pkg/skel: plugins now respond to VERSION command

To support CNI spec versioning, plugins must be able to report version
information to container runtimes.

pkg/version/version.go

@@ -0,0 +1,42 @@
+// Copyright 2016 CNI authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package version
+
+import (
+	"encoding/json"
+	"io"
+)
+
+// A PluginVersioner can encode information about its version
+type PluginVersioner interface {
+	Encode(io.Writer) error
+}
+
+// BasicVersioner is a PluginVersioner which reports a single cniVersion string
+type BasicVersioner struct {
+	CNIVersion string `json:"cniVersion"`
+}
+
+func (p *BasicVersioner) Encode(w io.Writer) error {
+	return json.NewEncoder(w).Encode(p)
+}
+
+// Current reports the version of the CNI spec implemented by this library
+func Current() string {
+	return "0.2.0"
+}
+
+// DefaultPluginVersioner reports the Current library spec version as the cniVersion
+var DefaultPluginVersioner = &BasicVersioner{CNIVersion: Current()}