Blobstore reader won't read large images

I am trying to read images from the Blobstore from the Go server side code. But if the image is large (as in: 0.5MB - 1.7MB) a large portion of the byte buffer becomes 0 which breaks the image.

The image works if I use serveUrl, but this is not an option for me in this case.

My first thought was that there was a size limit to the read, found this:

> In addition to systemwide safety quotas, a limit applies specifically
> to the use of the Blobstore: the maximum size of Blobstore data that
> can be read by the application with one API call is 32 megabytes.

The images I read are nowhere near 32MB.

Function I use to read from the Blobstore:

func BlobAsBase64(c appengine.Context, blobKey string) (*blobstore.BlobInfo, string, error) {
	info, err := blobstore.Stat(c, appengine.BlobKey(blobKey))
	if err != nil {
		return nil, "", err
	}

	imageBuffer := make([]byte, info.Size)
	reader := blobstore.NewReader(c, appengine.BlobKey(blobKey))
	if _, err = reader.Read(imageBuffer); err != nil {
		return nil, "", err
	}

	imageBase64 := base64.StdEncoding.EncodeToString(imageBuffer)

	return info, imageBase64, nil
}

What is the reason that my images become broken when I read them from the Blobstore?

I conjecture the reason this isn't working is because the reader.Read method is returning before it has filled the buffer. See the contract for io.Reader

> Read reads up to len(p) bytes into p. It returns the number of bytes
> read (0 <= n <= len(p)) and any error encountered. Even if Read
> returns n < len(p), it may use all of p as scratch space during the
> call. If some data is available but not len(p) bytes, Read
> conventionally returns what is available instead of waiting for more.

Note the last sentence in particular.

Instead of reader.Read(imageBuffer) try using ioutil.ReadAll which should fix your problem.