor.go

@@ -18,6 +18,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"math"
+	"math/rand"
 	"sort"
 	"sync"
 	"sync/atomic"
@@ -491,6 +492,7 @@ func (le *lessor) initAndRecover() {
 			itemSet: make(map[LeaseItem]struct{}),
 			expiry:  forever,
 			revokec: make(chan struct{}),
+			fresh:   true,
 		}
 	}
 	tx.Unlock()
@@ -508,6 +510,9 @@ type Lease struct {
 	mu      sync.RWMutex
 	itemSet map[LeaseItem]struct{}
 	revokec chan struct{}
+
+	// 'true' when lease is just recovered
+	fresh bool
 }
 
 func (l *Lease) expired() bool {
@@ -536,6 +541,13 @@ func (l *Lease) TTL() int64 {
 // refresh refreshes the expiry of the lease.
 func (l *Lease) refresh(extend time.Duration) {
 	t := monotime.Now().Add(extend + time.Duration(l.ttl)*time.Second)
+	// 'fresh' lease only created from 'initAndRecover'
+	// randomize expiry with 士10%, to prevent such recovered
+	// leases of same TTL from expiring all at the same time,
+	if l.fresh {
+		t.Add(time.Duration(int64(float64(l.ttl)*0.1*rand.Float64())) * time.Second)
+		l.fresh = false
+	}
 	atomic.StoreUint64((*uint64)(&l.expiry), uint64(t))
 }