Factor an API out into a module

This takes some of the machinery from CNI and from the rkt networking
code, and turns it into a library that can be linked into go apps.

Included is an example command-line application that uses the library,
called `cnitool`.

Other headline changes:

 * Plugin exec'ing is factored out

The motivation here is to factor out the protocol for invoking
plugins. To that end, a generalisation of the code from api.go and
pkg/plugin/ipam.go goes into pkg/invoke/exec.go.

 * Move argument-handling and conf-loading into public API

The fact that the arguments get turned into an environment for the
plugin is incidental to the API; so, provide a way of supplying them
as a struct or saying "just use the same arguments as I got" (the
latter is for IPAM plugins).

pkg/invoke/args.go

@@ -0,0 +1,76 @@
+// Copyright 2015 CoreOS, Inc.
+//
+// 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 invoke
+
+import (
+	"os"
+	"strings"
+)
+
+type CNIArgs interface {
+	// For use with os/exec; i.e., return nil to inherit the
+	// environment from this process
+	AsEnv() []string
+}
+
+type inherited struct{}
+
+var inheritArgsFromEnv inherited
+
+func (_ *inherited) AsEnv() []string {
+	return nil
+}
+
+func ArgsFromEnv() CNIArgs {
+	return &inheritArgsFromEnv
+}
+
+type Args struct {
+	Command       string
+	ContainerID   string
+	NetNS         string
+	PluginArgs    [][2]string
+	PluginArgsStr string
+	IfName        string
+	Path          string
+}
+
+func (args *Args) AsEnv() []string {
+	env := os.Environ()
+	pluginArgsStr := args.PluginArgsStr
+	if pluginArgsStr == "" {
+		pluginArgsStr = stringify(args.PluginArgs)
+	}
+
+	env = append(env,
+		"CNI_COMMAND="+args.Command,
+		"CNI_CONTAINERID="+args.ContainerID,
+		"CNI_NETNS="+args.NetNS,
+		"CNI_ARGS="+pluginArgsStr,
+		"CNI_IFNAME="+args.IfName,
+		"CNI_PATH="+args.Path)
+	return env
+}
+
+// taken from rkt/networking/net_plugin.go
+func stringify(pluginArgs [][2]string) string {
+	entries := make([]string, len(pluginArgs))
+
+	for i, kv := range pluginArgs {
+		entries[i] = strings.Join(kv[:], "=")
+	}
+
+	return strings.Join(entries, ";")
+}