objectstorage: add GetSize to Bucket interface

Add GetSize, which returns the size in bytes of an object by issuing a
HeadObject request. This will be used by the MantleBackup controller to
measure the size of the diff parts uploaded to the object storage.

Signed-off-by: Kohya Shiozaki <kouyan120706@gmail.com>

Author: skoya76

internal/controller/internal/objectstorage/objectstorage.go

@@ -7,4 +7,7 @@ type Bucket interface {
 
 	// Delete deletes the specified object. Delete will return nil if the object is not found.
 	Delete(ctx context.Context, path string) error
+
+	// GetSize returns the size of the specified object in bytes.
+	GetSize(ctx context.Context, path string) (int64, error)
 }

internal/controller/internal/objectstorage/objectstorage_mock.go

@@ -78,6 +78,21 @@ func (b *S3Bucket) Exists(ctx context.Context, key string) (bool, error) {
 	return true, nil
 }
 
+func (b *S3Bucket) GetSize(ctx context.Context, key string) (int64, error) {
+	output, err := b.s3Client.HeadObject(ctx, &s3.HeadObjectInput{
+		Bucket: &b.bucketName,
+		Key:    &key,
+	})
+	if err != nil {
+		return 0, fmt.Errorf("HeadObject failed: %s: %s: %s: %w", b.endpoint, b.bucketName, key, err)
+	}
+	if output.ContentLength == nil {
+		return 0, fmt.Errorf("HeadObject returned nil ContentLength: %s: %s: %s", b.endpoint, b.bucketName, key)
+	}
+
+	return *output.ContentLength, nil
+}
+
 func (b *S3Bucket) Delete(ctx context.Context, key string) error {
 	if _, err := b.s3Client.DeleteObject(ctx, &s3.DeleteObjectInput{
 		Bucket: &b.bucketName,