gcloud sdk go と context(cancel, timeout)の動作

bq_with_context.go

package main

import (
	"cloud.google.com/go/bigquery"
	"context"
	"fmt"
	"google.golang.org/api/iterator"
	"log"
	"os"
	"time"
)

func main() {
	doneCh := make(chan bool)
	ctx := context.Background()

	log.Println("start")
	// 3sec後にtimeout
	subCtx, cancelFunc := context.WithTimeout(ctx, time.Duration(3)*time.Second)
	defer cancelFunc()

	// // 以下のgoroutineを有効にすると、`context canceled` で終了
	// go func() {
	// 	// 2sec後にcancel
	// 	time.Sleep(time.Duration(2) * time.Second)
	// 	cancelFunc()
	// }()

	go func(subCtx context.Context) {
		projectID := os.Getenv("GCP_PROJECT")
		client, err := bigquery.NewClient(subCtx, projectID)
		if err != nil {
			message := fmt.Sprintf("Failed to create client: %v\n", err)
			log.Println(message)
			return
		}

		// timeout待ち
		time.Sleep(time.Duration(5) * time.Second)

		it := client.Datasets(subCtx)
		for {
			ds, err := it.Next()
			if err == iterator.Done {
				log.Println("iterator done.")
				break
			}
			if err != nil {
				log.Printf("iterate Dataset err: %v", err)
				break
			} else {
				fmt.Println("Dataset:", ds.DatasetID)
			}
		}

		select {
		case <-subCtx.Done():
			log.Println("subCtx Done")
		default:
			log.Println("default end.")
		}
		doneCh <- true
	}(subCtx)

	select {
	case <-doneCh:
		log.Println("doneCh end.")
	}
}

GCP_PROJECT="＊＊＊＊" GOOGLE_APPLICATION_CREDENTIALS=＊＊＊＊.json go run main.go
2020/02/23 13:28:17 start
2020/02/23 13:28:22 iterate Dataset err: context deadline exceeded
2020/02/23 13:28:22 subCtx Done
2020/02/23 13:28:22 doneCh end.

https://godoc.org/cloud.google.com/go/bigquery