context-example-001.go

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"net/http/httptest"
	"os"
	"strconv"
	"time"
)

type Foo struct {
	Foo int `json:"foo"`
}

func main() {
	if len(os.Args) != 3 {
		fmt.Printf("Usage: %s <sever-sleep-sec> <request-timeout-sec>\n", os.Args[0])
		return
	}
	sleepSec, err := strconv.Atoi(os.Args[1])
	if err != nil {
		log.Fatalf("[main] %s", err)
	}
	timeoutSec, err := strconv.Atoi(os.Args[2])
	if err != nil {
		log.Fatalf("[main] %s", err)
	}

	ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		for i := 1; i <= sleepSec; i++ {
			log.Printf("[server] %d", i)
			time.Sleep(1 * time.Second)
		}
		w.Write([]byte(`{"foo": 100}`))
	}))
	defer ts.Close()

	ctx, cancel := context.WithTimeout(context.Background(), time.Duration(timeoutSec)*time.Second)
	defer cancel()

	errCh := make(chan error, 1)
	resultCh := make(chan *Foo, 1)
	go func() {
		req, err := http.NewRequest(http.MethodGet, ts.URL, nil)
		if err != nil {
			errCh <- err
			return
		}
		req.WithContext(ctx)
		res, err := http.DefaultClient.Do(req)
		if err != nil {
			errCh <- err
			return
		}
		data, err := ioutil.ReadAll(res.Body)
		if err != nil {
			errCh <- err
			return
		}
		var foo Foo
		if err := json.Unmarshal(data, &foo); err != nil {
			errCh <- err
			return
		}
		resultCh <- &foo
	}()

	select {
	case result := <-resultCh:
		fmt.Printf("[main] Foo: %d\n", result.Foo)
	case err := <-errCh:
		fmt.Printf("[main] error: %s\n", err)
	case <-ctx.Done():
		fmt.Printf("[main] error: %s\n", ctx.Err())
	}

	// NOTE: looks not to be suspended. (because of httptest???)
	time.Sleep(2 * time.Second)
}