start.go

// Copyright 2024 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.

package telemetry

import (
	"io"
	"log"
	"os"
	"os/exec"

	"golang.org/x/telemetry/counter"
	"golang.org/x/telemetry/internal/crashmonitor"
)

// Config controls the behavior of [Start].
type Config struct {
	// ReportCrashes, if set, will enable crash reporting.
	// ReportCrashes uses the [debug.SetCrashOutput] mechanism, which is a
	// process-wide resource.
	// Do not make other calls to that function within your application.
	// ReportCrashes is a non-functional unless the program is built with go1.23+.
	ReportCrashes bool

	// Upload causes this program to periodically upload approved counters
	// from the local telemetry database to telemetry.go.dev.
	//
	// This option has no effect unless the user has given consent
	// to enable data collection, for example by running
	// cmd/gotelemetry or affirming the gopls dialog.
	//
	// (This feature is expected to be used only by gopls.
	// Longer term, the go command may become the sole program
	// responsible for uploading.)
	Upload bool

	// Log messages generated by crash monitoring or uploading will be
	// written to this optional writer.
	Logger io.Writer
}

// Start initializes telemetry using the specified configuration.
//
// Start opens the local telemetry database so that counter increment
// operations are durably recorded in the local file system.
//
// If [Config.Upload] is set, and the user has opted in to telemetry
// uploading, this process may attempt to upload approved counters
// to telemetry.go.dev. [But: this option doesn't appear to exist yet.]
//
// If [Config.ReportCrashes] is set, Start re-executes the current
// executable as a child process, in a special mode in which it acts
// as a crash monitor for the parent process (the application).
// In that mode, the call to Start will never return, so Start must
// be called immediately within main, even before such things as
// inspecting the command line. The application should avoid expensive
// steps or external side effects in init functions, as they will
// be executed twice (parent and child).
func Start(config Config) {
	counter.Open()
	if !config.ReportCrashes || !crashmonitor.Supported() {
		// TODO(matloob): Once support for uploading is added to start,
		// we'll just start the uploader instead of returning here.
		return
	}
	if os.Getenv(telemetryChildVar) != "" {
		child(config.Logger)
		panic("unreachable")
	}

	parent(config.Logger)
}

const telemetryChildVar = "X_TELEMETRY_CHILD"

func parent(logw io.Writer) {
	logger := log.New(logw, "", 0)
	// This process is the application (parent).
	// Fork+exec the telemetry child.
	exe, err := os.Executable()
	if err != nil {
		logger.Fatal(err)
	}
	cmd := exec.Command(exe, "** telemetry **") // this unused arg is just for ps(1)
	cmd.Env = append(os.Environ(), telemetryChildVar+"=1")
	cmd.Stderr = os.Stderr
	cmd.Stdout = os.Stderr
	pipe, err := cmd.StdinPipe()
	if err != nil {
		logger.Fatalf("StdinPipe: %v", err)
	}

	crashmonitor.Parent(pipe.(*os.File)) // (this conversion is safe)

	if err := cmd.Start(); err != nil {
		logger.Fatalf("can't start telemetry child process: %v", err)
	}
}

func child(logw io.Writer) {
	// TODO(matloob): add support for uploading.
	crashmonitor.Child(logw)
}