pool.go

import "time"

type Session struct {
	ID        string
	CreatedAt time.Time
}

type SessionPool struct {
	size     int
	ttl      time.Duration
	flow     chan Session
	producer func() (Session, error)
}

func NewSessionPool(producer func() Session, size int, ttl time.Duration) *SessionPool {
	s := &SessionPool{
		ttl:      ttl,
		size:     size,
		producer: producer,
		flow:     make(chan Session, size),
	}
	go s.renew()
}

func (s *SessionPool) Put(sess Session) {
	if time.Since(sess.CreatedAt) >= s.ttl {
		return
	}
	s.tryPutSession(sess)
}

func (s *SessionPool) Get(timeout time.Duration) Session {
	select {
	case session := <-s.flow:
		if time.Since(session.CreatedAt) < s.ttl {
			return session
		}
	default:
	}
	for i := 0; i < 3; i++ {
		go s.tryPutSession(s.producer())
	}
	return <-s.flow
}

func (s *SessionPool) renew() {
	t := time.NewTimer(s.ttl)
	for range t.C {
		for i := 0; i < s.size; i++ {
			s.trySkipSession()
		}
		for i := 0; i < s.size; i++ {
			go s.tryPutSession(s.producer())
		}
		t.Reset(s.ttl)
	}
}

func (s *SessionPool) trySkipSession() {
	select {
	case <-s.flow:
	default:
	}
}

func (s *SessionPool) tryPutSession(sess Session) {
	select {
	case s.flow <- sess:
	default:
	}
}