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).

types/types.go

@@ -0,0 +1,166 @@
+// 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 types
+
+import (
+	"encoding/json"
+	"net"
+	"os"
+)
+
+// like net.IPNet but adds JSON marshalling and unmarshalling
+type IPNet net.IPNet
+
+// ParseCIDR takes a string like "10.2.3.1/24" and
+// return IPNet with "10.2.3.1" and /24 mask
+func ParseCIDR(s string) (*net.IPNet, error) {
+	ip, ipn, err := net.ParseCIDR(s)
+	if err != nil {
+		return nil, err
+	}
+
+	ipn.IP = ip
+	return ipn, nil
+}
+
+func (n IPNet) MarshalJSON() ([]byte, error) {
+	return json.Marshal((*net.IPNet)(&n).String())
+}
+
+func (n *IPNet) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err != nil {
+		return err
+	}
+
+	tmp, err := ParseCIDR(s)
+	if err != nil {
+		return err
+	}
+
+	*n = IPNet(*tmp)
+	return nil
+}
+
+// NetConf describes a network.
+type NetConf struct {
+	Name string `json:"name,omitempty"`
+	Type string `json:"type,omitempty"`
+	IPAM struct {
+		Type string `json:"type,omitempty"`
+	} `json:"ipam,omitempty"`
+}
+
+// Result is what gets returned from the plugin (via stdout) to the caller
+type Result struct {
+	IP4 *IPConfig `json:"ip4,omitempty"`
+	IP6 *IPConfig `json:"ip6,omitempty"`
+}
+
+func (r *Result) Print() error {
+	return prettyPrint(r)
+}
+
+// IPConfig contains values necessary to configure an interface
+type IPConfig struct {
+	IP      net.IPNet
+	Gateway net.IP
+	Routes  []Route
+}
+
+type Route struct {
+	Dst net.IPNet
+	GW  net.IP
+}
+
+type Error struct {
+	Code    uint   `json:"code"`
+	Msg     string `json:"msg"`
+	Details string `json:"details,omitempty"`
+}
+
+func (e *Error) Error() string {
+	return e.Msg
+}
+
+func (e *Error) Print() error {
+	return prettyPrint(e)
+}
+
+// net.IPNet is not JSON (un)marshallable so this duality is needed
+// for our custom IPNet type
+
+// JSON (un)marshallable types
+type ipConfig struct {
+	IP      IPNet   `json:"ip"`
+	Gateway net.IP  `json:"gateway,omitempty"`
+	Routes  []Route `json:"routes,omitempty"`
+}
+
+type route struct {
+	Dst IPNet  `json:"dst"`
+	GW  net.IP `json:"gw,omitempty"`
+}
+
+func (c *IPConfig) MarshalJSON() ([]byte, error) {
+	ipc := ipConfig{
+		IP:      IPNet(c.IP),
+		Gateway: c.Gateway,
+		Routes:  c.Routes,
+	}
+
+	return json.Marshal(ipc)
+}
+
+func (c *IPConfig) UnmarshalJSON(data []byte) error {
+	ipc := ipConfig{}
+	if err := json.Unmarshal(data, &ipc); err != nil {
+		return err
+	}
+
+	c.IP = net.IPNet(ipc.IP)
+	c.Gateway = ipc.Gateway
+	c.Routes = ipc.Routes
+	return nil
+}
+
+func (r *Route) UnmarshalJSON(data []byte) error {
+	rt := route{}
+	if err := json.Unmarshal(data, &rt); err != nil {
+		return err
+	}
+
+	r.Dst = net.IPNet(rt.Dst)
+	r.GW = rt.GW
+	return nil
+}
+
+func (r *Route) MarshalJSON() ([]byte, error) {
+	rt := route{
+		Dst: IPNet(r.Dst),
+		GW:  r.GW,
+	}
+
+	return json.Marshal(rt)
+}
+
+func prettyPrint(obj interface{}) error {
+	data, err := json.MarshalIndent(obj, "", "    ")
+	if err != nil {
+		return err
+	}
+	_, err = os.Stdout.Write(data)
+	return err
+}