Fix the panic bug of lease

Signed-off-by: JmPotato <ghzpotato@gmail.com>

server/election/leadership_test.go

@@ -104,4 +104,10 @@ func (s *testLeadershipSuite) TestLeadership(c *C) {
 
 	c.Assert(leadership1.Check(), IsFalse)
 	c.Assert(leadership2.Check(), IsTrue)
+
+	// Test resetting the leadership.
+	leadership1.Reset()
+	leadership2.Reset()
+	c.Assert(leadership1.Check(), IsFalse)
+	c.Assert(leadership2.Check(), IsFalse)
 }

server/election/lease.go

@@ -67,6 +67,9 @@ func (l *lease) Grant(leaseTimeout int64) error {
 
 // Close releases the lease.
 func (l *lease) Close() error {
+	if l == nil {
+		return nil
+	}
 	// Reset expire time.
 	l.expireTime.Store(time.Time{})
 	// Try to revoke lease to make subsequent elections faster.
@@ -79,6 +82,9 @@ func (l *lease) Close() error {
 // IsExpired checks if the lease is expired. If it returns true,
 // current leader should step down and try to re-elect again.
 func (l *lease) IsExpired() bool {
+	if l == nil {
+		return true
+	}
 	if l.expireTime.Load() == nil {
 		return false
 	}