runtime/debug: add Example of SetCrashOutput in a crash monitor

Updates #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>

src/runtime/debug/example_monitor_test.go

@@ -0,0 +1,99 @@
+// 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...
+}