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>
diff --git a/src/runtime/debug/example_monitor_test.go b/src/runtime/debug/example_monitor_test.go
new file mode 100644
index 0000000..5a1f4e1
--- /dev/null
+++ b/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...
+}
