plugins: adds new no-op plugin that may be used as a test-double

Plugin can be configured to record all inputs and to respond with
arbitrary stdout or error message.  Will support upcoming integration
testing.

plugins/test/noop/main.go

@@ -0,0 +1,73 @@
+// 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.
+
+/*
+Noop plugin is a CNI plugin designed for use as a test-double.
+
+When calling, set the CNI_ARGS env var equal to the path of a file containing
+the JSON encoding of a Debug.
+*/
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/containernetworking/cni/pkg/skel"
+	"github.com/containernetworking/cni/plugins/test/noop/debug"
+)
+
+func debugBehavior(args *skel.CmdArgs, command string) error {
+	if !strings.HasPrefix(args.Args, "DEBUG=") {
+		fmt.Printf(`{}`)
+		os.Stderr.WriteString("CNI_ARGS empty, no debug behavior\n")
+		return nil
+	}
+	debugFilePath := strings.TrimPrefix(args.Args, "DEBUG=")
+	debug, err := debug.ReadDebug(debugFilePath)
+	if err != nil {
+		return err
+	}
+
+	debug.CmdArgs = *args
+	debug.Command = command
+
+	err = debug.WriteDebug(debugFilePath)
+	if err != nil {
+		return err
+	}
+
+	if debug.ReportError != "" {
+		return errors.New(debug.ReportError)
+	} else {
+		os.Stdout.WriteString(debug.ReportResult)
+	}
+
+	return nil
+}
+
+func cmdAdd(args *skel.CmdArgs) error {
+	return debugBehavior(args, "ADD")
+}
+
+func cmdDel(args *skel.CmdArgs) error {
+	return debugBehavior(args, "DEL")
+}
+
+func main() {
+	skel.PluginMain(cmdAdd, cmdDel)
+}