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?

Answer 1

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())
}

Answer 2

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.

Answer 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-

Answer 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
    }
}