lessor: extend leases on promote if expires will be rate limited

Instead of unconditionally randomizing, extend leases on promotion
if too many leases expire within the same time span. If the server
has few leases or spread out expires, there will be no extension.

Squashed previous commits for #8149.

This is a combination of 4 commits below:

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 #8096.

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

integration: remove lease exist checking on randomized expiry

Lease with TTL 5 should be renewed with randomization,
thus it's still possible to exist after 3 seconds.

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

lessor: extend leases on promote if expires will be rate limited

Instead of unconditionally randomizing, extend leases on promotion
if too many leases expire within the same time span. If the server
has few leases or spread out expires, there will be no extension.

Revert "integration: remove lease exist checking on randomized expiry"

This reverts commit 95bc33f. The new
lease extension algorithm should pass this test.

lease/lessor.go

@@ -32,15 +32,14 @@ const (
 	// NoLease is a special LeaseID representing the absence of a lease.
 	NoLease = LeaseID(0)
 
-	// maximum number of leases to revoke per iteration
-	// TODO: make this configurable?
-	leaseRevokeRate = 1000
+	forever = monotime.Time(math.MaxInt64)
 )
 
 var (
 	leaseBucketName = []byte("lease")
 
-	forever = monotime.Time(math.MaxInt64)
+	// maximum number of leases to revoke per second; configurable for tests
+	leaseRevokeRate = 1000
 
 	ErrNotPrimary    = errors.New("not a primary lessor")
 	ErrLeaseNotFound = errors.New("lease not found")
@@ -328,8 +327,53 @@ func (le *lessor) Promote(extend time.Duration) {
 	for _, l := range le.leaseMap {
 		l.refresh(extend)
 	}
+
+	if len(le.leaseMap) < leaseRevokeRate {
+		// no possibility of lease pile-up
+		return
+	}
+
+	// adjust expiries in case of overlap
+	leases := make([]*Lease, 0, len(le.leaseMap))
+	for _, l := range le.leaseMap {
+		leases = append(leases, l)
+	}
+	sort.Sort(leasesByExpiry(leases))
+
+	baseWindow := leases[0].Remaining()
+	nextWindow := baseWindow + time.Second
+	expires := 0
+	// have fewer expires than the total revoke rate so piled up leases
+	// don't consume the entire revoke limit
+	targetExpiresPerSecond := (3 * leaseRevokeRate) / 4
+	for _, l := range leases {
+		remaining := l.Remaining()
+		if remaining > nextWindow {
+			baseWindow = remaining
+			nextWindow = baseWindow + time.Second
+			expires = 1
+			continue
+		}
+		expires++
+		if expires <= targetExpiresPerSecond {
+			continue
+		}
+		rateDelay := float64(time.Second) * (float64(expires) / float64(targetExpiresPerSecond))
+		// If leases are extended by n seconds, leases n seconds ahead of the
+		// base window should be extended by only one second.
+		rateDelay -= float64(remaining - baseWindow)
+		delay := time.Duration(rateDelay)
+		nextWindow = baseWindow + delay
+		l.refresh(delay + extend)
+	}
 }
 
+type leasesByExpiry []*Lease
+
+func (le leasesByExpiry) Len() int           { return len(le) }
+func (le leasesByExpiry) Less(i, j int) bool { return le[i].Remaining() < le[j].Remaining() }
+func (le leasesByExpiry) Swap(i, j int)      { le[i], le[j] = le[j], le[i] }
+
 func (le *lessor) Demote() {
 	le.mu.Lock()
 	defer le.mu.Unlock()

lease/lessor_test.go

@@ -210,6 +210,58 @@ func TestLessorRenew(t *testing.T) {
 	}
 }
 
+// TestLessorRenewExtendPileup ensures Lessor extends leases on promotion if too many
+// expire at the same time.
+func TestLessorRenewExtendPileup(t *testing.T) {
+	oldRevokeRate := leaseRevokeRate
+	defer func() { leaseRevokeRate = oldRevokeRate }()
+	leaseRevokeRate = 10
+
+	dir, be := NewTestBackend(t)
+	defer os.RemoveAll(dir)
+
+	le := newLessor(be, minLeaseTTL)
+	ttl := int64(10)
+	for i := 1; i <= leaseRevokeRate*10; i++ {
+		if _, err := le.Grant(LeaseID(2*i), ttl); err != nil {
+			t.Fatal(err)
+		}
+		// ttls that overlap spillover for ttl=10
+		if _, err := le.Grant(LeaseID(2*i+1), ttl+1); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// simulate stop and recovery
+	le.Stop()
+	be.Close()
+	bcfg := backend.DefaultBackendConfig()
+	bcfg.Path = filepath.Join(dir, "be")
+	be = backend.New(bcfg)
+	defer be.Close()
+	le = newLessor(be, minLeaseTTL)
+
+	// extend after recovery should extend expiration on lease pile-up
+	le.Promote(0)
+
+	windowCounts := make(map[int64]int)
+	for _, l := range le.leaseMap {
+		// round up slightly for baseline ttl
+		s := int64(l.Remaining().Seconds() + 0.1)
+		windowCounts[s]++
+	}
+
+	for i := ttl; i < ttl+20; i++ {
+		c := windowCounts[i]
+		if c > leaseRevokeRate {
+			t.Errorf("expected at most %d expiring at %ds, got %d", leaseRevokeRate, i, c)
+		}
+		if c < leaseRevokeRate/2 {
+			t.Errorf("expected at least %d expiring at %ds, got %d", leaseRevokeRate/2, i, c)
+		}
+	}
+}
+
 func TestLessorDetach(t *testing.T) {
 	dir, be := NewTestBackend(t)
 	defer os.RemoveAll(dir)