How to exit a go program honoring deferred calls?

Question

I need to use defer to free allocations manually created using C library, but I also need to os.Exit with non 0 status at some point. The tricky part is that os.Exit skips any deferred instruction:

package main

import "fmt"
import "os"

func main() {

    // `defer`s will _not_ be run when using `os.Exit`, so
    // this `fmt.Println` will never be called.
    defer fmt.Println("!")
    // sometimes ones might use defer to do critical operations
    // like close a database, remove a lock or free memory

    // Exit with status code.
    os.Exit(3)
}

Playground: http://play.golang.org/p/CDiAh9SXRM stolen from https://gobyexample.com/exit

So how to exit a go program honoring declared defer calls? Is there any alternative to os.Exit?

Solution 1

runtime.Goexit() is the easy way to accomplish that.

Goexit terminates the goroutine that calls it. No other goroutine is affected. Goexit runs all deferred calls before terminating the goroutine. Because Goexit is not panic, however, any recover calls in those deferred functions will return nil.

However:

Calling Goexit from the main goroutine terminates that goroutine without func main returning. Since func main has not returned, the program continues execution of other goroutines. If all other goroutines exit, the program crashes.

So if you call it from the main goroutine, at the top of main you need to add

defer os.Exit(0)

Below that you might want to add some other defer statements that inform the other goroutines to stop and clean up.

Solution 2

Just move your program down a level and return your exit code:

package main

import "fmt"
import "os"

func doTheStuff() int {
    defer fmt.Println("!")

    return 3
}

func main() {
    os.Exit(doTheStuff())
}

Solution 3

After some research, refer to this this, I found an alternative that:

• Doesn't impose a certain architecture like in https://stackoverflow.com/a/27629493/438563
• Doesn't require any global value like in https://stackoverflow.com/a/24601700/438563

We can take advantage of panic and recover. It turns out that panic, by nature, will honor defer calls but will also always exit with non 0 status code and dump a stack trace. The trick is that we can override last aspect of panic behavior with:

package main

import "fmt"
import "os"

type Exit struct{ Code int }

// exit code handler
func handleExit() {
    if e := recover(); e != nil {
        if exit, ok := e.(Exit); ok == true {
            os.Exit(exit.Code)
        }
        panic(e) // not an Exit, bubble up
    }
}

Now, to exit a program at any point and still preserve any declared defer instruction we just need to emit an Exit type:

func main() {
    defer handleExit() // plug the exit handler
    defer fmt.Println("cleaning...")
    panic(Exit{3}) // 3 is the exit code
}

It doesn't require any refactoring apart from plugging a line inside func main:

func main() {
    defer handleExit()
    // ready to go
}

This scales pretty well with larger code bases so I'll leave it available for scrutinization. Hope it helps.

Playground: http://play.golang.org/p/4tyWwhcX0-

Solution 4

For posterity, for me this was a more elegant solution:

func main() { 
    retcode := 0
    defer func() { os.Exit(retcode) }()
    defer defer1()
    defer defer2()

    [...]

    if err != nil {
        retcode = 1
        return
    }
}

View more solutions

Comments

[marcio]

So I shouldn't to use defer inside func main or any function that exits?

[Rob Napier]

More to the point, I don't recommend using os.Exit() in random places in the code. It makes testing very difficult besides the problem of error codes. peterSO's solution that @ctcherry links is ok, but it doesn't scale well IMO to a larger program. You'd have to make the code global. I believe you should keep main() fairly simple, and have it just take care of the OS-level things (like the final status code).

[marcio]

Hi, I found a way to handle this without impose any architecture. Anyway, +1 because it's a good advice to always try KISS first.

[marcio]

I had no idea runtime.Goexit() existed. Is this from some recent release?

[EMBLEM]

@marcio I did some digging in the Go repositories. I couldn't find exactly when it was introduced, but I did find this test that references it and is "Copyright 2013" from before you posted this question.

[EMBLEM]

@marcio I did a little more digging and found an archive of the documentation from 2009. Goexit is listed there.

[marcio]

I'm gonna mark this as the accepted answer. The other answers are definitely still valid, but this seems the simplest approach - for now :D

[xpt]

Best answer of the all so far! One question, how to deal with normal exit then? How to make sure the handleExit get called even with normal exit ? Ref: play.golang.org/p/QDJum4kOXk un-comment the //panic and see the difference.

[marcio]

I think a handle for a normal exit 0 event doesn't exist, but you can always panic(Exit{0}) and then handle it or maybe defer handleNormalExit() before anything else on main()?

[PRMan]

This really takes the best parts of the other answers.

[Macindows]

I was breaking my head exploring different ways around this issue; why os.exit does not also call the "defer" functions but in my case simply using "return" instead of os.exit solved the issue. Now I feel stupid I didnt think of it earlier

[Macindows]

on a side note, does defer1, defer2 execute in chronological order?

[Macindows]

Yes, this sounds more like the exceptions bubbling in java/C#

[Mike Gleason jr Couturier]

@Macindows This is Go standard: last to first. So defer2(), then defer1()

[rubens21]

This code will suppress panics. Also, if retcode is not changed (during a panic, for example), it will return always 0 (no erro). If you are sure your code is panic free, you are probably fine with that, though