pool.go
func (pool *WriterPool) Len() int {
	pool.mu.Lock()
	l := len(pool.givenOut)
	pool.mu.Unlock()
	return l
}
func (pool *WriterPool) Len() int {
	pool.mu.Lock()
	l := len(pool.givenOut)
	pool.mu.Unlock()
	return l
}
func ( pool * WriterPool ) Len ( ) int { pool . mu . Lock ( ) l := len ( pool . givenOut ) pool . mu . Unlock ( ) return l }
// Len is the number of currently given out throttled writers.
Len is the number of currently given out throttled writers .