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/exec.go

@@ -0,0 +1,66 @@
+// 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 (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+
+	"github.com/appc/cni/pkg/types"
+)
+
+func pluginErr(err error, output []byte) error {
+	if _, ok := err.(*exec.ExitError); ok {
+		emsg := types.Error{}
+		if perr := json.Unmarshal(output, &emsg); perr != nil {
+			return fmt.Errorf("netplugin failed but error parsing its diagnostic message %q: %v", string(output), perr)
+		}
+		details := ""
+		if emsg.Details != "" {
+			details = fmt.Sprintf("; %v", emsg.Details)
+		}
+		return fmt.Errorf("%v%v", emsg.Msg, details)
+	}
+
+	return err
+}
+
+func ExecPlugin(pluginPath string, netconf []byte, args CNIArgs) (*types.Result, error) {
+	if pluginPath == "" {
+		return nil, fmt.Errorf("could not find %q plugin", filepath.Base(pluginPath))
+	}
+
+	stdout := &bytes.Buffer{}
+
+	c := exec.Cmd{
+		Env:    args.AsEnv(),
+		Path:   pluginPath,
+		Args:   []string{pluginPath},
+		Stdin:  bytes.NewBuffer(netconf),
+		Stdout: stdout,
+		Stderr: os.Stderr,
+	}
+	if err := c.Run(); err != nil {
+		return nil, pluginErr(err, stdout.Bytes())
+	}
+
+	res := &types.Result{}
+	err := json.Unmarshal(stdout.Bytes(), res)
+	return res, err
+}