reset sync.OnceValue

[Hartmut Wieselburger]

Hi,

is there a way to reset sync.OnceValue(s), as you can do it with sync.Once?

A common use case is loading a config. I would like to switch from sync.Once to sync.OnceValue, but I need to a chance to reload my config, without restarting my service.

If this is possible with sync.OnceValue, please let me know. Otherwise the new OnceValue(s) are useless for me.

Regards, Harti

[burak serdar]

On Thu, Nov 21, 2024 at 1:14 PM 'Hartmut Wieselburger' via golang-nuts
<golan...@googlegroups.com> wrote:
>
> Hi,
> is there a way to reset sync.OnceValue(s), as you can do it with sync.Once?

How can you reset a sync.Once?

> A common use case is loading a config. I would like to switch from sync.Once to sync.OnceValue, but I need to a chance to reload my config, without restarting my service.
> If this is possible with sync.OnceValue, please let me know. Otherwise the new OnceValue(s) are useless for me.
>
> Regards, Harti
>

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[Hartmut Wieselburger]

var configOnce sync.Once
func Reset() {
configOnce = sync.Once{}
}

func Load() *Config {
configOnce.Do(func() {
...

[Volker Dobler]

You can do the same with OnceValue, but neither is a good idea

as it totally invalidates what sync.Once is good for: preventing

data races which your "reset" (in quotation marks as you do not

reset the sync.Once) provokes.

V.

[Hartmut Wieselburger]

| You can do the same with OnceValue

Can you give an example?

[Volker Dobler]

var fOnce func() T
func Reset() {
fOnce = sync.OnceValue(your code here)
}
But as I said: Both is _wrong_. Really totally wrong.

The OnceValue code should make this clear.

Stop doing this. Redesign.

V.

[burak serdar]

On Fri, Nov 22, 2024 at 1:53 AM 'Hartmut Wieselburger' via golang-nuts
<golan...@googlegroups.com> wrote:
>
> var configOnce sync.Once
> func Reset() {
> configOnce = sync.Once{}
> }

This is not resetting. You are creating a new sync.Once and assigning
that to a variable. That variable, configOnce, is shared among
multiple goroutines, because that is what Once is used for, to limit
the execution of a function called by multiple goroutines to a single
execution. The Reset function sets this shared variable without any
synchronization, so this is a data race.

Also note that reassigning a Once instance like this may not behave as
you expect. Any goroutine that is waiting on that Once instance will
continue waiting until the Once function is done. The behavior of a
goroutine that reads Once after Reset() is undefined. Since there is a
data race, there is no guarantee on which instance of Once another
goroutine will see. A goroutine may see the completed instance of Once
even after you Reset() it.

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[Hartmut Wieselburger]

Ok, thanks, once means once. So https://pkg.go.dev/sync/atomic#Value seems to be a better fit, do you agree?

[burak serdar]

You can switch an active configuration using atomic.Value, after
initializing it with a Once. If your config is not initialized,
atomic.Value will not block others.

On Mon, Nov 25, 2024 at 1:05 AM 'Hartmut Wieselburger' via golang-nuts

> To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/0479f342-0b35-49b4-be81-c631bfe15900n%40googlegroups.com.