CODEWITHSUNDEEP
goerror-handling
Nick Lee
Nick Lee

Reputation: 5959

Why recover() does not work in a nested deferred function?

I am testing panic/recover in Golang. This simple program works as expected:

package main

import "fmt"

func printRecover() {
    r := recover()
    fmt.Println("Recovered:", r)
}

func main() {
    defer printRecover()

    panic("OMG!")
}

Output:

Recovered: OMG!

However, if I wrap the function printRecover() in a bigger deferred function:

package main

import "fmt"

func printRecover() {
    r := recover()
    fmt.Println("Recovered:", r)
}

func main() {
    defer func() {
        printRecover()
    }()

    panic("OMG!")
}

It does not recover and let the panic go through:

Recovered: <nil>
panic: OMG!

goroutine 1 [running]:
main.main()
    /tmp/sandbox898315096/main.go:15 +0x60

Can someone explain the difference?

Upvotes: 6

Views: 1989

Answers (1)

Shettyh
Shettyh

Reputation: 1216

It is because recover will be nil if not directly called by deferred function

Here is excerpt from golang spec

The return value of recover is nil if any of the following conditions holds:

  1. panic's argument was nil;
  2. the goroutine is not panicking;
  3. recover was not called directly by a deferred function.

For more info check the full spec here

Upvotes: 13

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