为什么context.Context信号会影响新的goroutine？

Question

I'm using context.Context to control multiple goroutines. A goroutine listens context.Done(), then print now date string. And the process listens syscall.SIGHUP which cancel context and then create new goroutine with context created before.

I think when syscall.SIGHUP emitting, the process will print one date string, however got two.

This process is running on ubuntu18.04.

package main

import (
    "context"
    "fmt"
    "os"
    "os/signal"
    "syscall"
    "time"
)

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    go printDate(ctx)

    sg := make(chan os.Signal)
    signal.Notify(sg, syscall.SIGHUP)

    for {
        select {
        case <-sg:
            cancel()
            go printDate(ctx)
        }
    }
}

func printDate(ctx context.Context) {
    select {
    case <-ctx.Done():
        fmt.Println(time.Now().Format("2006-01-02 15:04:05"))
    }
}

I expect the output to be

2019-01-02 01:02:03

but actual output is:

2019-01-02 01:02:03
2019-01-02 01:02:03

Answer 1

After cancel() your ctx is done and stays done forever. So when you call go printDate(ctx) a second time you call printDate with a done Context and the case can run becasue ctx.Done() returns a closed channel and you can receive from a closed channel.