How to return at once in function when the context is cancel in GoLang?

December 4, 2021

package main

import (
    "context"
    "fmt"
    "time"
)

func main() {
    ctx := context.Background()
    c, fn := context.WithCancel(ctx)
    go doSth(c)
    time.Sleep(1 * time.Second)
    fn()
    time.Sleep(10 * time.Second)
}

func doSth(ctx context.Context) {
    fmt.Println("doing")
    time.Sleep(2 * time.Second)
    fmt.Println("still doing")
    select {
    case <-ctx.Done():
        fmt.Println("cancel")
        return
    }
}

OUTPUT:

doing
still doing
cancel

I don’t know how to make this doSth function return when the context it get is canncel.

In another word, I want the output of this function is:

OUTPUT:

doing
cancel

>Solution :

You can use a timer, which will send a message over a channel after the given duration. This allows you to add it in the select.

func main() {
    ctx := context.Background()
    c, fn := context.WithCancel(ctx)
    go doSth(c)
    time.Sleep(1 * time.Second)
    fn()
    time.Sleep(10 * time.Second)
}

func doSth(ctx context.Context) {
    fmt.Println("doing")
    timer := time.NewTimer(2 * time.Second)
    select {
    case <-timer.C:
        fmt.Println("still doing")
    case <-ctx.Done():
        fmt.Println("cancel")
    }
}