Questions in golang

I list some questions in Go in my daily life and the corresponding answer.

QA 1. the reader <-channel will be blocked until the writer write the channel?

in gocrawl, the worker reads the channel as follows

select {
    case <-this.stop:
            this.logFunc(LogInfo, "stop signal received.")
        return
    default:
        // Nothing, just continue...
}

However, I can't find the code where the crawler writes the chanel but only to find code illustrated below

for {
    // By checking this after each channel reception, there is a bug if the worker
    // wants to reenqueue following an error or a redirection. The pushPopRefCount
    // temporarily gets to zero before the new URL is enqueued. Check the length
    // of the enqueue channel to see if this is really over, or just this temporary
    // state.
    //
    // Check if refcount is zero - MUST be before the select statement, so that if
    // no valid seeds are enqueued, the crawler stops.
    if this.pushPopRefCount == 0 && len(this.enqueue) == 0 {
        this.logFunc(LogInfo, "sending STOP signals...")
        close(this.stop)
        return nil
    }
        ......

I am puzzed about it and wonder does close chanel will make <-channel work. So I make a test ad demenstrated

package main

import (
    "fmt"
    "sync"
    "time"
)

func worker(wg *sync.WaitGroup, c <-chan int) {
    fmt.Println("begin worker")

    time.Sleep(1e9)
    temp, ok := <-c
14     fmt.Printf("read singal : %d and ok= %t 
", temp, ok)
    wg.Done()

    fmt.Println("end worker")
}

func main() {
    fmt.Println("begin main")

    var wg sync.WaitGroup
    wg.Add(1)

    cint := make(chan int, 1)
    go worker(&wg, cint)
    time.Sleep(3e9)
    fmt.Println("close channel in main")
    close(cint)

    wg.Wait()

    fmt.Println("end main")
}

the result is

conclusion:

if the writter calls close(channel), the reader with expression v, ok :=  <-channel will get v=0 and ok=false.

ok= false means the channel is closed.

the writer closing the channel makes the reader "read" the value 0 from the channel.

after further thought, I find that close(channel) boardcast the signal to all the readers, which is a good mechanism. 

in the case of gocrawler, the master can boardcast the "stop" signal to all its workers just use only one chan "stop chan struct{}" instead of using "map[host] chan struct{}" for each of the worker.