close http response body to avoid potential memory leak

[allencloud]

It is an amazing thing for Swarm to use engine-api to contact with docker daemon. However in engine-api, after Do a request, sometimes it makes sure that response body is closed via ensureReaderClosed:

err = json.NewDecoder(resp.body).Decode(&dels)
ensureReaderClosed(resp)

In some cases, engine api returns an io.ReadCloser like below:

    resp, err := cli.tryImageCreate(ctx, query, options.RegistryAuth)
    if err != nil {
        return nil, err
    }
    return resp.body, nil

In the case above, Swarm should close the resp.body.

This PR did:

1. change pullResponse into pullResponseBody to make it more readable, as this is a type of io.ReadCloser or http.Response.Body
2. close http response body to avoid potential memory leak

Signed-off-by: Sun Hongliang [email protected]

[MHBauer]

That is what the docs say.

https://github.com/docker/engine-api/blob/master/client/image_pull.go#L17
https://github.com/docker/engine-api/blob/master/client/image_load.go#L13

I don't see the need to rename pullResponse to pullResponseBody. But that is just my opinion.

LGTM as is.

[allencloud]

@MHBauer
Thanks for your providing reference. 👍 :)

[vieux]

LGTM ping @abronan @dongluochen

[abronan]

LGTM