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runtime/debug: add Example of SetCrashOutput in a crash monitor
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Updates golang#42888

Change-Id: I72e408301e17b1579bbea189bed6b1a0154bd55f
Reviewed-on: https://go-review.googlesource.com/c/go/+/548121
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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adonovan authored and ezz-no committed Feb 17, 2024
1 parent 6f310a5 commit 87caf03
Showing 1 changed file with 99 additions and 0 deletions.
99 changes: 99 additions & 0 deletions src/runtime/debug/example_monitor_test.go
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// Copyright 2023 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 debug_test

import (
"io"
"log"
"os"
"os/exec"
"runtime/debug"
)

// ExampleSetCrashOutput_monitor shows an example of using
// [debug.SetCrashOutput] to direct crashes to a "monitor" process,
// for automated crash reporting. The monitor is the same executable,
// invoked in a special mode indicated by an environment variable.
func ExampleSetCrashOutput_monitor() {
appmain()

// This Example doesn't actually run as a test because its
// purpose is to crash, so it has no "Output:" comment
// within the function body.
//
// To observe the monitor in action, replace the entire text
// of this comment with "Output:" and run this command:
//
// $ go test -run=ExampleSetCrashOutput_monitor runtime/debug
// panic: oops
// ...stack...
// monitor: saved crash report at /tmp/10804884239807998216.crash
}

// appmain represents the 'main' function of your application.
func appmain() {
monitor()

// Run the application.
println("hello")
panic("oops")
}

// monitor starts the monitor process, which performs automated
// crash reporting. Call this function immediately within main.
//
// This function re-executes the same executable as a child process,
// in a special mode. In that mode, the call to monitor will never
// return.
func monitor() {
const monitorVar = "RUNTIME_DEBUG_MONITOR"
if os.Getenv(monitorVar) != "" {
// This is the monitor (child) process.
log.SetFlags(0)
log.SetPrefix("monitor: ")

crash, err := io.ReadAll(os.Stdin)
if err != nil {
log.Fatalf("failed to read from input pipe: %v", err)
}
if len(crash) == 0 {
// Parent process terminated without reporting a crash.
os.Exit(0)
}

// Save the crash report securely in the file system.
f, err := os.CreateTemp("", "*.crash")
if err != nil {
log.Fatal(err)
}
if _, err := f.Write(crash); err != nil {
log.Fatal(err)
}
if err := f.Close(); err != nil {
log.Fatal(err)
}
log.Fatalf("saved crash report at %s", f.Name())
}

// This is the application process.
// Fork+exec the same executable in monitor mode.
exe, err := os.Executable()
if err != nil {
log.Fatal(err)
}
cmd := exec.Command(exe, "-test.run=ExampleSetCrashOutput_monitor")
cmd.Env = append(os.Environ(), monitorVar+"=1")
cmd.Stderr = os.Stderr
cmd.Stdout = os.Stderr
pipe, err := cmd.StdinPipe()
if err != nil {
log.Fatalf("StdinPipe: %v", err)
}
debug.SetCrashOutput(pipe.(*os.File)) // (this conversion is safe)
if err := cmd.Start(); err != nil {
log.Fatalf("can't start monitor: %v", err)
}
// Now return and start the application proper...
}

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