Call plugins with custom transports.

Small refactor to be able to use custom transports
to call remote plugins.

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

plugins/transport/transport.go

@@ -0,0 +1,36 @@
+package transport
+
+import (
+	"io"
+	"net/http"
+	"strings"
+)
+
+// VersionMimetype is the Content-Type the engine sends to plugins.
+const VersionMimetype = "application/vnd.docker.plugins.v1.2+json"
+
+// RequestFactory defines an interface that
+// transports can implement to create new requests.
+type RequestFactory interface {
+	NewRequest(path string, data io.Reader) (*http.Request, error)
+}
+
+// Transport defines an interface that plugin transports
+// must implement.
+type Transport interface {
+	http.RoundTripper
+	RequestFactory
+}
+
+// newHTTPRequest creates a new request with a path and a body.
+func newHTTPRequest(path string, data io.Reader) (*http.Request, error) {
+	if !strings.HasPrefix(path, "/") {
+		path = "/" + path
+	}
+	req, err := http.NewRequest("POST", path, data)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Add("Accept", VersionMimetype)
+	return req, nil
+}