"defer recover( )" can not recover a panic.

[Fannie Zhang]

Hi all,

 

In the following case, recover() cannot recover a panic.

playground: https://go.dev/play/p/cPkUZb6mbmt

 

package main

 

import "fmt"

 

func main() {

  d := div(1, 0)

  fmt.Println("Returned from div(): ", d)

}

 

func div(a, b int64) int64 {

  defer recover()

  if b == 0 {

    panic("integer divide by zero")

  }

  return a/b

}

 

 

The gorecover() function (below) checks whether argp is equal to p.argp, if they match, the caller is the one who can recover.

 

func gorecover(argp uintptr) interface{} {

                gp := getg()

                p := gp._panic

                if p != nil && !p.goexit && !p.recovered && argp == uintptr(p.argp) {

                                p.recovered = true

                                return p.arg

                }

                return nil

}

 

But for the above case, argp = div()’arpg = main()’sp +8, p.argp = gopanic()’argp = div()’sp + 8. They are not equal, causing gorecover to fail to recover and return nil.

Of course, I know the rationality and necessity of this check, but from my understanding of the above case, the recover in div deferred function should be able to recover the panic that occurred in the div?

What do you think?

If you think it should be able to recover, then in this case, the argp passed to gorecover should be recover()'argp, which is div()'sp+8.

 

Thanks & Best regards,

Fannie Zhang

 

 

[Jan Mercl]

On Fri, Dec 10, 2021 at 9:13 AM Fannie Zhang <Fannie...@arm.com> wrote:

> In the following case, recover() cannot recover a panic.

Because the specs say the recover() works iff called in a deferred
function: https://go.dev/play/p/e-YxlmwC7gl

[Fannie...@arm.com]

So you mean that "defer recover()" is not that recover is not in the deferred function? But this case https://go.dev/play/p/CH9GkpGFbPd works. I think that these two examples are quite similar. 

Thank you,

Fannie

[Jan Mercl]

On Fri, Dec 10, 2021 at 11:54 AM Fannie...@arm.com <Fannie...@arm.com> wrote:

> So you mean that "defer recover()" is not that recover is not in the deferred function? But this case https://go.dev/play/p/CH9GkpGFbPd works. I think that these two examples are quite similar.

They are substantially different. In this new example recover() _is_
called in a deferred function. That's why it now works.

Exactly as the specs say.

[Fannie...@arm.com]

Ok, I got it. I misunderstood the defer function. Thank you for your explanation.

Fannie