spec: recover() in nested defer

[guangda-hu]

What version of Go are you using (go version)?

Go playground

What did you do?

package main

import (
	"fmt"
)

func main() {
	defer func() {
		defer func() {
			fmt.Println(recover())
		}()
	}()
	panic("abcd")
}

See https://play.golang.org/p/0BULKAck6b6.

The recover returns nil, and the program panics.

What did you expect to see?

From the spec:

Suppose a function G defers a function D that calls recover and a panic occurs in a function on the same goroutine in which G is executing. When the running of deferred functions reaches D, the return value of D's call to recover will be the value passed to the call of panic.

In this case, G is the function deferred in main, and D is the func() { fmt.Println(recover()) } which is deferred in G. When there is the panic in the same goroutine the G runs, the recover does not recover the panic.

I did some experiments and it seems the recover in D only recovers panics inside G, assuming D is deferred in G. So probably the spec can be more accurate.

[ianlancetaylor]

The spec is accurate. In your example, there is no panic that occurs in the goroutine in which G is executing. A panic occurred before G started.

This is working as intended. It is intended because any other approach would make it impossible for the deferred function (G) to reliably call a function that itself uses panic/recover.

Closing. If you want to discuss this further, please use a forum (https://golang.org/wiki/Questions), not the issue tracker. Thanks.