Complete AWS S3 downloader

aws/s3.go

@@ -28,7 +28,8 @@ type S3Client struct {
 	bucket    string
 	key       string
 
-	uploader uploader // for testing via dependency injection
+	uploader   uploader   // for testing via dependency injection
+	downloader downloader // for testing via dependency injection
 }
 
 // NewS3Client returns an instance of an S3Client.
@@ -81,7 +82,13 @@ func (s *S3Client) Download(ctx context.Context, writer io.WriterAt) error {
 		return err
 	}
 
-	downloader := s3manager.NewDownloader(sess)
+	// If a downloader was not provided, use a real S3 downloader.
+	var downloader downloader
+	if s.downloader == nil {
+		downloader = s3manager.NewDownloader(sess)
+	} else {
+		downloader = s.downloader
+	}
 
 	_, err = downloader.DownloadWithContext(ctx, writer, &s3.GetObjectInput{
 		Bucket: aws.String(s.bucket),

aws/s3_test.go

@@ -4,10 +4,12 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"io"
 	"strings"
 	"testing"
 
 	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/service/s3"
 	"github.com/aws/aws-sdk-go/service/s3/s3manager"
 )
 
@@ -116,6 +118,92 @@ func TestS3ClientUploadFail(t *testing.T) {
 	}
 }
 
+func TestS3ClientDownloadOK(t *testing.T) {
+	region := "us-west-2"
+	accessKey := "your-access-key"
+	secretKey := "your-secret-key"
+	bucket := "your-bucket"
+	key := "your/key/path"
+	expectedData := "test data"
+
+	mockDownloader := &mockDownloader{
+		downloadFn: func(ctx aws.Context, w io.WriterAt, input *s3.GetObjectInput, opts ...func(*s3manager.Downloader)) (int64, error) {
+			if *input.Bucket != bucket {
+				t.Errorf("expected bucket to be %q, got %q", bucket, *input.Bucket)
+			}
+			if *input.Key != key {
+				t.Errorf("expected key to be %q, got %q", key, *input.Key)
+			}
+			n, err := w.WriteAt([]byte(expectedData), 0)
+			if err != nil {
+				t.Errorf("error writing to writer: %v", err)
+			}
+			return int64(n), nil
+		},
+	}
+
+	client := &S3Client{
+		region:     region,
+		accessKey:  accessKey,
+		secretKey:  secretKey,
+		bucket:     bucket,
+		key:        key,
+		downloader: mockDownloader,
+	}
+
+	writer := aws.NewWriteAtBuffer(make([]byte, len(expectedData)))
+	err := client.Download(context.Background(), writer)
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if string(writer.Bytes()) != expectedData {
+		t.Errorf("expected downloaded data to be %q, got %q", expectedData, writer.Bytes())
+	}
+}
+
+func TestS3ClientDownloadFail(t *testing.T) {
+	region := "us-west-2"
+	accessKey := "your-access-key"
+	secretKey := "your-secret-key"
+	bucket := "your-bucket"
+	key := "your/key/path"
+
+	mockDownloader := &mockDownloader{
+		downloadFn: func(ctx aws.Context, w io.WriterAt, input *s3.GetObjectInput, opts ...func(*s3manager.Downloader)) (n int64, err error) {
+			return 0, fmt.Errorf("some error related to S3")
+		},
+	}
+
+	client := &S3Client{
+		region:     region,
+		accessKey:  accessKey,
+		secretKey:  secretKey,
+		bucket:     bucket,
+		key:        key,
+		downloader: mockDownloader,
+	}
+
+	writer := aws.NewWriteAtBuffer(nil)
+	err := client.Download(context.Background(), writer)
+	if err == nil {
+		t.Fatal("Expected error, got nil")
+	}
+	if !strings.Contains(err.Error(), "some error related to S3") {
+		t.Fatalf("Expected error to contain %q, got %q", "some error related to S3", err.Error())
+	}
+}
+
+type mockDownloader struct {
+	downloadFn func(ctx aws.Context, w io.WriterAt, input *s3.GetObjectInput, opts ...func(*s3manager.Downloader)) (n int64, err error)
+}
+
+func (m *mockDownloader) DownloadWithContext(ctx aws.Context, w io.WriterAt, input *s3.GetObjectInput, opts ...func(*s3manager.Downloader)) (n int64, err error) {
+	if m.downloadFn != nil {
+		return m.downloadFn(ctx, w, input, opts...)
+	}
+	return 0, nil
+}
+
 type mockUploader struct {
 	uploadFn func(ctx aws.Context, input *s3manager.UploadInput, opts ...func(*s3manager.Uploader)) (*s3manager.UploadOutput, error)
 }