A little weird thing in using finalizer

[T L]

The following program might print:

31
37
47
47
37
31

or

31
37
47
47

but with the highest possibility to print

31
37
47

Is not normal?

package main

import (
    "fmt"
    "math/rand"
    "runtime"
    "time"
    "unsafe"
)

type Foo struct {
    x int
    a int
}

func main() {
    for i := 0; i < 3; i++ {
        f := NewFoo(i)
        println(f.a)
    }

    runtime.GC()
   
    time.After(time.Second)
}

func do(i *uint) {
    runtime.SetFinalizer(i, func(f *uint) {
        fmt.Println(*f)
    })
}

//go:noinline
func NewFoo(i int) *Foo {
    f := &Foo{a: rand.Intn(50)}
   
    do((*uint)(unsafe.Pointer(&f.a)))

    return f
}

[Ian Lance Taylor]

Finalizers are not guaranteed to run.

That said, I'm not sure why you are calling time.After. That doesn't
make finalizers any more likely to run. You'll probably see fairly
reliable results if you change that to call time.Sleep instead.

Ian

[T L]

With time.Sleep, it always prints:

31
37
47
47
37
31

Why is each of the finalizers called twice?

[Brian Candler]

On Saturday, 25 April 2020 06:35:22 UTC+1, T L wrote:

Why is each of the finalizers called twice?

Isn't it just printed once in main and once in finalizer?

https://play.golang.org/p/7aY9NA1AFNa

[T L]

Aha, I'm sorry I forgot that println line. I thought all the lines are printed in finalizers. ;D