You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: external.go
    path of file: ./repos/streamdal/apps/server/vendor/golang.org/x/tools/go/packages
    
    the code of the file until where you have to start completion: // Copyright 2018 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file enables an external tool to intercept package requests.
// If the tool is present then its results are used in preference to
// the go list command.

package packages

import (
	"bytes"
	"encoding/json"
	"fmt"
	"os"
	"os/exec"
	"strings"
)

// The Driver Protocol
//
// The driver, given the inputs to a call to Load, returns metadata about the packages specified.
// This allows for different build systems to support go/packages by telling go/packages how the
// packages' source is organized.
// The driver is a binary, either specified by the GOPACKAGESDRIVER environment variable or in
// the path as gopackagesdriver. It's given the inputs to load in its argv. See the package
// documentation in doc.go for the full description of the patterns that need to be supported.
// A driver receives as a JSON-serialized driverRequest struct in standard input and will
// produce a JSON-serialized driverResponse (see definition in packages.go) in its standard output.

// driverRequest is used to provide the portion of Load's Config that is needed by a
    
func findExternalDriver(cfg *Config) driver {
	const toolPrefix = "GOPACKAGESDRIVER="
	tool := ""
	for _, env := range cfg.Env {
		if val := strings.TrimPrefix(env, toolPrefix); val != env {
			tool = val
		}
	}
	if tool != "" && tool == "off" {
		return nil
	}
	if tool == "" {
		var err error
		tool, err = exec.LookPath("gopackagesdriver")
		if err != nil {
			return nil
		}
	}
	return func(cfg *Config, words ...string) (*driverResponse, error) {
		req, err := json.Marshal(driverRequest{
			Mode:       cfg.Mode,
			Env:        cfg.Env,
			BuildFlags: cfg.BuildFlags,
			Tests:      cfg.Tests,
			Overlay:    cfg.Overlay,
		})
		if err != nil {
			return nil, fmt.Errorf("failed to encode message to driver tool: %v", err)
		}

		buf := new(bytes.Buffer)
		stderr := new(bytes.Buffer)
		cmd := exec.CommandContext(cfg.Context, tool, words...)
		cmd.Dir = cfg.Dir
		cmd.Env = cfg.Env
		cmd.Stdin = bytes.NewReader(req)
		cmd.Stdout = buf
		cmd.Stderr = stderr

		if err := cmd.Run(); err != nil {
			return nil, fmt.Errorf("%v: %v: %s", tool, err, cmd.Stderr)
		}
		if len(stderr.Bytes()) != 0 && os.Getenv("GOPACKAGESPRINTDRIVERERRORS") != "" {
			fmt.Fprintf(os.Stderr, "%s stderr: <<%s>>\n", cmdDebugStr(cmd), stderr)
		}

		var response driverResponse
		if err := json.Unmarshal(buf.Bytes(), &response); err != nil {
			return nil, err
		}
		return &response, nil
	}
}