packer-cn/vendor/github.com/mitchellh/panicwrap

panicwrap

panicwrap is a Go library that re-executes a Go binary and monitors stderr output from the binary for a panic. When it finds a panic, it executes a user-defined handler function. Stdout, stderr, stdin, signals, and exit codes continue to work as normal, making the existence of panicwrap mostly invisible to the end user until a panic actually occurs.

Since a panic is truly a bug in the program meant to crash the runtime, globally catching panics within Go applications is not supposed to be possible. Despite this, it is often useful to have a way to know when panics occur. panicwrap allows you to do something with these panics, such as writing them to a file, so that you can track when panics occur.

panicwrap is not a panic recovery system. Panics indicate serious problems with your application and should crash the runtime. panicwrap is just meant as a way to monitor for panics. If you still think this is the worst idea ever, read the section below on why.

Features

• SIMPLE!
• Works with all Go applications on all platforms Go supports
• Custom behavior when a panic occurs
• Stdout, stderr, stdin, exit codes, and signals continue to work as expected.

Usage

Using panicwrap is simple. It behaves a lot like fork, if you know how that works. A basic example is shown below.

Because it would be sad to panic while capturing a panic, it is recommended that the handler functions for panicwrap remain relatively simple and well tested. panicwrap itself contains many tests.

package main

import (
	"fmt"
	"github.com/mitchellh/panicwrap"
	"os"
)

func main() {
	exitStatus, err := panicwrap.BasicWrap(panicHandler)
	if err != nil {
		// Something went wrong setting up the panic wrapper. Unlikely,
		// but possible.
		panic(err)
	}

	// If exitStatus >= 0, then we're the parent process and the panicwrap
	// re-executed ourselves and completed. Just exit with the proper status.
	if exitStatus >= 0 {
		os.Exit(exitStatus)
	}

	// Otherwise, exitStatus < 0 means we're the child. Continue executing as
	// normal...

	// Let's say we panic
	panic("oh shucks")
}

func panicHandler(output string) {
	// output contains the full output (including stack traces) of the
	// panic. Put it in a file or something.
	fmt.Printf("The child panicked:\n\n%s\n", output)
	os.Exit(1)
}

How Does it Work?

panicwrap works by re-executing the running program (retaining arguments, environmental variables, etc.) and monitoring the stderr of the program. Since Go always outputs panics in a predictable way with a predictable exit code, panicwrap is able to reliably detect panics and allow the parent process to handle them.

WHY?! Panics should CRASH!

Yes, panics should crash. They are 100% always indicative of bugs and having information on a production server or application as to what caused the panic is critical.

User Facing

In user-facing programs (programs like Packer or Docker), it is up to the user to report such panics. This is unreliable, at best, and it would be better if the program could have a way to automatically report panics. panicwrap provides a way to do this.

Server

For backend applications, it is easier to detect crashes (since the application exits) and having an idea as to why the crash occurs is equally important; particularly on a production server.

At HashiCorp, we use panicwrap to log panics to timestamped files with some additional data (configuration settings at the time, environmental variables, etc.)

The goal of panicwrap is not to hide panics. It is instead to provide a clean mechanism for capturing them and ultimately crashing.