Context cancellation example in Go

scratch.go

package main

import (
	"context"
	"fmt"
	"math/rand"
	"os"
	"time"
)

var (
	contextTimeOut time.Duration
	jobDuration time.Duration
)

func init() {
	rand.Seed(time.Now().UnixNano())
	contextTimeOut = time.Second * 10
	jobDuration = time.Second * time.Duration(rand.Intn(20))
}

func main() {
	fmt.Printf("timeout of %.f seconds\n", contextTimeOut.Seconds())
	ctx, cancel := context.WithTimeout(context.Background(), contextTimeOut)
	defer cancel()

	s := 0
	err := foo(ctx)
	if err != nil {
		s = 1
	}

	fmt.Println(err)
	os.Exit(s)
}

// foo describes a long cancellable job
func foo(ctx context.Context) error {
	// out simulate a this job output
	out := make(chan struct{})
	defer close(out)

	// this is the job
	go func() {
		defer fmt.Println("job completed")

		// Wait a random time between now and 20 seconds
		fmt.Printf("job duration will be %.f seconds\n", jobDuration.Seconds())
		time.Sleep(jobDuration)

		// Publish to simulate end of the job
		out <- struct{}{}
	}()

	// If ctx cancelled of timed out, return the error
	// Else, no error to return
	for {
		select {
		case <-ctx.Done():
			return ctx.Err()
		case <-out:
			return nil
		}
	}
}