What is the effects for this empty select-case-default code block？

I'm trying to understand a pool library codes, and when instancing a pool struct, call a function named startCleanerLocked(t Duration), in this function, there's one empty select...case...default... code block, I cann't understand what is the effect for this code block.

Pool Interface is:

// Pool interface.
type Pool interface {
	Get(ctx context.Context) (io.Closer, error)
	Put(ctx context.Context, c io.Closer, forceClose bool) error
	Close() error
}

List Struct implement Pool Interface,

type List struct {
	// New is an application supplied function for creating and configuring a
	// item.
	//
	// The item returned from new must not be in a special state
	// (subscribed to pubsub channel, transaction started, ...).
	New func(ctx context.Context) (io.Closer, error)

	// mu protects fields defined below.
	mu     sync.Mutex
	cond   chan struct{}
	closed bool
	active int
	// clean stale items
	cleanerCh chan struct{}

	// Stack of item with most recently used at the front.
	idles list.List

	// Config pool configuration
	conf *Config
}

when Create a new pool, startCleanerLocked(t Duration) function be called:

// NewList creates a new pool.
func NewList(c *Config) *List {
	// check Config
	if c == nil || c.Active < c.Idle {
		panic("config nil or Idle Must <= Active")
	}
	// new pool
	p := &List{conf: c}
	p.cond = make(chan struct{})
	p.startCleanerLocked(time.Duration(c.IdleTimeout))
	return p
}

and in startCleanerLocked(t Duration), there is a select...case...default:

// startCleanerLocked
func (p *List) startCleanerLocked(d time.Duration) {
	if d <= 0 {
		// if set 0, staleCleaner() will return directly
		return
	}
	if d < time.Duration(p.conf.IdleTimeout) && p.cleanerCh != nil {
		select {
		case p.cleanerCh <- struct{}{}:
		default:
		}
	}
	// run only one, clean stale items.
	if p.cleanerCh == nil {
		p.cleanerCh = make(chan struct{}, 1)
		go p.staleCleaner()
	}
}

what's the effect for this code block:

select {
	case p.cleanerCh <- struct{}{}:
	default:
}

Seems it's nothing to do...

and in staleCleaner(), there is a same empty select..case...case, also cannot undestand its effect:

// staleCleaner clean stale items proc.
func (p *List) staleCleaner() {
	ticker := time.NewTicker(100 * time.Millisecond)
	for {
		select {
		case <-ticker.C:
		case <-p.cleanerCh: // maxLifetime was changed or db was closed.
		}
		p.mu.Lock()
		if p.closed || p.conf.IdleTimeout <= 0 {
			p.mu.Unlock()
			return
		}
		for i, n := 0, p.idles.Len(); i < n; i++ {
			e := p.idles.Back()
			if e == nil {
				// no possible
				break
			}
			ic := e.Value.(item)
			if !ic.expired(time.Duration(p.conf.IdleTimeout)) {
				// not need continue.
				break
			}
			p.idles.Remove(e)
			p.release()
			p.mu.Unlock()
			ic.c.Close()
			p.mu.Lock()
		}
		p.mu.Unlock()
	}
}

select {
case p.cleanerCh <- struct{}{}:
default:
}

This is a non-blocking select statement. (because there is a default: case)

If there is a receiver goroutine at the other end of the p.cleanerCh channel, i.e. there is a goroutine that's currently "waiting" at a receive operation i.e. <-p.cleanerCh, then the case p.cleanerCh <- struct{}{} is executed immediately which effectively unblocks the receive operation <-p.cleanerCh and then the goroutine can proceed to execute whatever statements follow.

If there is no receiver goroutine then the default: case is immediately executed and the surrounding startCleanerLocked function can proceed to execute whatever statement follow the select statement.

select {
case <-ticker.C:
case <-p.cleanerCh: // maxLifetime was changed or db was closed.
}

This is a blocking select statement. (because there is no default: case)

This select statement blocks the for loop until one of the two communication cases is ready to receive.