proposal: sync: add a Reset method for sync.Once

[SilverRainZ]

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

go version go1.10.3 linux/amd64

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

Assuming that there are some structure which has Open and Close method. Some initial stuff should be done for only once - so they share a sync.Once to do this.

type St struct {
    openOnce *sync.Once
    closeOnce *sync.Once
}

func(st *St) Open(){
    st.openOnce.Do(func() { ... }) // init
    ...
}

func(st *St) Close(){
    st.closeOnce.Do(func() { ... }) // deinit
    ...
}

But in this case, St can not be opened again after closed because the "sync.Once.Do()" is actually run once.

What did you expect to see?

I hope this struct can be opened for multiple times, I think adding a Reset method is an idea - then we can reset the openOnce in the Close method.

What did you see instead?

There is not a method to reset the state of sync.Once.

[ianlancetaylor]

Use ponce *sync.Once, and reset it by writing ponce = new(sync.Once).

Changing a pointer makes sense, but resetting a sync.Once does not. The whole point of a sync.Once is that it only happens once.