This code helps to upload/download Google Cloud Objects of large size without dealing actually with object's bytes in Go

gcs_resumable_uploads.go

package main

import (
	"bytes"
	"cloud.google.com/go/storage"
	"context"
	"fmt"
	"github.com/motemen/go-loghttp"
	"github.com/pkg/errors"
	"github.com/spf13/viper"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/option"
	htransport "google.golang.org/api/transport/http"
	"io/ioutil"
	"log"
	"net/http"
	"time"
)
var storageClient *http.Client

func initStorageClient() {
	ctx := context.Background()
	var err error

	// obtain first JSON credential file for service account
	storageClient, _, err = htransport.NewClient(ctx, option.WithScopes(storage.ScopeFullControl),
		option.WithCredentialsFile(viper.GetString("GOOGLE_CREDENTIALS_FILE")))
	if err != nil {
		log.Fatalln(err)
	}

  // just for logs, wrapper could be removed
	storageClient.Transport = &loghttp.Transport{
		Transport: storageClient.Transport,
		LogRequest: func(req *http.Request) {
			log.Printf("--> %s %s", req.Method, req.URL)
		},
	}
}
// obtains url ready to send data
// save this url per each object for 1 week maximum
// anyone having this URL able to upload given object, so take care about security
func getResumableUploadUrl(bucket, object string) (*string, error) {
	req, err := http.NewRequest("POST", fmt.Sprintf("https://storage.googleapis.com/upload/storage/v1/b/%s/o?uploadType=resumable&name=%s", bucket, object), bytes.NewBuffer(nil))
	if err != nil {
		return nil, err
	}
	resp, err := storageClient.Do(req)

	if err != nil {
		return nil, err
	}
	location := resp.Header.Get("Location")
	if location == "" {
		return nil, errors.New("no redirect url")
	}
	return &location, nil
}
// gets signed url to download object
func getDownloadUrl(bucket, object string) (*string, error) {

	jsonKey, err := ioutil.ReadFile(viper.GetString("GOOGLE_CREDENTIALS_FILE"))
	if err != nil {
		return nil, fmt.Errorf("cannot read the JSON key file, err: %v", err)
	}

	conf, err := google.JWTConfigFromJSON(jsonKey)
	if err != nil {
		return nil, fmt.Errorf("google.JWTConfigFromJSON: %v", err)
	}

	opts := &storage.SignedURLOptions{
		Scheme:         storage.SigningSchemeV4,
		Method:         "GET",
		GoogleAccessID: conf.Email,
		PrivateKey:     conf.PrivateKey,
		Expires:        time.Now().Add(15 * time.Minute),
	}

	u, err := storage.SignedURL(bucket, object, opts)
	if err != nil {
		return nil, fmt.Errorf("unable to generate a signed URL: %v", err)
	}
	return &u, nil
}