GC finalizer does not run when object contains reference to itself

[Elemer Pixard]

Hi,

In my application I use finalizers to release C resources 

acquired by Go objects when these objects

are garbage collected. I noticed that some objects were not

being garbage collected (finalizer was not running) when

they reference themselves (GC cycle ?)

The program: https://play.golang.org/p/i2-rEutbWZ

also listed below, reproduces the problem.

The finalizers do not run If the subscription to

the internal dispatcher receiver

is NOT commented out, otherwise they run ok.

Is this correct behavior ?

Will these object ever be garbage collected ?

Any suggestions to  solve this problem ?

Regards.

package main

import (

"fmt"

"runtime"

"time"

)

type Dispatcher struct {

ID        int

callbacks []func()

}

func (d *Dispatcher) Subscribe(cb func()) {

d.callbacks = append(d.callbacks, cb)

}

func (d *Dispatcher) Dispatch() {

for _, cb := range d.callbacks {

cb()

}

}

func (d *Dispatcher) callback() {

fmt.Printf("Dispatcher:%d callback\n", d.ID)

}

func callback() {

fmt.Printf("Stand alone callback\n")

}

func main() {

// List of dispatchers

var disps []*Dispatcher

// Creates some dispatchers

for i := 1; i <= 5; i++ {

d := &Dispatcher{ID: i}

runtime.SetFinalizer(d, func(d *Dispatcher) {

fmt.Printf("Dispatcher:%d finalizer\n", d.ID)

})

// Subscribe to external function

d.Subscribe(callback)

// Subscribe to internal method.

// It will prevent finalizer to run (??)

//d.Subscribe(d.callback)

disps = append(disps, d)

}

// Test dispatch

for _, d := range disps {

d.Dispatch()

}

// Remove references to all dispatchers and force GC

// Finalizers should be executed.

disps = nil

runtime.GC()

// Give some time

time.Sleep(2)

}

[Ian Lance Taylor]

On Wed, May 25, 2016 at 3:24 PM, Elemer Pixard <elp...@gmail.com> wrote:
>
> In my application I use finalizers to release C resources
> acquired by Go objects when these objects
> are garbage collected. I noticed that some objects were not
> being garbage collected (finalizer was not running) when
> they reference themselves (GC cycle ?)
>
> The program: https://play.golang.org/p/i2-rEutbWZ
> also listed below, reproduces the problem.
> The finalizers do not run If the subscription to
> the internal dispatcher receiver
> is NOT commented out, otherwise they run ok.
>
> Is this correct behavior ?
> Will these object ever be garbage collected ?
> Any suggestions to solve this problem ?

The docs for SetFinalizer explain that cyclic structures are not
guaranteed to be finalized. This is not ideal but it is working as
expected.

In your example you should separate the Go struct that holds a C
pointer into a separate type that has just a single field. That
should avoid the cycle.

Ian

[keith....@gmail.com]

It gets even more interesting.  Self-pointers are ok (the object is still finalizeable), but a self-loop with another object is not.

Changing false to true in the code below changes the finalizer behavior.

package main

import (

"fmt"

"runtime"

"time"

)

type Dispatcher struct {

link *Dispatcher

}

func main() {

for i := 1; i <= 5; i++ {

d := &Dispatcher{}

runtime.SetFinalizer(d, func(d *Dispatcher) {

fmt.Println("Dispatcher finalizer")

})

if false {

d.link = d

} else {

e := &Dispatcher{}

e.link = d

d.link = e

}

}

runtime.GC()

time.Sleep(1000000)