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/ipam/ipam.go

@@ -0,0 +1,75 @@
+// 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 ipam
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/appc/cni/pkg/invoke"
+	"github.com/appc/cni/pkg/ip"
+	"github.com/appc/cni/pkg/types"
+
+	"github.com/vishvananda/netlink"
+)
+
+func ExecAdd(plugin string, netconf []byte) (*types.Result, error) {
+	if os.Getenv("CNI_COMMAND") != "ADD" {
+		return nil, fmt.Errorf("CNI_COMMAND is not ADD")
+	}
+	return invoke.ExecPlugin(invoke.Find(plugin), netconf, invoke.ArgsFromEnv())
+}
+
+func ExecDel(plugin string, netconf []byte) error {
+	if os.Getenv("CNI_COMMAND") != "DEL" {
+		return fmt.Errorf("CNI_COMMAND is not DEL")
+	}
+	_, err := invoke.ExecPlugin(invoke.Find(plugin), netconf, invoke.ArgsFromEnv())
+	return err
+}
+
+// ConfigureIface takes the result of IPAM plugin and
+// applies to the ifName interface
+func ConfigureIface(ifName string, res *types.Result) error {
+	link, err := netlink.LinkByName(ifName)
+	if err != nil {
+		return fmt.Errorf("failed to lookup %q: %v", ifName, err)
+	}
+
+	if err := netlink.LinkSetUp(link); err != nil {
+		return fmt.Errorf("failed to set %q UP: %v", ifName, err)
+	}
+
+	// TODO(eyakubovich): IPv6
+	addr := &netlink.Addr{IPNet: &res.IP4.IP, Label: ""}
+	if err = netlink.AddrAdd(link, addr); err != nil {
+		return fmt.Errorf("failed to add IP addr to %q: %v", ifName, err)
+	}
+
+	for _, r := range res.IP4.Routes {
+		gw := r.GW
+		if gw == nil {
+			gw = res.IP4.Gateway
+		}
+		if err = ip.AddRoute(&r.Dst, gw, link); err != nil {
+			// we skip over duplicate routes as we assume the first one wins
+			if !os.IsExist(err) {
+				return fmt.Errorf("failed to add route '%v via %v dev %v': %v", r.Dst, gw, ifName, err)
+			}
+		}
+	}
+
+	return nil
+}