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).

libcni/api.go

@@ -0,0 +1,68 @@
+// 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 libcni
+
+import (
+	"strings"
+
+	"github.com/appc/cni/pkg/invoke"
+	"github.com/appc/cni/pkg/types"
+)
+
+type RuntimeConf struct {
+	ContainerID string
+	NetNS       string
+	IfName      string
+	Args        [][2]string
+}
+
+type NetworkConfig struct {
+	Network *types.NetConf
+	Bytes   []byte
+}
+
+type CNI interface {
+	AddNetwork(net *NetworkConfig, rt *RuntimeConf) (*types.Result, error)
+	DelNetwork(net *NetworkConfig, rt *RuntimeConf) error
+}
+
+type CNIConfig struct {
+	Path []string
+}
+
+func (c *CNIConfig) AddNetwork(net *NetworkConfig, rt *RuntimeConf) (*types.Result, error) {
+	return c.execPlugin("ADD", net, rt)
+}
+
+func (c *CNIConfig) DelNetwork(net *NetworkConfig, rt *RuntimeConf) error {
+	_, err := c.execPlugin("DEL", net, rt)
+	return err
+}
+
+// =====
+
+func (c *CNIConfig) execPlugin(action string, conf *NetworkConfig, rt *RuntimeConf) (*types.Result, error) {
+	pluginPath := invoke.FindInPath(conf.Network.Type, c.Path)
+
+	args := &invoke.Args{
+		Command:     action,
+		ContainerID: rt.ContainerID,
+		NetNS:       rt.NetNS,
+		PluginArgs:  rt.Args,
+		IfName:      rt.IfName,
+		Path:        strings.Join(c.Path, ":"),
+	}
+	return invoke.ExecPlugin(pluginPath, conf.Bytes, args)
+}

libcni/conf.go

@@ -0,0 +1,85 @@
+// 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 libcni
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sort"
+)
+
+func ConfFromBytes(bytes []byte) (*NetworkConfig, error) {
+	conf := &NetworkConfig{Bytes: bytes}
+	if err := json.Unmarshal(bytes, &conf.Network); err != nil {
+		return nil, fmt.Errorf("error parsing configuration: %s", err)
+	}
+	return conf, nil
+}
+
+func ConfFromFile(filename string) (*NetworkConfig, error) {
+	bytes, err := ioutil.ReadFile(filename)
+	if err != nil {
+		return nil, fmt.Errorf("error reading %s: %s", filename, err)
+	}
+	return ConfFromBytes(bytes)
+}
+
+func ConfFiles(dir string) ([]string, error) {
+	// In part, adapted from rkt/networking/podenv.go#listFiles
+	files, err := ioutil.ReadDir(dir)
+	switch {
+	case err == nil: // break
+	case os.IsNotExist(err):
+		return nil, nil
+	default:
+		return nil, err
+	}
+
+	confFiles := []string{}
+	for _, f := range files {
+		if f.IsDir() {
+			continue
+		}
+		if filepath.Ext(f.Name()) == ".conf" {
+			confFiles = append(confFiles, filepath.Join(dir, f.Name()))
+		}
+	}
+	return confFiles, nil
+}
+
+func LoadConf(dir, name string) (*NetworkConfig, error) {
+	files, err := ConfFiles(dir)
+	switch {
+	case err != nil:
+		return nil, err
+	case len(files) == 0:
+		return nil, fmt.Errorf("no net configurations found")
+	}
+	sort.Strings(files)
+
+	for _, confFile := range files {
+		conf, err := ConfFromFile(confFile)
+		if err != nil {
+			return nil, err
+		}
+		if conf.Network.Name == name {
+			return conf, nil
+		}
+	}
+	return nil, fmt.Errorf(`no net configuration with name "%s" in %s`, name, dir)
+}