gistfile1.go

var limitSem = make(chan int, 100) // TODO(dsymonds): Use environment variable.

func limitRelease() {
	// non-blocking
	select {
	case <-limitSem:
	default:
		// This should not normally happen.
		log.Print("appengine: unbalanced limitSem release!")
	}
}

func limitDial(network, addr string) (net.Conn, error) {
	limitSem <- 1

	// Dial with a timeout in case the API host is MIA.
	// The connection should normally be very fast.
	conn, err := net.DialTimeout(network, addr, 500*time.Millisecond)
	if err != nil {
		limitRelease()
		return nil, err
	}
	lc := &limitConn{Conn: conn}
	return lc, nil
}

type limitConn struct {
	relonce sync.Once
	net.Conn
}

func (lc *limitConn) Close() error {
	defer lc.relonce.Do(func() {
		limitRelease()
	})
	return lc.Conn.Close()
}