Golang download multiple files in parallel using goroutines

Question

Is it possible to download and save files in parallel using goroutines?

Below is my code which downloads files from my dropbox:

package main

import (
    "encoding/json"
    "fmt"
    "io"
    "io/ioutil"
    "net/http"
    "net/url"
    "os"
    "path/filepath"
)

const app_key string = "<app_key>"
const app_secret string = "<app_secret>"

var code string

type TokenResponse struct {
    AccessToken string `json:"access_token"`
}

type File struct {
    Path string
}

type FileListResponse struct {
    FileList []File `json:"contents"`
}

func download_file(file File, token TokenResponse) {

    download_file := fmt.Sprintf("https://api-content.dropbox.com/1/files/dropbox/%s?access_token=%s", file.Path, token.AccessToken)

    resp, _ := http.Get(download_file)
    defer resp.Body.Close()

    filename := filepath.Base(file.Path)
    out, err := os.Create(filename)
    if err != nil {
        panic(err)
    }
    defer out.Close()

    io.Copy(out, resp.Body)
}

func main() {

    authorize_url := fmt.Sprintf("https://www.dropbox.com/1/oauth2/authorize?response_type=code&client_id=%s", app_key)

    // Get code
    fmt.Printf("1. Go to: %s\n", authorize_url)
    fmt.Println("2. Click 'Allow' (you might have to log in first)")
    fmt.Println("3. Copy the authorization code.")
    fmt.Printf("Enter the authorization code here: ")
    fmt.Scanf("%s", &code)
    // End get code

    // Get access token
    data := url.Values{}
    data.Add("code", code)
    data.Add("grant_type", "authorization_code")
    data.Add("client_id", app_key)
    data.Add("client_secret", app_secret)

    resp, _ := http.PostForm("https://api.dropbox.com/1/oauth2/token", data)
    defer resp.Body.Close()

    contents, _ := ioutil.ReadAll(resp.Body)

    var tr TokenResponse

    json.Unmarshal(contents, &tr)
    // End get access token

    // Get file list
    file_list_url := fmt.Sprintf("https://api.dropbox.com/1/metadata/dropbox/Camera Uploads?access_token=%s", tr.AccessToken)

    resp2, _ := http.Get(file_list_url)
    defer resp2.Body.Close()

    contents2, _ := ioutil.ReadAll(resp2.Body)

    var flr FileListResponse
    json.Unmarshal(contents2, &flr)
    // End get file list

    for i, file := range flr.FileList {

        download_file(file, tr)

        if i >= 2 {
            break
        }
    }
}

It doesn't work when I prefix the download_file function with the go command.

go download_file(file, tr)

Answer 1

That's because your main goroutine is exiting. You need to add a WaitGroup to wait until all the goroutines exit. For example,

var wg sync.WaitGroup
for i, file := range flr.FileList {
    wg.Add(1)

    go download_file(file, tr, wg)

    if i >= 2 {
        break
    }
}
wg.Wait()

...
func download_file(file File, token TokenResponse, wg sync.WaitGroup) {
    ...
    wg.Done()
}