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// 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 (
// 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 driver.
type driverRequest struct {
Mode LoadMode `json:"mode"`
// Env specifies the environment the underlying build system should be run in.
Env []string `json:"env"`
// BuildFlags are flags that should be passed to the underlying build system.
BuildFlags []string `json:"build_flags"`
// Tests specifies whether the patterns should also return test packages.
Tests bool `json:"tests"`
// Overlay maps file paths (relative to the driver's working directory) to the byte contents
// of overlay files.
Overlay map[string][]byte `json:"overlay"`
// findExternalDriver returns the file path of a tool that supplies
// the build system package structure, or "" if not found."
// If GOPACKAGESDRIVER is set in the environment findExternalTool returns its
// value, otherwise it searches for a binary named gopackagesdriver on the PATH.
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, words...), stderr)
var response driverResponse
if err := json.Unmarshal(buf.Bytes(), &response); err != nil {
return nil, err
return &response, nil