// reverseHealthCheck requests the status page of each idle buildlet.                                                                                              
// If the buildlet fails to respond promptly, it is removed from the pool.                                                                                         
func (p *reverseBuildletPool) reverseHealthCheck() {
        p.mu.Lock()
        responses := make(map[*reverseBuildlet]chan error)
        for _, b := range p.buildlets {
                if b.inUseAs == "health" { // sanity check                                                                                                         
                        panic("previous health check still running")
                }
                if b.inUseAs != "" {
                        continue // skip busy buildlets                                                                                                            
                }
                b.inUseAs = "health"
                res := make(chan error, 1)
                responses[b] = res
                client := b.client
                go func() {
                        _, err := client.Status()
                        res <- err
                }()
        }
        p.mu.Unlock()
        time.Sleep(5 * time.Second) // give buildlets time to respond                                                                                              
        p.mu.Lock()

        var buildlets []*reverseBuildlet
        for _, b := range p.buildlets {
                res := responses[b]
                if b.inUseAs != "health" || res == nil {
                        // buildlet skipped or registered after health check                                                                                       
                        buildlets = append(buildlets, b)
                        continue
                }
                b.inUseAs = ""
                err, done := <-res     // <------------ HERE