Implement docker pull with standalone client lib.

Signed-off-by: David Calavera <david.calavera@gmail.com>

client/image_create.go

@@ -13,11 +13,14 @@ func (cli *Client) ImageCreate(options types.ImageCreateOptions) (io.ReadCloser,
 	query := url.Values{}
 	query.Set("fromImage", options.Parent)
 	query.Set("tag", options.Tag)
-
-	headers := map[string][]string{"X-Registry-Auth": {options.RegistryAuth}}
-	resp, err := cli.post("/images/create", query, nil, headers)
+	resp, err := cli.tryImageCreate(query, options.RegistryAuth)
 	if err != nil {
 		return nil, err
 	}
 	return resp.body, nil
 }
+
+func (cli *Client) tryImageCreate(query url.Values, registryAuth string) (*serverResponse, error) {
+	headers := map[string][]string{"X-Registry-Auth": {registryAuth}}
+	return cli.post("/images/create", query, nil, headers)
+}

client/image_pull.go

@@ -0,0 +1,34 @@
+package lib
+
+import (
+	"io"
+	"net/http"
+	"net/url"
+
+	"github.com/docker/docker/api/types"
+)
+
+// ImagePull request the docker host to pull an image from a remote registry.
+// It executes the privileged function if the operation is unauthorized
+// and it tries one more time.
+// It's up to the caller to handle the io.ReadCloser and close it properly.
+func (cli *Client) ImagePull(options types.ImagePullOptions, privilegeFunc RequestPrivilegeFunc) (io.ReadCloser, error) {
+	query := url.Values{}
+	query.Set("fromImage", options.ImageID)
+	if options.Tag != "" {
+		query.Set("tag", options.Tag)
+	}
+
+	resp, err := cli.tryImageCreate(query, options.RegistryAuth)
+	if resp.statusCode == http.StatusUnauthorized {
+		newAuthHeader, privilegeErr := privilegeFunc()
+		if privilegeErr != nil {
+			return nil, privilegeErr
+		}
+		resp, err = cli.tryImageCreate(query, newAuthHeader)
+	}
+	if err != nil {
+		return nil, err
+	}
+	return resp.body, nil
+}

client/privileged.go

@@ -0,0 +1,9 @@
+package lib
+
+// RequestPrivilegeFunc is a function interface that
+// clients can supply to retry operations after
+// getting an authorization error.
+// This function returns the registry authentication
+// header value in base 64 format, or an error
+// if the privilege request fails.
+type RequestPrivilegeFunc func() (string, error)

types/client.go

@@ -179,6 +179,14 @@ type ImageListOptions struct {
 	Filters   filters.Args
 }
 
+// ImagePullOptions holds information to pull images.
+type ImagePullOptions struct {
+	ImageID string
+	Tag     string
+	// RegistryAuth is the base64 encoded credentials for this server
+	RegistryAuth string
+}
+
 // ImageRemoveOptions holds parameters to remove images.
 type ImageRemoveOptions struct {
 	ImageID       string