Example of using Golang errgroup with context.WithTimeout()

main.go

package main

import (
	"context"
	"fmt"
	"io"
	"log"
	"net/http"
	"os"
	"time"

	"golang.org/x/sync/errgroup"
)

var (
	httpClient             = http.Client{}
	baseURL                = "https://funkeinteraktiv.b-cdn.net"
	currentDataEndpoint    = "/current.v4.csv"
	historicalDataEndpoint = "/history.light.v4.csv"
	currentDataURL         = fmt.Sprintf("%s%s", baseURL, currentDataEndpoint)
	historicalDataURL      = fmt.Sprintf("%s%s", baseURL, historicalDataEndpoint)
)

func saveData(ctx context.Context, url, file string) error {
	errChan := make(chan error)

	go func() {
		req, err := http.NewRequestWithContext(ctx, "GET", url, nil)
		if err != nil {
			errChan <- err
			return
		}

		req.Header.Add("user-agent", "Mozilla/5.0")

		res, err := httpClient.Do(req)
		if err != nil {
			errChan <- err
			return
		}

		defer res.Body.Close()

		f, err := os.Create(file)
		if err != nil {
			errChan <- err
			return
		}
		defer f.Close()

		_, err = io.Copy(f, res.Body)
		if err != nil {
			errChan <- err
			return
		}
		errChan <- nil
	}()

	for {
		select {
		case <-ctx.Done():
			return ctx.Err()
		case err := <-errChan:
			if err != nil {
				return fmt.Errorf("saveData: %w", err)
			}
			return nil
		}
	}

	return nil
}

func saveOriginalData() error {
	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
	g, ctx := errgroup.WithContext(ctx)
	defer cancel()

	fn := func(ctx context.Context, url, file string) func() error {
		return func() error {
			return saveData(ctx, url, file)
		}
	}

	g.Go(fn(ctx, currentDataURL, "./data/current.csv"))
	g.Go(fn(ctx, historicalDataURL, "./data/historical.csv"))

	return g.Wait()
}

func main() {
	err := saveOriginalData()
	if err != nil {
		log.Fatal(err)
	}
}

good

I have some questions: say L85 starts go-routine A and L86 starts go-routine B.

• Will L27~L57 starts new go-routines? Say A start X and B start Y.
• Suppose X runs 100ms then failed due to 5xx. And Y runs 5 seconds then successes.
• When will A/B/Y be finished/cancelled?
• In my understanding, 1) X fails first after 100ms due to 5xx, 2) then A fails due to L63, which returns error to errgroup in L65. 3) then B fails due to L61. 4) Y successes after 5s cause no one stops it.
  Am I right? Should Y be stopped correctly?