Golang io.copy twice on the request body

I would suggest using an io.TeeReader if you want to push all reads from the blob through the sha1 concurrently.

bodyReader := io.TeeReader(body, hash)

Now as the bodyReader is consumed during upload, the hash is automatically updated.

You can't do that directly but you can write a wrapper that does the hashing on io.Copy

// this works for either a reader or writer, 
//  but if you use both in the same time the hash will be wrong.
type Hasher struct {
    io.Writer
    io.Reader
    hash.Hash
    Size uint64
}

func (h *Hasher) Write(p []byte) (n int, err error) {
    n, err = h.Writer.Write(p)
    h.Hash.Write(p)
    h.Size += uint64(n)
    return
}

func (h *Hasher) Read(p []byte) (n int, err error) {
    n, err = h.Reader.Read(p)
    h.Hash.Write(p[:n]) //on error n is gonna be 0 so this is still safe.
    return
}

func (h *Hasher) Sum() string {
    return hex.EncodeToString(h.Hash.Sum(nil))
}

func (h *UploadHandle) Read() (io.Reader, string, int64, error) {
    var b bytes.Buffer

    hashedReader := &Hasher{Reader: h.Contents, Hash: sha1.New()}
    n, err := io.Copy(&b, hashedReader)

    if err != nil {
        return nil, "", 0, err
    }

    return &b, hashedReader.Sum(), n, nil
}

// updated version based on @Dustin's comment since I complete forgot io.TeeReader existed.

func (h *UploadHandle) Read() (io.Reader, string, int64, error) {
    var b bytes.Buffer

    hash := sha1.New()
    n, err := io.Copy(&b, io.TeeReader(h.Contents, hash))

    if err != nil {
        return nil, "", 0, err
    }

    return &b, hex.EncodeToString(hash.Sum(nil)), n, nil
}

You have two options.

The most direct way is to use io.MultiWriter.

But if you need the hash to produce the multipart output, then you will have to copy to a bytes.Buffer and then write the buffer back to each writer.